RC Systems ST-72, Instruction Manual

Page: 66 / 142

ST-72 User's Manual
Document: UM-1007 Revision Level A
60
the one time only Initial Setup as described below, all subsequent calibrations are by the
ST-72’s electronic Cal Mode menus.
*Catalytic sensors connected directly to the ST-72 should be limited to ranges of 0-
1000ppm.
3.2.4 BRIDGE SENSOR INPUT BOARD INITIAL SETUP
Catalytic bead sensors vary widely in power requirements and sensitivity. It is therefore
important to configure each channel to match the sensor with which it will operate.
1. Prior to connecting sensors, apply power to the system. Note this board requires
24VDC power be connected to its TB3 or TB4 terminals 1 & 2 as shown in Figure
3-4. Measure the voltage between each channel’s A and R terminals and set the
Voltage Adjust potentiometers for the correct sensor excitation voltage. This
may range from 1.5 volts to 7.5 volts depending upon sensor specifications.
Sensors may be damaged by accidental over voltage conditions. It is
recommended the
Voltage Adjust potentiometer screws be covered by a
dollop of RTV or similar material after completion of this procedure to avoid
accidental over voltage conditions.
2. Remove system power and connect sensor wires to the R-C-A terminals. Reapply
system power and confirm correct voltage across each sensor’s A & R terminals.
Note: If sensor wires are long, it may be necessary to measure the excitation
voltage at the sensor end to compensate for I
2
R losses in the wiring.
3. With the minus voltmeter lead on TB3 common, connect the plus lead to the
channel’s red test point. With zero air on that sensor, adjust its
Balance
potentiometer for .4 volts at the test point.
4. Apply 50% span gas to the sensor and allow the test point voltage to stabilize.
Two volts = 100% input to the A – D Converter and .4 volts = 0%. Therefore, 1.2
volts = 50%. Place the 3 position
Coarse Gain jumper into the position which
reads between .8 volts and 1.2 volts on the test point with 50% gas on the
sensor. Gain settings for each jumper position are as follows: no jumper = 1,
LOW = 7, MED = 21, HI = 41. Multiple jumpers have an additive affect upon gain,
so the LOW and MED jumpers together provide a gain of 28.
Initial setup is now complete and normally only requires repeating if a sensor is
replaced. Final calibration of this channel may now be performed using the ST-72’s
electronic Cal Mode feature described in
section 2.4.4.4 .
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