Group3 DTM-133 User manual

01 April 2010
Group3 Technology Limited
2 Charann Place, Avondale, Auckland 1026
P.O. Box 71-111, Rosebank, Auckland 1348, New Zealand.
Phone: [phone] Fax: [phone]
email: [email protected]
web: www.group3technology.com
DTM-133
DIGITAL TESLAMETER
with IEEE-488 GPIB Communication
V1.0
USER’S MANUAL

Thank you for purchasing and using a Group3 digital teslameter. We hope you will join the many hundreds of users
worldwide who are enthusiastic about our products.
Group3 has been designing and building magnetic field measuring equipment since 1983. We are constantly
upgrading our products and support documentation. We welcome input from our customers, so if there are aspects
of the instrument which you particularly like, or which you would like to see improved, please contact your Group3
supplier (seeback page for acomplete list) or Group3 directly with your suggestions to
[e-mail] .
The Group3 website, http://www.group3technoIogy.com contains details of all our products. This site is
regularly updated, so check it from time to time to learn about recent developments.

CONTENTS
1. General Description
1-1
2. Specification of Systems
2-1
3. Setting Up
3-1
3.1 Introduction
3-1
3.2 Installing the Panel Mount Option
3-1
3.3 Connecting the Hall Probe
3-2
3.4 Connecting the Power Source
3-3
3.5 Internal DIP Switches
3-5
3.6 Analog Output
3-5
3.7 Grounding
3-5
3.8 Installation Techniques for Electrically Noisy Environments
3-6
4. Operating Instructions
4-1
4.1 Zeroing
4-1
4.2 Installing the Probe
4-2
4.3 Reading the Field Value
4-3
4.4 Display Modes, Using Front Panel Keys
4-4
4.5 Digital Filtering
4-7
5. IEEE-4988 Interfacing Option
5-1
5.1 IEEE-488 Bus Connection
5-1
5.2 IEEE-488 Board Switch Settings
5-3
5.3 Using the IEEE-488 GPIB Interface
5-4
5.4 Triggered Operation
5-19
LIST OF FIGURES
3-1
Fig. 1 Panel Cut-out Dimensions
3-4
Fig. 2 Power Input Connections for the -L Option
4-3
Fig. 3 Probe Dimensions
5-2
Fig. 4 IEEE-4888 Standard Connector
5-2
Fig. 5 Location of IEEE-488 Board Switches
5-5
Fig. 6 A Typical IEEEE-488 System
LIST OF TABLES
Table 1 Internal DIP Switch Functions
3-5
Table 2 Analog Output Connector Pin Assignments
3-5
Table 3 IEEE-488 Connector Pin Assignments
5-1
Table 4 IEEE-488 Board DIP Switch Functions
5-3
Table 5 String Terminator Switch Settings
5-4
Table 6 IEEE-488 Command Codes
5-8
Table 7 DTM-133-_G IEEE-4888 Command Codes
5-12
Table 8 DTM-133-_G Commands - alphabetic listing
5-13
Table 9 DTM-133-_G Commands - listing by function
5-16
DTM-133 User’s Manual Contents -1

Contents -2 DTM-133 User’s Manual

1. GENERAL DESCRIPTION
The DTM-133 Digital Teslameter offers accurate measurement of magnetic flux density, with direct digital readout
in tesla or gauss. The instruments are light and compact, and the probes are easy to use. The DTM-133 has been
engineered to withstand the severe electrical interference produced by high voltage discharge.
Options provide serial communications (RS-232C and fiber optic) or IEEE-488 interfacing for system applications.
FEATURES
Measures magnetic fields over four ranges up to 3 tesla with polarity indication; resolution up to 1 part in 6000
Range selection is manual or by selectable auto-ranging
Used with special miniature Hall probe - easy to attach to magnet pole or other hardware. Probe holders are
available as optional accessories.
Accuracy and temperature specifications include total system performance, probe and instrument. This is the only
meaningful indication of measurement accuracy.
Probe is calibrated with field characteristics stored in memory chip contained in cable plug.
Accuracy is better than +0.03% for the complete system probe and instrument.
Temperature coefficient is better than 100ppm/”C for the overall system.
Accuracy is verified against nuclear magnetic resonance (NMR) standard.
Probe calibration is verified at many field points on every probe, and a printed calibration table is supplied with every
probe.
Front panel keys select the desired field range and invoke the peak hold function to read the peak field value. The
keys are also used to reset the peak hold to zero the system, and to switch on auto-ranging. Peak hold is
implemented digitally, has zero sag
Digital filtering of the displayed field reading suppresses short-term fluctuations. The filtering characteristic is non-
linear; small field variations within a narrow window centered on the currently displayed value are filtered; large
field changes are displayed immediately. Filter window and time-constant may be changed by remote command
when serial or IEEE-488 option is fitted. Filtering is switched internally.
DTM-133 (GPIB) User’s Manual 1-1

With digital communications, the DTM-133 can deliver 30 readings per second.
Two digital communication options: serial (RS-232G and fiber optic) and IEEE-488 General Purpose Interface Bus.
With the serial option, a single teslameter may be connected to standard RS-232C equipment, or up to 31 units may
be interconnected on the Group3 Communication Loop (G3CL) and driven from computer or terminal.
Fiber optic ports duplicate functions of RS-232C signals, for electrical noise immunity and voltage isolation. Fiber
optic links may be up to 60 meters in length using Hewlett- Packard HFBR-3500 series improved fiber optic cables.
The IEEE-488 option fully supports all relevant GPIB functions and commands. including full talker-listener capability.
serial and parallel polling, service request, and talker-only.
ASCII control commands are accepted to modify the output data format to change the rate of data transmission or
to request transmission of a single field reading. Other commands select the field range. select and peak hold
functions turn on and off digital filtering and modify the filter characteristics. System status may be determined
remotely.
The system can be operated in triggered mode where field measurements by one or more teslameters are triggered in
synchronism with each other by external command.
Internal switches select serial data format and baud rate device address, string terminators, filtering, field units in
gauss or tesla, data format service request action, EOI action and perform system reset.
An analog output gives instantaneous field value (0 to 9 kHz) not corrected for probe non-linearity.
Model variations are available without display and keys for true ’black box' magnetic- field-to-computer interfacing.
A panel mount model with display is available.
1-2 DTM-133 User’s Manual

2. SPECIFICATIONS OF A DTM-133 SYSTEM
Measurements Field
magnetic field density in tesla or gauss
ranges
0.3
0.6
1.2
3
tesla full-scale,
3
6
12
30
kilogauss full-
scale,
with polarity indication and selectable auto ranging maximum
calibrated field
+2.2 tesla +22 kilogauss
Resolution
1 in 12,000 of bipolar span with digital filtering on
ranqe
resolution
gauss
tesla
0.3 tesla
0.5
0.00005
0.6 tesla
1
0.0001
1.2 tesla
2
0.0002
3.0 tesla
5
0.0005
Accuracy
DTM-133 with LPT-130 or MPT-132 probe:
+(0.03% of reading + 0.03% of full-scale) max at 25°C
Temperature stability
DTM-133 with LPT-130 probe:
caIibration: -100 ppm of reading/'C max. add -3ppm/°C for
each meter of probe cable
zero drift: +(8 microtesla + 0.0015% of full-scale)/°C max
Temperature stability
DTM-133 with MPT-132 probe:
calibration:
-100 ppm of reading/°C typical
-140 ppm of reading/°C max. add -
3ppm/°C for each meter of probe cable
zero drift:
+(15 microtesla + 0.0010% of full-scale)/ °C typical
+(40 microtesla + 0.0015% of full-scale)/ °C max.
Time stability
+0.1% max. over 1 year
Measurement rate
30 fully corrected measurements per second
Display rate
10 display updates per second
Response time
full-scale change of field reading settles to within resolution
in less than 0.2 second (filtering off - see below)
Peak hold mode
displays maximum field since mode entered or reset
peak hold is implemented digitally with zero sag or decay
DTM-133 User’s Manual 2-1

Probes standard sensitivity transverse types: LPT-130 , MPT-132
high sensitivity transverse types: LPT-230, MPT-230
2-2 DTM-133 User’s Manual
Display Indicators
7-character 7-segment alphanumeric display
8 back-lit legends for:
0.3, 0.6, 1.2, or 3.0 tesla range selected,
peak hold mode on, digital filtering on, tesla/gauss units
Display modes
field, peak hold field
Digital filtering
field value filtering smooth’s out small fluctuations in the reading;
large, rapid field changes are not filtered; internally switch selected.
Keys
2 keys select range, access peak hold display, load defaults
zero field display, reset peak hold, select auto-ranging,
Analog output
instantaneous field analog:
full-scale output: +3V nominal source
impedance: 1000a accuracy: I-10%
bandwidth: 9kHz at -3dB, roll-off 3-pole 60dB/decade
On-board switches
digital filtering on/off, units (tesla or gauss)
Memory backup
user-entered data stored indefinitely in non-volatile memory
Power source
DTM-133
DTM-133-_S
DTM-133-
_G
ac:
min
8V
0.65
0.75
1.1
A rms
Max
15V
0.3
0.35
0.5
A rms
dc:
min
10V
0.45
0.5
0.75
A dc
Max
19V
0.17
0.2
0.3
A dc
(because a switchmode regulator is used, input current falls as the voltage rises)
ac line input plugpack supplied.
Power fuse on processor board: 1amp anti-surge 5 x 20mm
To obtain maximum spark protection, use PS12D7 power supply
and ferrite kit 11000036. See section 3.8.
L option: 115/208/230 V ac power input.
Enclosure
aluminum, 217 x 125 x 50 mm, textured finish, charcoal grey color,
tilt stand fitted to bench models
Ambient field
Maximum operating field for electronics package:
10 millitesla with single-range probe,
0.5 millitesla with multi-range probe.
Temperature range
0 to 50°C operating, absolute maximum temperature of probe 60°C
Instrument weight
1.2 kg,
shipping weight:
2.5 kg

Additional specifications with communication options
Digital interfacing serial option: RS-232C and fiber optic;
parallel option: IEEE-488 General Purpose Interface Bus
System orientation Group3 Communication Loop (G3CL) using serial ports,
simple loop for 31 devices no multiplexer required:
GPIB with IEEE-488 option.
Digital data format ASCII input commands and output responses
Commands requests for field value; setting and inspection of display and control modes;
field measurement triggering; entry of numerical values; setting units. output data format,
and filter characteristics; test commands
Output responses field value in tesla or gauss followed by optional T or G and carriage return/line feed:
numerical and system status data requested by commands: messages
Serial bit rate 16 standard rates. switch selected, 50, 110, 134.5 1 TO, 200, 300,
600 900, 1050, 1200, 1800, 2000 2400. 4800. 9600, 19200 baud
Fiber optic cable Hewlett-Packard HFBR-3500, 60 meters max
IEEE-488 functions SH1 source handshake capability
AH1 acceptor handshake capability
T5 talker (basic talker, serial poll, talk-only mode, unaddressed to talk if addressed to
listen)
TE0 no address extension talker capability
L4 listener (basic listener, unaddressed to listen if addressed to talk)
LEO no address extension listener capability
SR1 service request capability
RLO no remote local capability
PP1 parallel poll capability (configured by controller)
DC 1 device clear capability
DT1 device clear capability
CO no controller capability
GPIB connector standard Amphenol 57-20240 with metric standoffs
DTM-133 User’s Manual 2-3

ORDER CODES
Basic teslameters,
capable of four measurement ranges 0.3, 0.6, 1.2, 3.0 tesla full scale,
0.03, 0.06, 0.12, 0.3 tesla full scale for high sensitivity probes
support all LPT and MPT series probes, plugpack supplied except for option -L.
DTM-133 (recommended probes are LPT-130, LPT-230, MPT-132, MPT-230)
Options
Bench styleinstrument withdisplay: addsuffix -D
Panel-mount version: addsuffix -P
Withoutdisplay,plugpack powered: addsuffix-N
Without display, line voltage power: add suffix -L
Serial data input/output, RS-232C & fiber optic: add suffix-S
IEEE-488 GPIB capability: add suffix G
Example: DTM-133-DS
Probes
Four ranges, standard 2-meter cable
LPT-130
| standard sensitivity
LPT-230
| high sensitivity
MPT-132
| probes
MPT-230
| probes
Single range probes: add range suffix -03, -06, -12,-30.
Special probe cable lengths: add length suffix -XMor -XS,
forXsubstitutecablelengthinmeters, 30max.
M denotes unshielded cable; Sdenotesshieldedcable.
Example: LPT-130-2S
Accessories
fiber optic cable fitted with connectors,
60-meter length maximum
probe holders
fiber optic repeater, bidirectional, model FOR-2PP
fiber optic to RS-232C adaptor, model FTR
serial/GPIB adaptor, model COM-488
digital display for remote control & readout of field values, model DPM
rack panels, 3.5 inches high (2U), for rack mounting 1, 2, or 3 DTMs or DPMs
ferrite kit 11000036 for spark protection
power supply PS12D7 for spark protection
2-4 DTM-133 User’s Manual
One of these options
must be specified
Must select
one option

3. SETTING UP
3.1 INTRODUCTION
This chapter provides instructions for the basic operation of all members of the Group3 DTM-133 family of digital
teslameters and their companion LPT-130, LPT-230, MPT- 132, and MPT-230 Hall probes. If your teslameter is a
DTM-133- S (serial communications option) or DTM-133- G (IEEE-488 option), a later chapter will describe the use of
the relevant digital communications features. For a summary of all current members of the product family, see page 2-
4.
These instructions are written for a teslameter with front panel display and keys. Users of teslameters without
display and keys should ignore sections of this manual referring to these features. All other aspects of operation are
identical.
Before using your teslameter for the first time, please read through sections 3.2, 3.3, 4.1, 4.2, and 4.3 of this manual.
This will give a quick introduction to basic operation of the instrument.
3.2 INSTALLING THE PANEL MOUNT OPTION
Model DTM-133-P is supplied fitted with a special front bezel which has threaded studs to allow panel mounting. A
panel mount support bracket (part 17000058) is included to help support the teslameter. Group3 can supply 19-inch
wide, 2U (3.5") high rack panels to hold one, two, or three teslameters (parts 17000025, 17000026, and 17000027,
respectively). Alternatively, the user can mount the teslameter in any panel of thickness up to 3/16" (4.76mm).
Dimensions for the cutout and drilled holes are shown in Fig. 1.
DTM-133 User’s Manual 3-1
Fig. 1 Panel Cutout Dimension

To fit the teslameter to the panel, first remove the nuts and washers from the bezel studs. Push the teslameter
through the panel from the front, making sure all the studs fit through the small holes. While holding the teslameter in
place, place the support bracket under the teslameter from the rear, pushing it up to the panel with the studs through
the holes in the bracket. Put the flat washers on the studs, then the lock-washers, and finally screw on the nuts.
Make sure the teslameter is resting on the bracket, then tighten the nuts, preferably using a long-stemmed nut driver.
3.3 CONNECTING THE HALLPROBE
Before handling the probe, please read the following:
Group3 Hall probes are built to be as robust as possible for a small, precision device. However, it is most important
that certain precautions be taken when handling and installing probes so that they are not damaged or destroyed, and
to preserve their accurate calibration.
Mount the probe head so there is no pressure which will tend to bend or depress its ceramic rear surface. If the probe
head is clamped, make sure the surface in contact with the ceramic is flat and covers the whole of the ceramic
surface. Do not apply more force than is required to hold the probe in place. Any strain on the ceramic will alter the
probe’s calibration, and excessive force will destroy the Hall element inside.
When the probe head is mounted, the cable should be clamped firmly nearby so it cannot be torn away from the probe
head if accidentally pulled. The flexible section adjacent to the probe head can be carefully folded to allow the cable to
come away in any direction but avoid repeated flexing of this section.
Keep the cable out of the way of foot traffic. Do not pinch the cable or drop sharp or heavy objects on it. A severed
cable cannot be re-joined without altering the probe's performance and requires factory repair and re-calibration.
The DTM-133 must be used with a Group3 Hall probe. Probe models LPT-130, LPT- 230, MPT-132, or MPT-230 are
the most suitable for use with the DTM-133. The probe may be one supplied with your teslameter, or it may have
been obtained separately. In any case, calibration is preserved when probes are exchanged between instruments.
The standard probe cable length is 2 meters. Probes with non-standard cable lengths up to 30 meters may be ordered
from your Group3 supplier. The cable used for Group3 probes is shielded to reduce pickup of induced noise from
external sources. Such noise may reduce the accuracy of the instrument, cause malfunctioning, or in extreme
circumstances even result in damage to the internal circuitry. See section 3.8.
With the DTM unpowered, plug the probe connector into the instrument. The pin side of the plug is inserted into the
large opening in the rear of the DTM, with the plug’s label uppermost when the instrument is standing right way up. It
is easy to find the correct mating position for the plug, and then push it fully home, but if any difficulty is experienced
at first, remove the DTM’s top cover by loosening the central screw and lifting the cover off. Now it is possible to see
when the plug is centrally located and its overhang slides over the card-edge receptacle, ensuring that its pins engage
correctly. Tighten the connector retaining screws finger tight. Do not leave these screws loose as they form part of
the shielding system around the teslameter. The teslameter should always be used with both covers attached.
3-2 DTM-133 User's Manual

Always disconnect power from the teslameter before connecting or disconnecting the probe. If the probe connector is
inserted or withdrawn with power on, data stored in memory may be corrupted, leading to erroneous field readings. If
this happens, the instrument should be powered down, and then repowered while both keys are held down. This will
restore default operating conditions.
When no probe is connected to the DTM, the display reads noProbE.
3.4 CONNECTING THE POWER SOURCE
All teslameter versions, except for the L option. are supplied with a plug-pack. Connect the plug-pack to a convenient
ac power source, first checking the voltage marked on the plug-pack and insert the cable connector into the power
receptacle on the DTM rear panel.
Instead of the plug-pack, the unit can be powered by any convenient source of ac or dc (either polarity) which meets
the specification on page 2-2. The cable connector required for power connection to the DTM is a standard coaxial
plugpack connector with 2.1mm center hole and is generally available from electronics suppliers.
For extra immunity to damage and operational disturbance caused by serious high voltage sparking near the
teslameter, the use of the Group3 model PS12D7 off-line switch-mode power supply and the Group3 ferrite kit part
no. 11000036 is recommended. These accessories will greatly reduce the amount of electrical transient energy
entering the teslameter. The ferrite kit includes a suppressor which fits to the probe cable near the point of entry to
the teslameter to reduce the effects of transients picked up on the probe cable. For a full discussion of techniques to
promote trouble free operation in electrically noisy environments, see section 3.8 of this manual.
Powering the L option teslameter -
The L option will accept power input from the ac power line.
Access to the power input terminals of the L option is obtained by taking off the orange cover; remove the 3 fixing
screws to release the cover.
Use 3-conductor power cord. For safety from electrical shock, it is essential to provide a reliable ground connection to
the DTM case. Make sure the ground wire is connected as shown in Fig. 2. Strip about 60 mm (2.5 in) of outer jacket
from the cord, and strip 5 mm (3/16 inch) of insulation from the 3 wires. Pass the cord through the grommeted hole in
the cover. Loosen the screw securing the cable clamp and pass the cord through the clamp. Tighten the clamp on the
outer jacket. Terminate the wires and fit links according to the supply voltage as set out in Fig. 2 below. Replace the
orange cover, making sure that wires are not pinched in the process. For safety reasons, do not operate the unit with
the cover off.
DTM-133 User’s Manual 3-3

Note that input power protection is provided by a thermal fuse wound into the power transformer. This fuse will open
in the event of transformer overheating rather than on excess current. The power input must be connected as shown
to include the thermal fuse in the circuit correctly. If a fault causes transformer overheating and subsequently the fuse
opens, the transformer must be replaced with the genuine Group3 part.
Fig. 2. Power Input Connections of the -L option
If desired, the wiring may be protected by installing an external fuse in the ac power feed. Suggested fuse ratings are
200 mA for 115 volts, or 100 mA for 208- and 230-volt operation.
When the unit is first powered up, the display shows Group 3 for 2 seconds before field measurements appear. If the
Hall probe is not plugged in, the field reading display is replaced with noProbE.
3-4 DTM-133 User’s Manual

3.5 INTERNAL DIP SWITCHES
The main circuit board under the top cover of the DTM-1 33, has a set of two DIP switches. The switch functions are
listed in Table 1 below. For access to the switches, loosen the central screw and lift the cover off.
switch function
1digital filtering
2 field units
OFF ON
disabled enabled
tesla gauss
Table 1. Internal DIP Switch Functions
The functions of switches 1 and 2 are described in full in section 4. The switches are scanned once a second, so the
effects of changed settings can be observed immediately.
3.6 ANALOG OUTPUT
An analog output signal is available at the rear of the teslameter. This output is the Hall probe signal amplified to 3
volts full-scale and gives an indication of the instantaneous field value from dc to 9kHz (-3dB), with a roll-off of
60dB/decade above 9 kHz. Field direction is indicated by the output voltage polarity. There is a small zero offset (10
millivolts maximum), arising from the probe zero-field output and amplifier offsets. The output impedance is 1000 ohm
with a 1 nF capacitor to common for noise filtering.
The cable connector required is a Molex receptacle type M5051-2 fitted with M2759 terminals. Pin assignments are
given below.
The analog output is not corrected for linearity errors.
pin signal
1 ground
2dc output
Table 2. Analog Output Connector Pin Assignments
3.7 GROUNDING
All parts of the teslameter's metal case are connected together to form an integral electric shield around the circuitry
inside. When the probe connector is plugged into the teslameter and the retaining screws are tightened, the probe
connector case and the teslameter case are connected together and form an integral shield around the circuitry inside.
The cable shield is added to the case shield and extends protection from electrical interference almost up to the probe
head.
DTM-133 User’s Manual 3-5

Because there is an internal connection between teslameter circuit common and the probe connector case, when the
probe connector is engaged, and the retaining screws tightened the teslameter circuit common will be connected to
the case. Do not make an additional connection between circuit common and the case at any point, including at the
RS-232C connector or at the G3CL connectors on serial teslameters, or at the GPIB connector on teslameters with
the IEEE-488 option. Such additional connection will form a ground loop and may introduce errors in the measured
field value.
The shielding provided with the above arrangement should be sufficient protection against EMI in most cases,
especially when the probe cable is shielded. Sometimes it may be found helpful to ground the teslameter case to a
good electrical ground point. Connection can be made to the case by inserting an appropriate lug or terminal under the
head of one of the rear panel fixing screws.
Further protection from transient interference can be obtained by using model PS12D7 power supply in place of the
usual plugpack supplied with the teslameter, and by installing the Group3 ferrite kit part no. 11000036. See section
3.8 of this manual.
For electrical safety, the case of the L version must be grounded through the third wire of the power input cord.
3.8 INSTALLATION TECHNIQUES FOR ELECTRICALLY NOISY ENVIRONMENTS
The DTM-133 is a precision electronic measuring device. Because of the nature of the measurements, it is asked to
do, it is frequently exposed to conditions that are considerably worse than are normally encountered by precision
instruments. Therefore, the teslameter has been carefully engineered to be as immune as possible to sparks and other
forms of interference through the use of several kinds of power input filtering and a special high-isolation switch mode
power module built into its circuitry. The design has been verified by extensive testing, using high energy sparking in
close proximity to both the teslameter instrument case and the probe. Nevertheless, due care should always be taken
when installing the teslameter system.
The teslameter and its probe must be protected from any chance of receiving a direct hit by a high voltage discharge.
The probe should have shielded cable if the meter is to be used in an electrically noisy environment. The cable shield is
an RFI screen, not a high current path, so if there is any possibility of an arcing discharge hitting the probe area, then
the probe head and part or all of the cable must be enclosed in a metal tube (non- magnetic near the probe head) or
shielded in some other way.
The probe cable should be routed away from any power, high current or high voltage wiring. It should be shielded from
any capacitively coupled noise effects. If the cable runs close to any section of the apparatus that could be subjected
to a very rapid change of potential when a spark discharge occurs, then the probe cable may need additional shielding
to prevent capacitive coupling of the noise.
The retaining jack screws designed to hold the probe connector onto the teslameter must be screwed up finger tight,
as they form part of the electrical connection of the shield system. The woven braid of the probe cable is terminated
to the probe connector case. The retaining screws then connect the probe connector case to the teslameter case.
3-6 DTM-133 User’s Manual

The teslameter itself should be sited in a sheltered location, where it will not be exposed to spark discharges or
radiated or capacitively coupled noise. The teslameter case is made of metal for shielding reasons. However, of
necessity it is less than perfect, as apertures have to be left in the case for the display and various connectors etc.
The unit is a precision measuring device, and should be treated with care, not subjected to adverse environmental
conditions.
The plugpacks supplied with each teslameter should be plugged in to a clean mains power supply. Noise on the mains
will work its way through the transformers and disturb the teslameter. Simple mains filters are readily available if
there is only one mains supply for the whole machine. Route the low voltage lead away from high current or high
voltage wiring. Ideally cut the low voltage lead to the minimum length required for the installation, and re-connect the
plug to it.
If you are using the serial communication features of the teslameter, take advantage of the noise immunity of the fiber
optic facilities available, rather than using the wired RS- 232C connection. Fiber optics were included in the DTM-133
for the express purpose of providing noise free communication in hostile applications. The fiber optic cables used with
the DTM-133 are economical and convenient to use - simpler in fact than wiring. To interface the fiber optic cables to
your computer or other data acquisition system, use a Group3 model FTR fiber optic adaptor.
Grounding The Teslameter Case
The probe shield is terminated to the probe connector case, which is then connected by the retaining screws to the
teslameter chassis. At this point the entire shield system is floating. In some installations it is beneficial to have the
system floating, but most frequently it is sensible to have the shields grounded.
If the teslameter is panel mounted, then the case is almost certainly electrically connected to the control rack and
grounded that way. However, if the teslameter is a bench unit, then the rubber and plastic feet on it will isolate the
case. If the case does need to be grounded, then loosen one of the screws on the back panel and put a grounding lug
under the head of the screw. It is most convenient to use a 1/4inch (6.35mm) quick connect tab. The grounding wire
can then be easily disconnected if the teslameter has to be moved. Use a heavy gauge, short wire to ground the unit
to a substantial grounding point nearby. If the teslameter is sitting on metalwork, then it should really be grounded to
that metalwork, so it is at the same potential.
DTM-133 User's Manual 3-7

Further Preventative Measures
If problems are still encountered, despite following the precautions detailed above, then there are some further things
to try.
Tests have shown that, in an electrically noisy environment, the main path of noise entry to the teslameter is through
the low voltage power supply input. The trouble could come from mains borne transients working their way through
the plug pack transformer, or from interference picked up on the low voltage lead itself. The quickest and simplest fix
for this problem is to wind the power lead several times through a ferrite core. Use a thick-walled ferrite tube of
substantial size - a simple small toroid is not nearly as effective. A suggested ferrite is the TDK part number
HF70RH26x29x13. This is a tubular ferrite, 29 mm long, 26mm outside diameter, and 13mm inside diameter. Winding
the power lead four times through this core, really close to the teslameter, significantly reduces noise upsets.
If the analog outputs are wired up, then shielded twisted pair should be used for all wiring, routed away from any high
current or high voltage cabling. In a really noisy environment, it can be beneficial to put this analog cabling through a
ferrite tube for a few turns to suppress induced noise.
The probe cable itself can be passed through a ferrite core. The internal diameter will need to be sufficient to pass the
probe head through. An MPT (miniature) probe head is nearly the same size as shielded cable (6.5mm diameter), but an
LPT probe head needs an internal ferrite diameter of 14mm or more. Alternatively, a split core ferrite variety can be
used, such as TDK part HF70RU16x28x9. The core should be placed where the probe cable enters the probe
connector, and optionally a second ferrite can be placed where the cable shield layer ends, approximately 300mm
back from the probe head.
Group3 can supply an alternative power supply to be used instead of the usual plug pack. The alternative power
supply is model PS12D7. It is a universal voltage (85 - 270V 50/60Hz) input, 12Vdc 7W output unit with excellent
input-output isolation for noise and transients. The PS12D7 is DIN rail mounted. In conjunction with the PS12D7 we
recommend the use of our ferrite kit, part no. 11000036 which implements the ferrite filtering measures described
above. The kit consists of a 1.2-meter length of twin cord with a ferrite tube fitted. This cord is intended to connect
between the PS12D7 and the teslameter. The kit also contains a split ferrite tube and housing for fitting to the probe
cable.
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4. OPERATING INSTRUCTIONS
4.1 ZEROING
The DTM-133 digital teslameter has a very stable zero field reading. Nevertheless, it is good practice to zero the
instrument on all ranges immediately prior to making critical field measurements. The zeroing process takes out
residual zero errors in the Hall probe and the instrument's preamplifier "front-end".
Zeroing is mandatory if a different probe is to be used since the instrument was last zeroed. You should also zero the
instrument when using it for the first time.
Before zeroing the system, connect the probe and apply power as described in sections 3.3 and 3.4. Allow 30 minutes
for the instrument and probe to stabilise.
For absolute zeroing, place the probe in a zero-field region, either in a zero-field chamber or inside a suitable magnetic
shield, so that the probe is shielded from the earth’s magnetic field and other stray fields.
If desired, a relative zero setting may be done; the instrument is zeroed after the probe is placed in its measurement
position. Thus, any ambient field is automatically subtracted from subsequent measurements. The probe should not be
moved once zeroing is complete. About 5% of full-scale may be zeroed out without reducing full-scale span below
specification.
The zero-field reading is affected slightly by the presence of metal against the probe's back surface. If the probe is to
be used clamped to a metal surface, or in a probe holder, it should be zeroed in the same situation. Allow the probe to
stabilize thermally for a minute or two before zeroing.
Ambient temperature also has a slight effect on the probe zero. The probe should be zeroed at a temperature as close
as possible to the temperature during service.
If the teslameter is set for auto ranging (see section 4.4.1), zeroing is implemented simply by momentarily pressing
both keys together. The display will flash ZEro while the teslameter cycles through all four ranges in turn, waiting for
each range to stabilise before zeroing it. The whole process takes about 10 seconds. The range indicators show which
range is selected. If a single-range probe is connected, only the one range will be selected and zeroed.
If auto ranging is disabled, each range must be manually selected and zeroed. A range is selected by pressing the
RANGE key. The four range indicators show the selected range. The RANGE key selects the ranges in turn in the
sequence 0.3, 0.6, 1.2, and 3.0 tesla. If a single-range probe is in use, the RANGE key will have no effect.
The zeroing process is implemented by pressing and releasing both keys together. The display will read ZEro for a
moment, indicating that zeroing has occurred.
DTM-133 User's Manual 4-1

The zeroing process should now be repeated for all the remaining ranges. Press the RANGE key to select another
range, and zero this range by pressing both keys together, as above. After changing ranges, wait 1 or 2 seconds
before zeroing. Continue until all the ranges have been zeroed.
Once the zeroing process has been completed, the internal processor will apply the appropriate correction to
whichever range is selected.
Zero offsets calculated during the zeroing process are stored in non-volatile memory and are retained while the power
is off. Therefore, the teslameter does not need re- zeroing simply because it has been powered down.
4.2 INSTALLING THE PROBE
Group3 Hall probes are built to be as robust as possible for a small, precision device. However, it is most important
that certain precautions be taken when handling and installing probes so that they are not damaged or destroyed, and
to preserve their accurate calibration.
Mount the probe head so there is no pressure which will tend to bend or depress its ceramic rear surface. If the probe
head is clamped, make sure the surface in contact with the ceramic is flat and covers the whole of the ceramic surface.
Do not apply more force than is required to hold the probe in place. Any strain on the ceramic will alter the probe's
calibration, and excessive force will destroy the Hall element inside.
When the probe head ismounted, the cable should beclamped firmly nearby soit cannotbetornawayfromtheprobeheadif
accidentally pulled. Theflexible section adjacent to the probe head can be carefully folded to allow the cable to come away in any
direction but avoid repeated flexing of this section.
Keep the cable out of the way of foot traffic. Do not pinch the cable or drop sharp or heavy objects on it. A severed
cable cannot be re-joined without altering the probe’s performance and requires factory repair and re-calibration.
The LPT-130 and LPT-230 probes can be fitted to a Group3 probe holder part number 17000049. MPT-132 and MPT-
230 probes fit probe holder part number 17000081. The holder protects the probe and provides additional cable strain
relief.
The probe will measure the component of the field which is normal to the flat surface of the probe case. The point of
maximum sensitivity is marked by a target printed onthe top of the probe case. A positive indication will be obtained
when the magnetic field vector enters this side of the probe. The target represents the tail of the vectorarrow.
4-2 DTM-133 User’s Manual