ACM ST200 User manual

ST200
ST300
SERIES
INSTRUCTIONS
AMC-ST200 Electrochemical
AMC-ST350 Solid State
AMC-ST360 Catalytic
INSTALLATION AND OPERATING
INSTRUCTIONS FOR THE AMC-ST200, ST350,
ST360 TRANSMITTERS
IMPORTANT:
Please read these installation and operating instructions
completely and carefully before starting.
.........................................................................................................................................Copyright 1999
The Armstrong Monitoring Corporation (AMC)
[phone] -Å
[phone] -(Fax)
1 [phone] -Å(U.S. Toll Free)

Armstrong Monitoring Corporation
Model ST Series Non-Intrusive Transmitter
-ii -
WARRANTY .......................................................................................................1
LIABILITY...........................................................................................................1
MODIFICATIONS AND SUBSTITUTIONS......................................................1
PRODUCT RETURN ...........................................................................................2
PRODUCT INFORMATION...............................................................................2
FACTORY CALIBRATION................................................................................2
SECTION I.......................................................................................................................3
GENERAL DESCRIPTION .............................................................................................3
1.1 DESCRIPTION..............................................................................................3
1.2 SPECIFICATIONS........................................................................................5
1.2.1 SENSOR VOLTAGE (ST350/360 Series)...................................................5
1.2.2 SIGNAL OUTPUT......................................................................................5
1.2.3 POWER SUPPLY .......................................................................................5
1.2.4 CALIBRATION RANGE............................................................................5
1.2.5 CALIBRATION RESOLUTION.................................................................5
1.2.6 ACCURACY...............................................................................................6
1.2.7 AMBIENT TEMPERATURE RANGE ........................................................6
1.2.8 TEMPERATURE DRIFT.............................................................................6
1.2.9 HOUSING:..................................................................................................6
SECTION II .....................................................................................................................7
INSTALLATION .............................................................................................................7
2.1 MOUNTING..................................................................................................7
2.2 ELECTRICAL CONNECTIONS...................................................................7
SECTION III....................................................................................................................9
OPERATING PROCEDURES .........................................................................................9
3.1 NORMAL MODE OPERATION...................................................................9
3.2 OVERRANGE................................................................................................9
3.3 ROUTINE CALIBRATIONS USING CAL MODE.......................................10
3.4 UNITY GAIN MODE ....................................................................................10
3.5 DELAY MODES ............................................................................................11
3.5.1 POWER UP DELAY............................................................................11
3.5.2 CAL MODE EXIT DELAY..................................................................11
3.5.3 AUTOMATIC CAL MODE EXIT TIMER...........................................12
3.5.4 ABORTING THE DELAY MODES.....................................................12
3.6 FAULT CONDITIONS..................................................................................12
3.7 BACK-UP OF CALIBRATION VALUES DURING POWER LOSS...........12
3.8 LCD READOUT CALIBRATION PROCEDURE .........................................12

Armstrong Monitoring Corporation
Model ST Series Non-Intrusive Transmitter
-ii -
SECTION IV....................................................................................................................15
MODEL ST350 & 360 SOLID STATE AND CATALYTIC SERIES ..............................15
4.1 GENERAL DESCRIPTION ...........................................................................15
4.2 INITIAL START-UP AND CALIBRATION PROCEDURE .........................15
4.2.1 SENSOR VOLTAGE ADJUSTMENT.................................................16
4.2.1 MONITORING THE VOUT TEST POINT.........................................16
4.2.2 BALANCE ADJUSTMENT.................................................................17
4.2.3 ST350/ST360 SERIES INITIAL FIXED GAIN ADJUSTMENT.........17
4.3 SENSOR FAULT SUPERVISION.................................................................18
SECTION V.....................................................................................................................20
MODEL ST200 ELECTROCHEMICAL SERIES ............................................................20
5.1 GENERAL DESCRIPTIONS.........................................................................20
5.2 INITIAL START-UP AND CALIBRATION PROCEDURE .........................20
5.2.1 SENSOR RESPONSE COEFFICIENT...............................................21
5.2.2 SELECTING PRE-AMP GAIN RESISTOR R13.................................21
5.2.3 INITIAL FIXED GAIN ADJUSTMENT..............................................21
5.3 SENSOR FAULT SUPERVISION:................................................................23
SECTION VI....................................................................................................................24
OPTIONAL FEATURES..................................................................................................24
6.1 GENERAL DESCRIPTION ...........................................................................24
6.2 ISOLATED 4-20 MILLIAMPERE OUTPUT OPTION .................................24
6.3 MODBUS® RS-485 SERIAL INTERFACE OPTION ..................................25
6.3.1 RTU ADDRESS ...................................................................................26
6.3.2 DATA REGISTERS AND FUNCTION CODE...................................26
6.4 ALARM RELAYS OPTION ..........................................................................26
6.4.1 LATCHING ALARMS:........................................................................27
6.4.2 FAIL SAFE ALARMS:.........................................................................27
SECTION VII...................................................................................................................29
CUSTOMER SUPPORT..................................................................................................29
7.1 TECHNICAL SERVICE:................................................................................29
7.2 HEAD OFFICE..............................................................................................29
SECTION VIII .................................................................................................................30
PHYSICAL DIMENSIONS.............................................................................................30

Armstrong Monitoring Corporation
Model ST Series Non-Intrusive Transmitter
-1-
WARRANTY:
The AMC-ST Series Super Transmitter is warranted against defects in material and
workmanship for a period of two years from date of delivery (For sensor, see 1.2 sensor
Specifications). During the warranty period, we will repair or replace components that
prove to be defective in the opinion of The Armstrong Monitoring Corporation
(AMC). We are not liable for auxiliary interfaced equipment, nor consequential damage.
This warranty shall not apply to any product, which has been modified in any way, which
has been repaired by any other party other than qualified technician or authorized AMC
representative, or when such failure is due to misuse or conditions of use.
LIABILITY:
All AMC products must be installed and maintained according to instructions. Only
qualified technicians should install and maintain the equipment.
AMC shall have no liability arising from auxiliary interfaced equipment for consequential
damage, or the installation and operation of this equipment. AMC shall have no liability
for labour or freight costs, or any other costs or charges in excess of the amount of the
invoice for the products.
THIS WARRANTY IS IN LIEU OF ALL OTHER WARRANTIES,
EXPRESSED OR IMPLIED, AND SPECIFICALLY THE WARRANTIES OF
MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE.
THERE ARE NO WARRANTIES, WHICH EXTEND BEYOND THE
DESCRIPTION ON THE FACE THEREOF.
MODIFICATIONS AND SUBSTITUTIONS:
Due to an ongoing development program, AMC reserves the right to substitute
components and change specifications at any time without incurring and
obligations.

Armstrong Monitoring Corporation
Model ST Series Non-Intrusive Transmitter
-2-
PRODUCT RETURN:
All products returned for warranty service will be prepaid freight and they will only be
accepted with an R.M.A. number issued by AMC. All products returned to the client
will be freight collect.
PRODUCT INFORMATION:
Transmitter/Sensor unit Order Number ……..
Transmitter Part Number …………………….
Transmitter Serial Number …………………..
Sensor Part Number ………………………….
Power Supply Requirement 18-30 VDC; less than 4 Watts
Signal Output Load 4-2OmA into 800 ohms max.
FACTORY CALIBRATION:
Gas Type ………………………………………
Zero Gas, at 4mA Signal ………………………
Gas Concentration at 20 mA Signal …………..

Armstrong Monitoring Corporation
Model ST Series Non-Intrusive Transmitter
-3-
SECTION I
GENERAL DESCRIPTION
1.1 DESCRIPTION:
This manual describes the AMC microprocessor based ST Series Non-Intrusive
Transmitters. Three models of the ST Series are described within this manual.
The ST360 Series accepts catalytic bead combustible sensors. The ST350 Series is a
non-intrusive transmitter used with solid state sensors. The ST200 Series accepts
electrochemical toxic and oxygen sensors.
Several options are available to enhance ST Series Non-Intrusive Transmitters
capabilities. These include an isolated 4-20 mA output, an RS-485 Modbus® serial
interface, or an alarm providing 5 A form C relays for fault, warn and high alarm
conditions.
The AMC ST Series Non-Intrusive Transmitters accept a wide range of gas detection
sensors. These sensors are directly connected to the ST Series without need of other
transmitters or electronics. Supplied transmitters are factory calibrated. A 4-20 mA
output and 3 1/2 digit LCD readout provide analog and visual indications of gas
concentration. Magnetic sensors allow complete "end to end" calibrations to be
performed to the transmitter section without opening the explosion-proof enclosure.
This is especially useful when the area is classified as potentially hazardous and
declassification is required to open enclosures.
FAULT conditions, such as LEL sensor opens and shorts, missing EC sensor, or
negative drift below 1.6 mA (-15% of full scale), are detected and flagged by the ST
Series. During these FAULT conditions the 4-20 mA output is held at 0 mA and the
KEYPAD/FAULT LED indicator is illuminated.

Armstrong Monitoring Corporation
Model ST Series Non-Intrusive Transmitter
-4-
The only tool required to perform calibrations is a small magnet, which is provided on a
key chain. The integral 3 1/2 digit LCD readout may be calibrated for direct reading in
engineering units such as percent of LEL (lower explosive limit) or PPM (parts per
million).
Before calibration is possible, the ST Series must be properly activated by entering the
CAL MODE. This is done by briefly holding the magnet to a location on the panel
marked by a small dot. The ST Series will respond by illuminating a left hand arrow on
the LCD readout. This serves as a "pass-key" and makes it difficult for unauthorized
personnel to tamper with settings. During CAL MODE, the 4-20 mA output is held at
1.5 mA to prevent alarms or other instrumentation from being affected by calibration
levels.
Calibration of the system may now be performed by appropriately exciting the sensor and
holding the magnet close to the UP/DOWN ZERO or UP/DOWN SPAN controls, as
required to obtain correct readings on the front panel LCD readout. ZERO and SPAN
controls do not interact when the zero adjustment is performed first. The RESET control
is provided to place the ST Series in a UNITY GAIN mode of operation. This returns
the ZERO and SPAN controls to the center of their adjustment range thereby returning
offset and gain values to the same starting point each time UNITY GAIN mode is
entered. This is useful for tracking the sensor's sensitivity, which may deteriorate with
age. If a RESET is applied before calibration, the SPAN gas may be applied and signal
shift observed. When signal shift falls to half the original value of when the sensor was
new, it is approaching time to replace that particular sensor.
A non-volatile memory device provides indefinite battery back-up of calibration values
during power interruptions.

Armstrong Monitoring Corporation
Model ST Series Non-Intrusive Transmitter
-5-
FIGURE 1.1
1.2 SPECIFICATIONS:
1.2.1 SENSOR VOLTAGE (ST350/360 SERIES):
Adjustable Voltage; 2 W Max.
1.2.2 SIGNAL OUTPUT:
4-20 mA into 800 Ohms Max. with 24VDC Power Standard
1.2.3 POWER SUPPLY:
18-30 VDC; 4 W Max.
1.2.4 CALIBRATION RANGE:
ZERO -+15 % of Full Scale
SPAN –Turn UP to GAIN of 2, DOWN to GAIN of 0.5
1.2.5 CALIBRATION RESOLUTION:
0.1% of Full Scale

Armstrong Monitoring Corporation
Model ST Series Non-Intrusive Transmitter
-6-
1.2.6 ACCURACY:
±0.1% of Full Scale ±1 Count
1.2.7 AMBIENT TEMPERATURE RANGE:
-40 to +60 °C
1.2.8 TEMPERATURE DRIFT:
Less than 0.1% /°C over Ambient Temperature range.
1.2.9 HOUSING:
Explosion Proof Class 1, Groups B, C, D & Class 2, Groups E, F, G with CSA and FM
approval.

Armstrong Monitoring Corporation
Model ST Series Non-Intrusive Transmitter
-7-
SECTION II
INSTALLATION
2.1 MOUNTING:
The ST Series is packaged in an explosion-proof housing with two 3/4 inch N.P.T.
conduit hubs located in line and across the housing from each other (see figure 8.1). This
housing has symmetrical mounting holes for the electronics, which are drilled so the
conduit hubs may be oriented across the bottom, top, left side or right side. Simply
mount the housing as desired then remove the four screws which hold the ST Series I/O
printed circuit board in place. Orient the I/O PCB so the LCD readout will align properly
and replace the mounting screws.
Note: To gain access to the I/O PCB for wiring or mounting purposes, loosen the
two captive thumb screws in the ST Series front panel and remove the
PANEL/CPU PCB assembly as far as is allowed by the ribbon cable. The front
panel and attached PCB may then be removed from the ST Series housing leaving
the I/O PCB fully exposed. To replace the front panel assembly, align the two
thumb-screws with their mating stand-offs and firmly hand tighten. Be sure the
front panel is centered in the ST Series housing opening.
2.2 ELECTRICAL CONNECTIONS:
Please see the above note for instructions how to access the ST Series I/O terminals for
field wiring.
The ST Series is a three-wire device. There is a positive 24VDC power wire (other
power supplies are available by special order), and a positive output signal wire which is
typically 4-20 mA. The third wire is the system common and serves as power supply and
signal returns. These three wires connect to the three point terminal block labeled TB2.
An earth ground lug inside the explosion-proof enclosure is provided for earth grounding
of shield wires if the three-wire cable is shielded.

Armstrong Monitoring Corporation
Model ST Series Non-Intrusive Transmitter
-8-
Since the ST Series is designed to function with ambient gas sensors, there is another
three point terminal block labeled TB1 and provided for sensor wires. ST360 Series
TB1 terminals are labeled R, C, and A. Ris for the reference element, Cis for the
junction of the reference and active elements, and A is for the active element. Ais the
same as system common. The sensor voltage level may be measured across Aand R.
For AMC-ST200 Series units, TB1 terminals are labeled S, R, and C. Sis for the
SENSE electrode, Rfor the REFERENCE electrode and Cfor the COUNTER
electrode.
TB1 and TB2 may accept wire sizes up to 14 AWG. It is suggested the wires be
stripped 1/4 inch and the bare copper wire be tinned with solder to prevent loose strands
from shorting.
P1
TB1TB2
RIBBONCABLE
TOCPUASSY.
VOUT
JP2
BA
JP1
1 32 4
U2U7
U4
U5
U1
SCR
R19
ST-46ECI/O
ST-46EC I/O PCB FOR
ELECTROCHEMICAL SENSORS
P1
TB1
TB2
RIBBONCABLE
TOCPUASSY.
VOUT
JP1
1 32 4
U2
U4
U5
RAC
ST-46ECI/O
U3
VOLTSBAL
R2R11
U1
CATALYTIC BEAD SENSORS
ST-46LEL I/O PCB FOR
FIGURE 2.1 FIGURE 2.2

Armstrong Monitoring Corporation
Model ST Series Non-Intrusive Transmitter
-9-
SECTION III
OPERATING PROCEDURES
3.1 NORMAL MODE OPERATION:
The NORMAL MODE is present any time the LCD’s left hand arrow or the FAULT /
KEYPAD LED is not illuminated. During this time the ST Series functions as a 4-20
mA transmitter with a digital LCD readout. LCD readouts are typically factory
calibrated to display engineering units being monitored by the sensor.
During NORMAL MODE the 4-20 mA output and the LCD readout do not track
input values below 0% of full scale. This prevents erroneous and momentary negative
noise from being transmitted and displayed. This feature does require the NORMAL
MODE be exited and the CAL MODE entered prior to exposing the sensor to ZERO
gas for checking ZERO drift. Negative ZERO drift will only be displayed during the
CAL MODE.
3.2 OVERRANGE:
The analog input to the ST Series is converted to a 0.4 -2 volt signal on the I/O PCB
and then applied to a 10 bit analog to digital (A-D) converter on the CPU PCB. If gain
settings are too high on the I/O PCB, or if gas values are too high, it is possible for
upscale inputs to exceed 2 volts. The A-D converter saturates at readings above 103%
of full scale, or, about 2.06 V. At this point an OVERRANGE condition is indicated by
a fast (3-4 times per second) flashing of the left hand arrow. If OVERRANGE occurs
during a SPAN calibration the calibration should be halted since the analog voltage
being applied to the A-D converter is too high. Either the SPAN gas is incorrect, or,
the FIXED GAIN jumper setting on the I/O PCB is too.

Armstrong Monitoring Corporation
Model ST Series Non-Intrusive Transmitter
-10-
3.3 ROUTINE CALIBRATIONS USING CAL MODE:
Routine calibrations of gas readings are easily performed using the magnet tool provided
with each ST Series Non-Intrusive Calibrating Transmitter. To enter CAL MODE,
briefly hold the magnet close to the small dot located on the lower left of the front panel.
The left hand arrow on the upper left side of the LCD will illuminate and the 4-20 mA
output is locked at 1.5 mA, indicating the ST Series is ready for calibration. CAL
MODE is indicated by a steady arrow. Note: It is important the arrow NOT be
flashing during calibrations since a fast flash rate (3-4 times per second) indicates a
saturated A-D converter and may require a lower gain setting be selected on the
I/O PCB. Expose the sensor to a ZERO gas and observe the LCD readout. If it does
not return to the correct ZERO reading a ZERO adjustment is needed. Hold the magnet
close to the UP ZERO or DOWN ZERO sensor and adjust the reading to the correct
ZERO value.
Then expose the sensor to an appropriate SPAN gas, such as 50% of the gas being
monitored. If the LCD does not display the correct SPAN value a SPAN adjustment is
needed. With the arrow still on, hold the magnet close to the UP SPAN or DOWN
SPAN sensors and adjust the reading to the correct SPAN value.
The monitor is now calibrated.
REMEMBER TO DEACTIVATE THE CAL MODE BY HOLDING THE MAGNET
CLOSE TO THE SMALL DOT AGAIN. THE 4-20 mA OUTPUT IS HELD AT 1.5
mA WHILE ACTIVATED FOR CALIBRATION TO PREVENT ALARMS FROM
BEING TRIPPED BY CALIBRATION LEVELS.
3.4 UNITY GAIN MODE:
A RESET magnetic control is available during CAL MODE to allow the ZERO and
SPAN adjustments to be centered within their range. This is similar to setting a
potentiometer so the wiper terminal is exactly halfway between the clockwise and
counterclockwise terminals. This is identified as the UNITY GAIN mode. In UNITY
GAIN the ZERO controls have a +15% of full scale adjustment range. For example, in
UNITY GAIN, if the sensor's ZERO output has drifted so high that it reads 15% with
ZERO gas applied, the DOWN ZERO magnetic control could still bring the ST Series

Armstrong Monitoring Corporation
Model ST Series Non-Intrusive Transmitter
-11-
reading to ZERO. However, it will be at the end of its adjustment range. If the ZERO
adjustment required is greater than +10% of full scale, a BALANCE adjustment should
be performed as described in section 4.2.2.
In UNITY GAIN the SPAN controls have a 0.5 to 2 adjustment range. For example, in
UNITY GAIN, if a sensor's output sensitivity has been reduced to the point where a
50% SPAN gas provides only a 25% reading, the UP SPAN magnetic control could still
calibrate the reading to the proper value of 50%. However, it will be at the end of its
adjustment range.
The RESET control is also useful because it may be used to begin each calibration from
the same starting point. That point being UNITY GAIN. If each calibration begins at the
same point, it is possible to observe the cumulative affect of sensor drift over time. It is
known that gas detection sensors lose output signal strength as they age. Knowing the
signal strength of a sensor from its initial installation, and being able to track its
deterioration over time, may be useful in determining when to replace it before failure
occurs.
3.5 DELAY MODES:
The POWER UP DELAY and the CAL MODE EXIT DELAY described in this
section are both indicated by the slow flashing of the LCD’s left hand arrow. During
these delays the LCD readout is active, but the 4-20 mA output is held at 4 mA.
3.5.1 POWER UP DELAY:
The 4-20 mA output is held a 4 mA for 1 minute after power is applied. This is to allow
the sensor to stabilize briefly and reduce the possibility of causing an erroneous alarm
condition within the gas detection system.
3.5.2 CAL MODE EXIT DELAY:
SPAN values are typically the last gas applied during a routine calibration. If the CAL
MODE is exited too quickly after removing SPAN gas from the sensor, the reading may
still correspond to the SPAN value and trip alarms. For this reason the ST Series 4-20
mA output is also held at 4 mA for 1 minute after exiting the CAL MODE.

Armstrong Monitoring Corporation
Model ST Series Non-Intrusive Transmitter
-12-
3.5.3 AUTOMATIC CAL MODE EXIT TIMER:
The ST Series 4-20 mA output is held at 1.5 mA during the CAL MODE. This alerts
any loop monitoring devices that a special condition is present. Since it is possible for
an operator to forget to return the ST Series to the NORMAL MODE, a 5 minute
timer monitors the magnetic keypad during the CAL MODE. If no keystroke is made
during a 5 minute interval, the CAL MODE is exited and the 4-20 mA output becomes
active again.
3.5.4 ABORTING THE DELAY MODES:
Trouble shooting or other testing procedures may be easier without the delay periods
described above. These may be aborted by removing power, holding the magnet to the
UP ZERO control and then reapplying power. This procedure only works if the sensor
is not in a FAULT condition when power is applied. Any subsequent power up without
holding the magnet to the UP ZERO control returns the delay periods.
3.6 FAULT CONDITIONS:
The FAULT / KEYPAD LED is a dual-purpose indicator. If it is illuminated without a
magnet near the keypad it is signaling a FAULT condition. A FAULT also causes the
4-20mA output to be held at 0 mA. Negative sensor drift below -15% of full scale will
automatically cause the FAULT indication. Sensor failures, described in sections 4 &
5, will also cause the FAULT indication.
3.7 BACK-UP OF CALIBRATION VALUES DURINGPOWER LOSS:
An E
2non-volatile memory device provides unlimited periods of continuous storage of
calibration values during power interruptions.
3.8 LCD READOUT CALIBRATION PROCEDURE:
Note: This procedure is typically performed at the factory and normally need not
be repeated unless engineering units displayed by the LCD are to be changed.
When calibration of the LCD readout is necessary it is important to understand that
the only requirement is the display must read ‘0’ engineering units with 4 mA
outputs and ‘full scale’ engineering units with 20 mA outputs.

Armstrong Monitoring Corporation
Model ST Series Non-Intrusive Transmitter
-13-
A. Remove the two thumbscrews attaching the front panel assembly to the enclosure.
This assembly is connected to the I/O PCB via a ribbon cable long enough to remove
it from the enclosure without disconnection. Removal exposes ZERO (R20) &
SPAN (R19) potentiometers and J1 & J2 jumpers on the back of the display PCB
assembly as shown in figure 3.1. J1 allows decimal point placement and J2
determines full scale engineering unit values.
B. Determine the desired full scale readout, including decimal point position, and place
jumpers in J1 and J2 accordingly. No jumper in J2 allows LCD readings between
700 & 1999 counts. A jumper in position ‘B’ is for readings between 60 & 600
counts and ‘A’ is for readings between 20 & 50 counts. NOTE: These ‘count’
values do not consider decimal points. It is important to understand that a reading of
0-50 ppm may be obtained by reading 500 counts with a decimal point between the
0’s (50.0), or by reading only 50 counts with no decimal point. The choice depends
upon the resolution desired in the readout. J1-1 illuminates the right hand least
significant decimal point (XX.X). J1-2 illuminates the middle (X.XX) and J1-3
illuminates the left hand decimal point (.XXX).
C. Force the ST Series to provide a 4 mA output. This may be done by a number of
methods such as activating CAL MODE and utilizing the UP/DOWN ZERO or
UP/DOWN SPAN controls to provide a 4mA output when it is deactivated (the 4-
20 mA output is held at 1.5mA while activated).
D. Adjust the LCD ZERO control (R20) until the display reads the desired value for 4
mA outputs.
E. Force the ST Series to provide 20 mA output by exposing the sensor to the
appropriate level of gas or by simulating this value.
F. Adjust the LCD SPAN control (R19) until the display reads the desired value for 20
mA outputs.
IT IS IMPORTANT TO UNDERSTAND THAT R19 AND R20 HAVE NO
AFFECT UPON THE ST Series ANALOG OUTPUT. THEY ARE AVAILABLE
ONLY TO CALIBRATE THE LCD READOUT TO DISPLAY THE DESIRED
ENGINEERING UNITS.

Armstrong Monitoring Corporation
Model ST Series Non-Intrusive Transmitter
-14-
U8
MADEINUSA
ASSY10-0120
LCD ZERO
R20
LCD SPAN
R19
J1
J2A
B
P1
U9
1 2 3
U3
U7
U4
U10 U2U6U5
Q2
Q1
U1
RIBBONCABLE
TOI/OPCB
THUMBSCREW
CPUPCB(BACKSIDE)
C
P
U
P
C
B
F
R
O
N
T
P
A
N
E
L
FAULT/
KEYPADLED
LCD
SOCKETED
CPUIC
ST-46 FRONT PANEL/CPU ASSY.
FIGURE 3.1

Armstrong Monitoring Corporation
Model ST Series Non-Intrusive Transmitter
-15-
SECTION IV
MODEL ST350 & 360 SERIES
4.1 GENERAL DESCRIPTION:
Catalytic bead sensors interfaced to the ST360 Series are powered by a high efficiency
switching power supply with an adjustable output to accommodate many sensor voltage
requirements. An input bridge circuit with a balance adjustment allows matching of
various types of combustible sensors to the ST360 Series input.
TB1
RC A
CATALYTIC BEAD LEL SENSOR
(ST-46LEL)
REF ANA
FIGURE 4.1
Solid State sensors interfaced to the ST350 Series are powered by a switching power
supply with an adjustable output to accommodate many sensor voltage requirements. An
input circuit with a precision adjustment allows matching of various types of solid state
sensors to the ST350 Series input.
4.2 INITIAL START-UP AND CALIBRATION PROCEDURE:
Note: After all power and signal connections have been checked, apply power and
wait one hour for the system to stabilize. For the initial start-up procedure only,
the front panel assembly must be removed as described in section 2.1 to perform
the following adjustments.

Armstrong Monitoring Corporation
Model ST Series Non-Intrusive Transmitter
-16-
IMPORTANT: Be sure to reclassify the area non-hazardous before opening the
ST Series or any other electronic enclosures.
4.2.1 SENSOR VOLTAGE ADJUSTMENT:
The ST360 Series has been factory set to 2.0V, the ST350 is set to 2.7V. The sensor
operating voltage is applied to the Rand Aterminals of TB1 (see FIGURE 4.1). The
ST350/ST360 Series switching regulator power supply on the I/O PCB has an adjustable
output. To prevent sensor damage this voltage must be set to the correct value by
adjusting the SENSOR VOLTAGE ADJUST potentiometer, R11. Operating at sensor
voltages too low may cause incorrect readings and too high may damage the sensor.
Again, these have been set at the factory
4.2.1 MONITORING THE VOUT TEST POINT:
The VOUT test point on the I/O PCB may be monitored during the remainder of this
section to verify correct initial set-up. An alternative to this is to attach the LCD
assembly ribbon cable and monitor VOUT using the ST Series LCD. The LCD
assembly must be placed in UNITY GAIN by entering the CAL MODE and applying a
RESET to the magnetic keypad.
To use a voltmeter, the minus lead is attached to the power supply return, or 0 volt
terminal, and the plus lead to the VOUT test point. The active range of VOUT is 0.4 -
2 volts corresponding to 0-100% of full scale. Therefore, 0% = 0.4 volts, 25% = 0.8
volts, 50% = 1.2 volts, 75% = 1.6 volts and 100% = 2.0 volts. The I/O board may be
considered properly configured when VOUT is within 20% of the desired reading
without exceeding the desired value. For example, if 50% of LEL SPAN gas reads
between 1.0 and 1.2 volts, this is an acceptable value if 100% of LEL is full scale.

Armstrong Monitoring Corporation
Model ST Series Non-Intrusive Transmitter
-17-
4.2.2 BALANCE ADJUSTMENT:
Differences in element resistance from sensor to sensor necessitate a BALANCE
adjustment, which simply matches a particular sensor to the ST Series input bridge circuit.
This adjustment should only be made when a sensor is initially wired to the ST Series
I/O PCB. With the new sensor exposed to a ZERO gas (usually ambient air) adjust the
BALANCE potentiometer, R2, until the VOUT test point equals 0.4 volts. THIS IS AN
APPROXIMATE ADJUSTMENT. THE FINAL PRECISION ADJUSTMENT IS
MADE VIA THE MAGNETIC ZERO CONTROLS. The UP/DOWN ZERO
magnetic controls have a +15% of full scale adjustment range which the sensor's ZERO
drift would have to exceed before any BALANCE potentiometer adjustment becomes
necessary again.
4.2.3 ST350/ST360 SERIES INITIAL FIXED GAIN ADJUSTMENT:
The ST Series I/O PCB has 4 fixed ranges of sensitivity, which are selectable via JP1.
The JP1 positions are labeled 1, 2, 3 & 4. The VOUT test point on the I/O PCB has a
range of 0.4 -2 volts for 0 -100% of the measurement range. The BALANCE
potentiometer sets the 0.4 volt value as described in section 4.2.2 above. JP1 jumpers
set the coarse upscale SPAN values by affecting the gain of the bridge circuit. JP1 is set
correctly if a 50% of full scale gas reads between 1.0 & 1.2 volts on VOUT. Fine tuning
of these settings is done later by adjusting the ST Series magnetic controls. JP1 jumpers
are factory set and normally only require configuring if a new sensor is installed or if the
monitoring range is changed.
JP1 gain values are as follows:
JP1 with jumper in position 1 = GAIN = 51
JP1 with jumper in position 2 = GAIN = 26
JP1 with jumper in position 3= GAIN = 12.5
JP1 with jumper in position 4 = GAIN = 7
JP1 with no jumper = GAIN = 1
More than one jumper may be installed to allow additional gain values. Multiple jumpers
are additive in relation to the gain value. For example, if a gain of 20 is needed, jumpers
should be placed in positions 3 and 4 to provide a gain of 19.5.