Oxford Instruments ILM200 Service manual

Operator’s Handbook
ILM200
Family of Cryogen Level Meters
Issue 1.5
April 2000
File reference: ILM-15.DOC
Oxford Instruments
Superconductivity
Tubney Woods, Abingdon,
Oxon, OX13 5QX, England
Tel: [phone]
Fax: [phone]
E-mail:[email protected]
www.oxford-instruments.com

Contents
1 Introduction .............................................................................................................. 5
1.1 Use of this Manual...................................................................................... 5
1.2 Description of the ILM200 Family .............................................................. 5
1.3 Principles of Operation .............................................................................. 5
1.3.1 Helium Level Probe................................................................... 5
1.3.2 Nitrogen Level Probe................................................................ 6
1.3.3 Why Two Different Sensing Methods Are Used ..................... 6
2 Safety.........................................................................................................................7
2.1 Protective Ground ...................................................................................... 7
2.2 Working Environment................................................................................ 7
2.3 Repair and Adjustment .............................................................................. 7
3 Installation ................................................................................................................ 8
3.1 Supply Connections .................................................................................... 8
3.2 Probe Connections...................................................................................... 8
3.3 Compatibility with Earlier Probes.............................................................. 8
3.4 Auxiliary Port Connections......................................................................... 9
3.5 Relay Contact............................................................................................ 10
3.6 Connections for Automatic Magnet Run Down ..................................... 10
3.7 Analogue Outputs .................................................................................... 11
3.8 RS232 Serial Data Line Connections ........................................................ 11
3.9 The Oxford Instruments Isobus................................................................ 12
3.10 GPIB (IEEE-488) Connection (Optional) ................................................... 12
3.11 The GPIB to ISOBUS Gateway .................................................................. 13
4 Operation ................................................................................................................ 14
4.1 Front Panel Controls................................................................................. 14
4.2 First Time Operation................................................................................. 15
4.3 "Err" Display ............................................................................................. 16
4.4 Sample Rate Selection.............................................................................. 16
4.5 Calibration ................................................................................................ 16
4.5.1 Trimming the Calibration....................................................... 16
4.5.2 Setting the Default Calibration ............................................. 17
4.6 Storing Calibration and other Power-Up Defaults ................................. 18
5 Auto-Fill and Alarms............................................................................................... 19
5.1 Level Thresholds ....................................................................................... 19
5.2 Automatic Filling ...................................................................................... 19
5.3 Automatic Rate Switching ....................................................................... 20
5.4 Audible Alarm........................................................................................... 21
5.5 Automatic Shut Down.............................................................................. 21
6 Remote Operation .................................................................................................. 22
6.1 Introduction.............................................................................................. 22
6.2 Communication Protocols ........................................................................ 22
6.3 Commands and Responses....................................................................... 22
6.4 Numeric Parameters ................................................................................. 23

6.5 Use with Oxford Instruments ISOBUS...................................................... 23
6.6 The GPIB Interface.................................................................................... 24
6.6.1 Sending Commands via the GPIB........................................... 25
6.6.2 Accepting Responses via the GPIB ......................................... 25
6.6.3 The Status Byte, Use of a Serial Poll ...................................... 25
6.6.4 Use of the Service Request Line............................................. 26
6.6.5 Use of the Device Clear Function........................................... 26
6.6.6 Use of the Interface Clear (IFC) Function .............................. 26
6.6.7 Non-Implemented Features of the GPIB ............................... 26
6.6.8 Compatibility with IEEE-488.2................................................ 27
6.6.9 Use of the GPIB Interface as a GATEWAY to ISOBUS ........... 27
6.6.10 Writing a "Rugged" GPIB Control Program.......................... 27
7 Command List ......................................................................................................... 28
8 Command Syntax .................................................................................................... 29
8.1 User Commands........................................................................................ 29
8.2 Interpreting the Status Message ............................................................. 31
8.2.1 Example Sequence.................................................................. 33
9 Configuration and Test Mode................................................................................ 35
9.1 Use of Test Mode...................................................................................... 35
9.2 Entering Test Mode.................................................................................. 35
9.3 Test t.01 Testing the Display and Relays ................................................. 35
9.4 Test t.02 Testing the Keys ....................................................................... 35
9.5 Test t.03 Setting the GPIB Address ......................................................... 36
9.6 Test t.04 The Diagnostic Front Panel Display......................................... 36
9.7 Test t.05 Configuring the Channels........................................................ 36
9.8 Test t.06 Calibrating the Current Sources .............................................. 37
9.9 Test t.07 Setting Probe Active Length.................................................... 38
9.10 Test t.08 Setting Helium Probe Currents................................................ 38
9.11 Test t.09 Setting Helium Probe Pulse Widths......................................... 39
9.12 Test t.10 Setting Helium Probe Sample Rates........................................ 39
9.13 Test t.11 Setting Full, Fill and Low Thresholds....................................... 39
9.14 Test t.12 Setting Stepper Motor Operating Mode ................................ 40
9.15 Test t.13 Manual Adjustment of Needle Valve...................................... 40
9.16 Test t.14 Setting Helium Probe Wire Resistivity..................................... 40
9.17 A Final Reminder ...................................................................................... 40
10 Servicing .................................................................................................................. 41
10.1 Safety ........................................................................................................ 41
10.2 Circuit Description .................................................................................... 41
10.3 Reset Procedures....................................................................................... 42
10.3.1 Entry to Test Mode ................................................................. 42
10.3.2 Two Button Reset ................................................................... 43
10.3.3 Three Button Reset................................................................. 43

11 In Case of Difficulty ................................................................................................ 44
12 Specification............................................................................................................ 45
13 Quick Reference Guide........................................................................................... 47
13.1 Panel Controls and Lamps........................................................................ 47
13.2 Serial / GPIB Commands ........................................................................... 47
13.3 Interpreting the Status Message ............................................................. 48
13.4 Test Mode Menu....................................................................................... 49
14 Circuit Diagrams...................................................................................................... 52
Warnings
Before you operate this equipment for the first time, please make sure that you are
aware of the precautions which you must take to ensure your own safety. In particular
please read the Safety section of this manual.
Explosive Atmospheres
The ILM is intended quite specifically for use with liquid helium and liquid nitrogen. It is
not intended for use in an inflammable or explosive atmosphere. It must not be used
under any circumstances for monitoring the level of combustible liquids or in the
presence of combustible or explosive gases.
Important Note
This manual is part of the product that you have bought. Please keep it for the whole
life of the product and make sure that you incorporate any amendments which might be
sent to you. If you sell or give away the product to someone else please give them the
manual too. Before you attempt to install or operate this equipment for the first time,
please make sure that you are aware of the precautions that you must take to ensure
your own safety.
Oxford Instruments Superconductivity Limited, April 2000. All rights strictly reserved.

5
1Introduction
1.1 Use of this Manual
This manual provides operating and service information for the Oxford Instruments
ILM200 Family of Cryogen Level Meters. Sections 1-4 provide essential information and
should be read before operating the instrument for the first time.
The remainder of the manual provides more detail on specific aspects and may be
referred to as required. Section 11 attempts to identify some of the more common
operating pitfalls and may be useful if problems are encountered.
A Quick Reference guide is provided in Section 13 to help to remind you of the
commands and configuration parameters. Please feel free to copy this and keep a copy
beside the instrument if you wish.
1.2 Description of the ILM200 Family
ILM200 is a family of Intelligent Cryogen Level Meters with general application in systems
containing liquid helium or liquid nitrogen. The family comprises five instruments. The
last two digits of the type number specify the number of helium and nitrogen channels
respectively. Thus ILM221 is a three channel instrument with two helium channels and
one nitrogen channel. From now on, we shall use “ILM” to represent any member of the
family.
ILM uses a superconductive wire probe to measure the depth of liquid helium and a
capacitance probe to measure the depth of liquid nitrogen. ILM can have up to three
measurement channels, with dedicated displays used for each channel.
ILM is microprocessor based and incorporates all the logic needed to control an
automatic filling operation or to de-energise a magnet safely should the cryogen level
fall below a safe value. All members of the family include an RS232 Serial Computer
Interface as standard and if required may be fitted with an optional GPIB (IEEE-488)
Interface.
Manual operation of the ILM is by means of front panel push buttons and associated
status lamps.
1.3 Principles of Operation
1.3.1 Helium Level Probe
The probe consists of a length of superconductive wire extending from the bottom to the
top of the helium reservoir. Normally the probe will be mounted vertically, though other
geometries are possible to suit individual shapes of reservoir.

6
The probe relies on the wire below the liquid surface being more efficiently cooled than
that in the gas above the liquid. Thus the Joule heating of the resistive section of the
wire is sufficient to keep this above its critical temperature where it is in gas but not
where it is in liquid. To maintain this situation requires the correct value of current in the
wire. The graph accompanying the probe drawing at the end of this manual shows how
this current varies as a function of the temperature of the liquid in the reservoir. With no
current in the wire, the entire length will become superconducting. It is therefore
necessary to include a small heater resistor in thermal contact with the top of the wire to
drive the top end of the wire into its resistive state. Provided the current in the wire is
sufficient this resistive region will then propagate down the wire to the liquid surface.
When current has been flowing for sufficient time to ensure that the resistive region has
reached the liquid, the voltage across the wire is measured and will be directly
proportional to the length of wire in gas, from which the liquid level may be calculated.
When the measurement has been made, the current through the wire is turned off and
the measured reading displayed. The process is repeated at intervals varying from a few
seconds to a few hours, depending upon the expected rate of change in the level.
The maximum probe length that can be measured, depends upon the resistivity of the
probe wire. Prior to firmware version 1.08, ILM was designed to handle the standard
Oxford Instruments probe wire, with a typical resistivity of 178 ohms/metre. Since version
1.08 this parameter may be adjusted up to a maximum value of 200 ohms/metre. There
are now two types of wire in use:. For probes up to 1.4m active length, the standard wire
is nominally 166 ohms/metre and operates at a current of 130mA. For probes up to 2m
active length, a thicker wire is used, with a nominal resistivity of 61.2 ohms/metre,
operating at a current of 250mA.
1.3.2 Nitrogen Level Probe
The nitrogen probe is constructed of two concentric stainless steel tubes. The annular
space between these acts as a capacitor with the dielectric consisting of liquid or gaseous
nitrogen. Liquid nitrogen has a relative permittivity of 1.45, so that the capacitance
increases by around 45% for the portion of the probe that is in liquid. The probe head
incorporates an oscillator, which uses the probe capacitance to define its period. Hence
the oscillation frequency may be used as an indicator of liquid level. There are no
adjustments in the probe head for ILM. The working range of the oscillator (5 kHz to 65
kHz) can handle the full range of probe lengths up to 2 metres.
1.3.3 Why Two Different Sensing Methods Are Used
The capacitance method cannot be used routinely for liquid helium. (It can be used for
certain specialised applications where the reservoir is strictly isothermal.) The relative
permittivity of liquid helium is only 1.055, which is close to the figure for cold helium gas
at 4.2K. Thus the probe would be more responsive to temperature gradients within the
gas above the liquid surface, than to the position of the surface.
With modern high-Tc superconductors, it is possible to make a superconductive nitrogen
probe. However this would be much less robust than the capacitive probe, which works
well for nitrogen. Hence ILM uses the most appropriate sensing technique for each
cryogen. By allowing ILM to accept inputs from either type of probe, there is no cost
penalty in using two different sensing methods.

7
2Safety
The following general safety precautions must be observed during the operation, service
and repair of this instrument.
2.1 Protective Ground
To minimise shock hazard the instrument must be connected to an electrical ground. The
ground wire (green/yellow) in the instrument power cable must be connected to the
installation electrical ground system. Do not use extension cords without a protective
earth conductor. Do not disconnect the protective ground inside or outside the
instrument. Do not have external circuits connected to the instrument when its
protective ground is disconnected.
2.2 Working Environment
The ILM indicator unit is not designed to be water or splash proof. Therefore it should
not be exposed to rain or excessive moisture.
Neither the indicator unit nor the probes are designed to be used in areas where there
are flammable or explosive gases or fumes.
2.3 Repair and Adjustment
Some internal adjustments can be made to ILM. Although we do not encourage you to
make these adjustments we try to supply you with enough information to allow you to
do it safely. Disconnect the AC power supply before you remove the covers or fuses,
because dangerous voltages are accessible on the circuit board and other components. It
is not sufficient to switch off the main power switch. Capacitors inside the instrument
and the power connector filter, may remain charged after removal of AC power.
Discharge these carefully before you start work.
Some fault finding operations can only be carried out with power connected to the
instrument. If you have to reconnect the AC power supply with the protective covers
removed you must remember that you are putting your life at risk. You should only do
this type of work if you are suitably qualified and sufficiently skilled to understand all the
risks you are taking.

8
3Installation
3.1 Supply Connections
Before applying power to the instrument, ensure that the voltage selector on the rear
panel is correctly set for the intended supply voltage.
If necessary, open the voltage selector panel using the slot provided, withdraw the
voltage selector and replace it in the correct orientation for the intended voltage. Check
that the correct fuses are fitted, then close the voltage selector panel.
Fuse ratings are:
100-120v1.6A Type T (Slow Blow)
200-240v0.8A Type T (Slow Blow)
3.2 Probe Connections
Probes are connected by means of 9-way D-sockets on the rear panel. Up to 3 of these
may be fitted, depending upon the instrument type. Their position corresponds logically
to the associated display on the front panel. Probes 1 and 2 may be used for either
Helium or Nitrogen. Probe 3 may only be used for Nitrogen. Pin outs used are the same
for either probe and no damage will be caused if the wrong type of probe is plugged in
by mistake.
Pin Connections are:
Pin Name Helium Probe Nitrogen Probe
1 VHIGH V (Top) n/c
2 VLOW V (Bottom) n/c
3 (Unused)
4 FREQ IN Link to 5 OUTPUT FREQ
5 FREQ OUT Link to 4 n/c
6 IHIGH I (Top) n/c
7 ILOW I (Bottom) 0 V
8 +12 V n/c +12 V
9 CHASSIS GND SCREEN SCREEN
3.3 Compatibility with Earlier Probes
The design of the Helium Probe for the ILM family is unchanged from that for earlier
instruments, except for the type of connectors used. Old style probes may be used with
the new meter, provided a suitable interconnecting lead is used.
The design of the Nitrogen Probe, though based on the same principle, has changed in
detail. Old style probes are not compatible with ILM and should not be connected to it.

9
Users familiar with Oxford Instruments earlier Helium Level meters models HLM2 and
4016 should note that the direction of current flow through the helium level probe is
reversed with respect to these instruments. The top of the probe is now positive, where
it was previously negative. This has no effect on the operation of the probe. However
any users who may have built a dummy "test probe" incorporating an LED to monitor
current flow, will need to reverse the connections to the LED. As with earlier
instruments, it is important not to swap Voltage (V) and Current (I) leads, since the probe
incorporates a small start-up resistor in the top current lead ITOP and will not work reliably
if this is transposed with the VTOP lead.
3.4 Auxiliary Port Connections
An auxiliary port is provided in the form of a 15 way D socket on the rear panel. Four
digital outputs are available at this port, which may be used to drive relays to control
autofilling and external alarms. The outputs are open-collector transistors (Specification
as for ULN2803A) and can sink up to 500mA from a supply of up to 12V maximum. When
driving an inductive load, it is recommended that a diode is connected across the load to
absorb the stored energy. When the optional internal relays are fitted (see below), these
are connected to the auxiliary port outputs and provide an effective pull-up resistance to
the +11 volt line. The internal alarm sounder is similarly connected to the Relay 4 output.
For low power loads, current may be drawn from pin 15, which is connected via a diode
and fuse to the internal unregulated 11 volt line. A maximum of 500mA total may be
drawn from this source.
Four further digital output lines are available to drive a small stepper motor. These may
be used to control the filling of a small vessel by means of a motorised needle valve.
In addition to the digital outputs there are four digital inputs available. Three of these
may be taken high, to inhibit autofilling on the three channels (for example in the event
of an empty storage dewar). The fourth input K7, may be taken high to initiate an
Autofill, if one is possible. In the normal course of events, filling will start automatically
when the level falls below a FILL level, and will continue until it reaches the FULL level.
There may be occasions when it is desirable to start an autofill, before the level has fallen
all the way to the FILL threshold. If K7 is taken high filling will start on any channels
configured for autofilling, provided the level is below the FULL threshold, and will then
continue normally.

10
Pin connections at the auxiliary socket are:
Pin Signal Name Function
1 Output Bit 0 Stepper Motor
9 Output Bit 1 Stepper Motor
2 Output Bit 2 Stepper Motor
10 Output Bit 3 Stepper Motor
3 Output Bit 4 Fill Relay 1
11 Output Bit 5Fill Relay 2
4 Output Bit 6 Fill Relay 3
12 Output Bit 7 Alarm Relay 4
5 Input K4 Inhibit Autofill 1
13 Input K5 Inhibit Autofill 2
6 Input K6 Inhibit Autofill 3
14 Input K7 Initiate Autofill
7 +5 volt rail
15 Driver Protection / +11 volt rail
8 0 volt rail
3.5 Relay Contact
Connections Internal relays are available on ILM as an option. These have contacts rated
to carry up to 5 amps at up to 250 volts AC. They are intended for driving solenoid valves
or mains rated contactors directly. To ensure safety when using these relays, connections
to these relays are made via terminal blocks within the instrument. ILM should be
unplugged from its mains supply before removing the top cover to make connection to
the relay contacts. The incoming wires should be brought in via the cable gland on the
rear panel. After the connections have been made the cover should be replaced before
connecting ILM to the mains supply.
The connections are labelled TB1 to TB4 (corresponding to relays 1 to 4). Three terminals
are available for each relay. The terminal nearest the front of the instrument is the
Common contact. The middle terminal is the Normally Open contact and the terminal
nearest the rear panel is the Normally Closed contact. With the instrument switched off,
or the relay not active, Common is linked to Normally Closed. When the relay is active,
Common is linked to Normally Open.
3.6 Connections for Automatic Magnet Run Down
One common application of the relay contacts is to run down a magnet in the event that
the Helium level falls below the LOW threshold. Run down is initiated by providing a
contact closure to the magnet power supply. The contacts to be linked are pins 7 and 14
of the PARALLEL I/O socket on the IPS family of Oxford Instruments Power Supplies. (On
the earlier PS120-10 and PS120-3 power supplies, the same pin connections are used but
the socket is labelled AUXILIARY I/O on the rear panel.)

11
A pair of wires should be run from these pins to the COMMON and NORMALLY open
contacts (i.e. the front and middle terminals) of the relay to be used. Normally this will be
the relay associated with the channel being used for Helium level measurement.
However if a helium autofill system is also in use, another relay may be used for
rundown. Refer to section 9.6, for details on configuring the relays for different
purposes.
3.7 Analogue Outputs
Where none of the channels in an ILM have been configured to use the stepper motor
output lines for controlling autofilling, three of these four lines (bits 0 to 2) are
reallocated to provide pseudo analogue output signals representing the levels on the
three channels. These outputs are intended only to drive a chart recorder or similar for
trend recording. For precision data logging the computer interface should always be
used. The pseudo analogue signal is achieved by alternately pulling the output pin to 0v
and releasing it, with a time dependent waveform, the mean value of which represents
the analogue output. To use the output, a pull up resistor is required to a positive
voltage (not greater than 25V), followed by a passive filter with a time constant of
around 5 seconds. A typical output lead giving analogue outputs in the range 0 to 1V is
shown in circuit CNC6802 at the end of this manual. Note that no analogue outputs
are available if any channel is configured to use a stepper motor for autofill.
3.8 RS232 Serial Data Line Connections
The bi-directional serial data link from the computer is connected via a 25 way D-socket
on the rear panel. ILM is configured as a DCE with the standard pin outs given below.
The majority of computer RS232 interfaces are configured as a DTE and are fitted with a
25 way D plug. For this type of connector, a simple lead connecting pin 1 to pin 1, pin 2
to pin 2 etc. is all that is required. For computers fitted with a 9 way D plug for RS232,
(AT style COM port), a standard "AT lead" fitted with a 9 way socket and a 25 way plug is
required.
Pin connections at the ILM RS232 socket are:
Pin Signal Name Notes
1 FG Linked to Chassis Ground in ILM
2 TD Data from Computer to ILM
3 RD Data from ILM to Computer
4 RTS Linked to Pin 5 in ILM
5 CTS Linked to Pin 4 in ILM
6 DSR Linked to +5V when ILM is powered
7 SG Linked to Digital Ground in ILM
8 DCD Linked to +5V when ILM is powered
All other pins are open circuit.
ILM does not require signals to be present on any of the "modem control" lines, RTS or
DTR (pin 20). RTS is looped back as CTS and logic high levels are returned on DSR and
DCD to ensure maximum compatibility with any requirement of the computer.

12
Voltage levels for the transmitted and received data are:
Tx Data High > +5.5V
Tx Data Low < -5.5V
Rx Data High Threshold < +2.6V
Rx Data Low Threshold > +1.4V
Max Rx Input Voltage +/-30V
Data protocols are:
Baud Rate 9600
Tx Start Bits 1
Tx Data Bits 8
Tx Stop Bits 2
Rx Start Bits 1
Rx Data Bits 8
Rx Stop Bits 1 or 2
Parity None
3.9 The Oxford Instruments Isobus
A unique feature of ILM and other Oxford Instruments products, is the ability to connect
a number of instruments simultaneously, to a single RS232 port on a computer and to
control each one independently. This is done by means of an ISOBUS cable which carries
a single MASTER connector (25-way D socket) and up to eight, daisy-chained SLAVE
connectors (25-way D plugs). Each slave connector incorporates full optical isolation so
that the slaves are all isolated from the master and from each other. The slave
connectors draw their power from the individual instruments, via the DCD signal on pin
8. The master connector may draw its power from either DTR or RTS signals from the
computer.
To use ISOBUS, a special communication protocol is required, which is part of the
command structure of Oxford Instruments products and is described in section 6.5.
3.10 GPIB (IEEE-488) Connection (Optional)
If the optional GPIB interface is fitted, connections to the GPIB are made via a standard
24 way GPIB connector. Assignment of the connector pins conforms to the standard IEEE-
488.1. Connections should be made using a standard GPIB cable. GPIB connections
should never be made or broken whilst the monitor or any of the instruments
connected to the Bus are powered up. Failure to observe this precaution can result in
damage to one or more instruments.
The GPIB interface complies fully with IEEE-488.1-1987 as a talker/listener, able to
generate service requests and respond to serial poll and device clear commands. It does
not support parallel polling and has no trigger function. Open collector drivers are used
on the bus lines so it does not prevent parallel polling of other devices on the bus. Its
complete GPIB capability is specified by the Capability Identification Codes:-
SH1 AH1 T6 L4 SR1 RL0 PP0 DC1 DT0 C0 E1

13
Two lamps are fitted to the rear panel immediately below the GPIB connector, to assist in
diagnosing any GPIB communication problems. The RED lamp lights whenever the ILM is
addressed to TALK and the GREEN lamp lights whenever it is addressed to LISTEN. The
behaviour of the lamps is very dependent on the GPIB monitor in use. Some controllers
un-address an instrument at the end of any transaction, in which case the lamps will just
blink on for each transaction. Others leave instruments addressed between transactions
in which case one or other lamp may remain lit depending on whether ILM was last
addressed to talk or to listen.
Before any communication can occur, ILM must be given a unique GPIB address. By
default, ILM is supplied with its address set to 24. If this address is already in use by
another instrument on the bus, it can be changed from the front panel via the Test
Mode. This is described in section 9.5.
3.11 The GPIB to ISOBUS Gateway
An ILM fitted with a GPIB interface has the ability to act as a GATEWAY to an ISOBUS
cable, allowing other instruments to be linked to the GPIB without themselves requiring
GPIB interfaces. This can enable other Oxford Instruments products, for which an internal
GPIB interface is not available, to be linked. It offers the additional advantage of optical
isolation between these instruments and the GPIB.
To use the gateway, all that is required is GATEWAY MASTER ADAPTOR. This allows the
25 way ISOBUS MASTER socket to be linked to the 25 way RS232 socket on the ILM. The
adapter is a symmetrical 25-way plug to 25-way plug link, with pin connections as shown
below.
Beware of using 25-way plug to 25-way plug adapters, sold as "DCE-linkers" by some
suppliers. Several different conventions exist for these, not all of which will work as a
Gateway Master Adapter. The connections required are given in the table below. A
Gateway Master Adapter providing these connections may be obtained from Oxford
Instruments.
25 WAY PLUG 25 WAY PLUG
1 1
2 3
3 2
7 7
6 4
4 6
Note that the connections are symmetrical and the adapter may be plugged in either way
round.
The necessary protocols for use as a Gateway Master are included as standard in ILM and
are described in section 6.6.9.

14
4Operation
4.1 Front Panel Controls
The front panel controls are grouped together in logically related blocks.
POWER
The main ON/OFF switch. A green lamp illuminates whenever the instrument is switched
on.
ADJUST
The red RAISE and LOWER buttons provide the main means of adjusting any parameter.
In ILM their main use is for adjusting the display reading during the configuration and
calibration process. They have no effect on their own but are always used in conjunction
with one of the other buttons. Whenever a parameter is being adjusted, its current value
is shown on the display. Setting a value involves pressing RAISE or LOWER until the
required value is shown.
Operation of the RAISE and LOWER controls has been designed to allow large changes to
be made relatively quickly whilst at the same time enabling any value to be set exactly.
Pressing RAISE or LOWER briefly will cause the value to change by one unit. If the button
is held in, the last figure will start to change at about 5 units per second. After 2 seconds,
an approximately 10-fold increase in rate will occur, followed after another 2 seconds by
a further rate increase and so on. Altogether there are 4 different rates. Whenever RAISE
or LOWER is released, the next lower speed will be selected. This allows the user to
"home-in" on the required value in a logical way.
A secondary use of RAISE and LOWER is in conjunction with SILENCE, to enter the TEST &
CONFIGURATION mode, as described below.
STATUS
Certain functions of ILM, such as Alarm and Magnet Shut-Down in the event of low
cryogen levels, are common to all channels. ALARM and SHUT DOWN lamps in the
STATUS box allow the operation of these functions to be observed. In particular these
conditions remain latched, so that once an alarm or shut down is initiated, it will remain
in force, even if the cryogen level is restored. A SILENCE button is provided to clear the
alarm and shut down conditions. If the low level causing the condition has been cleared,
pressing the SILENCE button will silence the alarm and reset the lamps. If the low level is
still present, pressing SILENCE will stop the alarm sounding and release Relay 4.
However the ALARM and/or SHUT DOWN lamps will remain lit, and any of Relays 1 to 3
which are active will remain so. If the alarm has been silenced in this way, it indicates to
ILM that the operator is aware of the problem. The ALARM lamp will therefore
automatically cancel when the low level condition is removed. The SHUT DOWN
condition will not cancel until the SILENCE button is pressed, after the low level
condition has been cleared. The reason for this is that once a run down has been
initiated, it is not advisable to terminate this at some arbitrary magnet current without
operator intervention.

15
The SILENCE control button has a number of secondary SELECT functions which are
obtained by pressing this button whilst one or more other buttons are held depressed. If
SILENCE is pressed whilst both RAISE and LOWER are held in, ILM enters the TEST mode
(see Section 9). If SILENCE is pressed whilst one of the 0% or 100% calibration buttons
are held in, calibration and configuration data is STORED in the non-volatile memory and
so is retained at power-up.
HELIUM AND NITROGEN DISPLAY
Each channel has its own display section, with a display showing the cryogen level as a
percentage 0.0% to 100.0%. If problems are detected with the level sensing probe the
display will change to show "Err". The display also includes FILL and LOW lamps
indicating the status of the channel. In addition this section contains recessed calibration
buttons labelled 0% and 100% allowing the instrument to be calibrated to suit a
particular probe. Channels used for Helium include an additional RATE button and two
lamps indicating whether the unit is sampling at the SLOW or FAST rate. During the
actual sample pulse, both lamps light to indicate that current is flowing through the
probe.
One, two or three display sections may be present on an instrument depending on the
number of channels in use. If an instrument includes one or more channels which have
been configured as "unused", their display will remain blank.
4.2 First Time Operation
Switch on the instrument by means of the POWER switch. Check that the green POWER
lamp lights.
After about one second the left hand display will show "S" followed by a number, which
indicates the instrument's "ISOBUS" address (see below). Alternatively if the instrument
is fitted with an optional GPIB card the display will show "G" followed by a number,
indicating its GPIB address assuming this has been selected. After a pause, all displays
fitted will show a message indicating the use for which their channel has been
configured. This will be "He" for Helium, "N2" for Nitrogen or "---" for an unused
channel. If a channel has been configured for a continuously energised helium probe,
this will be displayed as "Hec".
After a further pause the normal channel display will appear, showing the cryogen level.
On nitrogen channels, the correct level will be displayed immediately. On helium
channels, irrespective of the selected sampling rate, the level will be sampled about 10
seconds after switch on. Unless autofilling is in progress the instrument will then default
to the slow rate.

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4.3 "Err" Display
If no probe is plugged into a channel which has been configured to be in use, the display
will show a steady "Err" display. The same display will be produced to indicate a fault in
the probe, its wiring, or the ILM itself. In the case of a nitrogen probe, if the display
flashes between "Err" and a 100% reading every second, the inner and outer tubes of
the probe have become shorted together. This can be caused by the probe becoming
bent, by the inner tube touching the bottom of the cryostat, or by condensation within
the probe.
4.4 Sample Rate Selection
Helium level is measured by passing a current down a superconductive wire, such that the
wire is driven into its resistive state where it is in gas and therefore less well cooled. The
measuring process thus introduces a significant heat load into the cryogen. In order to
minimise this heat load, the probe is normally only energised for a brief pulse of around
2 to 3 seconds. Between pulses the level is assumed not to vary greatly and the last
measured value is displayed. When cryogen is being refilled, the level can change quickly
and is sensible to sample at a fast rate with the wire being pulsed every 10 - 30 seconds.
At other times the level will only change slowly and it is sufficient to sample it once or
twice per hour. At this rate, the heat load due to the pulse is negligible.
Pressing the RATE button switches between FAST and SLOW modes of operation.
Whenever the channel is switched to the FAST rate, a sample is taken straight away.
Thus if you are already in FAST but wish to take a sample immediately, pressing the RATE
button twice will switch to SLOW and back to FAST and so initiate a pulse.
The interval between pulses may be configured for both FAST and SLOW rates over a
wide range of values, from within test mode (see section 9.12). It is also possible to
configure ILM to switch back to SLOW automatically if left in FAST for more than 15
minutes (see section 9.7).
4.5 Calibration
To obtain accurate level readings of Helium or Nitrogen, ILM must be calibrated for a
specific probe. There are two levels of calibration. In normal use, it should only be
necessary to make small trim changes to the calibration. When an instrument is initially
set up for use with a new probe, it is necessary to establish a default starting-point
calibration. This will normally be done before the unit leaves the factory and should only
need changing if the probe is replaced by one of a different length.
4.5.1 Trimming the Calibration
For an accurate calibration it is necessary to set two points accurately near the ends of
the probe range. This is done by means of the recessed 0% and 100% buttons. These
may be pressed with a pointed object, such as a pencil. Whilst holding the button
pressed, RAISE and LOWER are used to adjust the display to the required value.

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With the cryogen at a known level near zero, the 0% button should be pressed and
RAISE and LOWER used so that the known level is displayed. The process is should then
be repeated at a known level near full, using the 100% button. It is often convenient to
set the 0% point to exactly 0 when liquid starts to collect during the cryostat cooldown
and to set the 100% point to exactly 100% when the reservoir is completely full. The
display has been designed to read slightly below 0% and slightly above 100% to simplify
this adjustment.
Although the above method may be convenient, it is not necessary to have the levels at
exactly 0% and 100% to perform an accurate calibration, provided the actual level is
known at the two calibration points. ILM automatically remembers the point at which
each setting was made and after any adjustment to one end of the range, it
automatically recalculates the complete calibration to ensure that the reading at the
other end remains as it was last calibrated. This ensures that there is no interaction
between the two calibration points. If this recalibration fails, the display will briefly
show "Err" after the calibration button is released. In this case the newly calibrated
point will remain correct but the other end of the range may have been moved. The
usual reason for this is that the default calibration established for the probe is in error.
This may also show as an inability to set the required display number by means of RAISE
and LOWER. In both cases check that the correct active length has been entered.
Note that it is not accurate to set the 0% point for either Helium or Nitrogen probes
when the cryostat is still warm, since the properties of warm gas are not the same as
those of the cold gas which will be present in the reservoir above the liquid surface
during normal operation.
4.5.2 Setting the Default Calibration
When the instrument is first configured for a probe, the nominal active probe length
(in millimetres) must be supplied, using test mode t.07 as described in section 9.8. (The
active probe length is the working length of the probe. The physical length of the probe
will normally be longer than this since the probe will extend from the top of the cryogen
reservoir to the top of the cryostat.) For instruments with firmware version 1.08 or later,
it is also possible to configure any helium channels for the probe wire resistivity. This
is done by means of test mode t.14. If this is changed for any reason, it should be done
before setting the active probe length.
For a Nitrogen probe it is also necessary to establish the default 0% point. This is done
by pressing 0% and 100% buttons simultaneously whilst the probe is connected and is
immersed in air at room temperature, or preferably in cold nitrogen gas. This operation
sets the 0% and 100% trim adjustments to the midrange points, reads the probe and
defines this as the initial 0% point, then uses the entered length to compute a nominal
100% point.
For a Helium probe, it is not necessary to establish a 0% point in this way. The 100%
point is always defined for a Helium probe as zero resistance, so the entered length may
be used to calculate the 0% point. Pressing 0% and 100% simultaneously may used to
set the default midrange values to the 0% and 100% trim, but no probe reading is taken
during this process, so it does not matter whether the probe is in warm gas, cold gas or
liquid.

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In both cases when the two buttons are pressed together the display will show "dEF" to
indicate that default conditions have been set.
4.6 Storing Calibration and other Power-Up
Defaults
Whenever any data has been changed, which is intended to be retained after power
down, this must be deliberately STORED. This write operation is achieved by holding any
of the 0% or 100% calibration buttons pressed in, whilst pressing and releasing SILENCE.
The display will briefly show "Stor" indicating that the data has been correctly stored. It
does not matter which calibration button is pressed, the entire calibration of the
instrument for all channels will always be saved in a single operation.
If instead of showing "Stor", the display shows "Prot", this indicates that the memory is
protected by the hardware WRITE-ENABLE switch being in the OFF position. This is
Switch 1 of a small 2 way Dual-in-Line switch SW2 on the motherboard. Set it to the
"ON" position and try again. (Switch 2 of this switch is used to disable the internal
alarm.)
The switch need only be returned to the OFF position if it is desired to prevent any
possibility of the data being changed by someone tampering with the front panel.

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5Auto-Fill and Alarms
5.1 Level Thresholds
There are three threshold levels associated with each channel. These are used to control
an automatic filling operation, or to sound an audible alarm or de- energise a magnet in
the event of low cryogen level. The three levels are called FULL, FILL, and LOW, with
FULL normally being the highest level and LOW the lowest. All three levels may be
adjusted anywhere within the 0% to 100% range by means of the test mode. (See section
9.13.) The standard factory default settings are 90% for FULL, 20% for FILL and 10% for
LOW.
5.2 Automatic Filling
FILL and FULL levels are intended for use with automatic re-filling. Autofill can be
enabled or disabled by means of the configuration parameter for the channel, set in test
mode. When Autofill is enabled, filling will start when the level falls below the FILL
threshold. Thereafter filling will continue until the level rises above the FULL threshold.
When filling is in progress the FILL lamp will be lit.
Three inputs are provided on the Auxiliary Port, to inhibit this filling operation on each
of the three channels. The purpose of this is to provide a means of suppressing the filling
operation, if for example the storage vessel from which the cryogen is to be drawn, is
empty. Taking one of these lines high, to +5V will prevent the FILL lamp lighting on that
channel. (The LOW lamp will still light if its threshold is exceeded). If this feature is not
required, the inputs may be left unconnected.
For instruments fitted with firmware 1.07 and later, it is possible to use a fourth input on
the auxiliary port to initiate a fill, before the levels have reached the FILL threshold. This
might for example be attached to a time switch to ensure that a system was always filled
with cryogens during the night, when the refilling would cause least disturbance. Taking
this line high, will cause filling to start on all channels for which autofill is enabled, and
for which the level is below the FULL threshold.. If filling for any channel, has been
inhibited by one of the three control lines described above, this will prevent the Initiate
line starting a fill on that channel.
There are two alternative methods by which an autofill may be controlled. Selecting
which of these is to be used forms part of the Configuration Parameter set from within
Test Mode (see section 9.7).
The normal method of controlling an autofill is by means of the digital logic level at the
Auxiliary socket and/or the associated relay for the channel concerned. The logic line is
pulled low and the relay (if fitted) is energised whilst filling is in progress.

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An alternative method of controlling filling is to use a motorised needle valve. This may
for example be used for filling a small reservoir from a main helium bath. ILM supports
the use of a small stepper motor to drive such a needle valve. The motor is connected to
the auxiliary port. When the level falls below the FILL threshold the motor will slowly
open the needle valve. When it rises above the FULL threshold the motor will slowly
close the needle valve. Whilst the level is between the two thresholds the needle valve
setting will not change. By bringing the two thresholds close together a relatively
constant level may be maintained within the small reservoir.
The operation of the stepper motor may be configured from test mode, by means of test
12. If this parameter is set to 0, there is no motion of the needle valve when ILM is first
switched on, provided the level is between the FILL and FULL levels, and a "mid-travel"
position is assumed for the needle valve. From this initial position the ILM will provide
up to 32767 step pulses in either direction. The motor and gear box used should be
selected such that this number of pulses is more than sufficient to drive the valve from
one end of its travel to the other and the needle valve should be fitted with a slipping
clutch or a low-torque motor used, so that attempting to drive the needle beyond its
normal travel will cause no damage. For instruments with firmware version 1.03 and
earlier, this is the only mode that is available.
For instruments with firmware 1.04 and later, setting the test 12 parameter to any value
other than 0, will result in the needle valve being automatically driven towards the
closed position at power up (a similar procedure to that employed for the gas flow
control on the ITC5 family of temperature controllers). When this is complete, a "zero" or
"fully closed" position is assumed for the needle valve. The value of the test 12
parameter then determines how many steps are required to fully open the valve. A value
of 1 corresponds to 65536 steps, a value of 2 to 32768 steps etc. up to a maximum value
of 7, corresponding to 1024 steps.
5.3 Automatic Rate Switching
When autofilling is used for a Helium vessel, it is advisable to switch to the FAST sampling
rate for the filling operation. Otherwise the vessel is likely to be full before the next
sample occurs! A separate, optional part of the configuration parameter (see section 9.7)
allows automatic control of the sampling rate. The strategy is that whenever the level is
below the FILL threshold, the sampling rate switches to FAST. It will then remain in FAST
until the level is above the FILL threshold and the level has not risen for at least 15
minutes. This indicates that either the fill is complete or the process has stopped for
some reason. In either case there is no need to remain in the FAST rate, so ILM switches
back to LOW.
Note that even on systems which do not have an autofill, it can be useful to have
automatic rate switching active, since it prevents cryogen wastage if the operator should
leave the rate set to FAST by mistake. If automatic rate switching is being used in this
application, the FILL threshold should be set to -1%, so that ILM will never see a level
below the FILL threshold and so will never automatically switch to the FAST rate.