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The sensor screws into one hub of the enclosure and plugs into the bottom electronics card via a six pin connector. Cabling to the controller connects to a three pin spring loaded terminal strip. 7.5.2 TROUBLE ANALYSIS Electrical adjustment, or replacement of the sensor will be necessary under the following conditions: • Controller displays the following error messages: CHK BRIDGE VOLT SENSOR FAILURE LOW SENSITIVITY • False readings or alarms are received due to sensor inaccuracy. Warning: : During sensor adjustments the concentration reading on the controller will be inaccurate and alarm level concentrations may be displayed. If false activation of the alarm relays will cause a problem disconnect the relay wiring Comparing to the "GND" (ground) test point measure prior to adjustment or turn the module off using the "BRIDGE VOL" (bridge voltage) and adjust to 2.00 VDC CHANGE MODULE mode. using "BRIDGE ADJ" potentiometer. Turn counter- NOTE clockwise to increase. Although all the necessary data can be collected with a voltmeter at the sensor module, some helpful information can be displayed or printed. See diagnostic codes 0004 7.5.3.2 ZERO ADJUSTMENT and 0008 in Appendix C. Comparing to ground measure SIGNAL OUT and adjust to 0.18 VDC by turning ZERO ADJ potentiometer. Turn 7.5.3 ADJUSTMENT PROCEDURE counter clock-wise to increase. Prior to reading voltages and making adjustments perform a visual inspection to confirm that there are no physical 7.5.3.3 SPAN ADJUSTMENT problems such as water in the electronics enclosure, wiring damage or corrosion. 1. Determine the concentration of the calibration gas (CG) in % LEL. Use Figure 7-6 to locate test points during the following procedures: 2. Use the following formula to determine the response to the calibration gas at SIGNAL OUT. Caution: The area must be free of flammable vapors or gases during any adjustments or maintenance procedures. Voltage (Signal Out) = (CG+12)/60 For Example: 7.5.3.1 BRIDGE VOLTAGE ADJUSTMENT If CG = 50% L.E.L Confirm that no combustible gas is present and remove Signal Out = (50+12)/60 sensor module cover. = 1.03 volts SERVICE Page: 69 SMC sierra monitor corporation Sentry Instruction Manual - Version 6 3. Using a Sierra Monitor Model 1200-26 Gas Calibrator with a Sierra Monitor Model 535800 calibration adapter, or Model 5360 Gas Delivery Fitting. Apply the calibration gas at a flow rate of 100 cc/min. until the signal out voltage stops changing (approx. 2 minutes). Then adjust span adj. potentiometer to the calculated value. 4. Remove the calibration gas and wait for the voltage to drop. If the voltage is below 0.30 VDC the procedure is complete. If the voltage is above 0.30 VDC repeat the zero and span adjustments. 5. Calibrate the sensor using the instruction manual procedure. • A LOW SENSITIVITY message during calibration indicates one of the two conditions for a combustible sensor: • The zero gas voltage must be less than .3 volts. • The new span change (span voltage less zero voltage) must exceed 30% of the calculated span change. For Example: Zero gas voltage = .20 VDC Span voltage = .43 VDC Span change is .43-.2 = .23 VDC As the calculated span value (from step 5 above) is 1.03, the calculated span change is: 1.03-.2 = .87 By calculation: 30% of .87 = .26 The new span change (.23) is less than 30% of the calculated span change so a LOW SENSITIVITY message will be displayed. The LOW SENSITIVITY calculation is made to insure that the calibration does not adjust the electronics so that the "gain" is large enough to cause false alarms due to minor drift or due to minimal electronic noise. 7.5.4 SENSOR REPLACEMENT If any of the above adjustments cannot be completed successfully the sensor and/or electronics will require replacement. To check the sensor unplug it from the electronics and remove from the housing so that continuity and resistance tests can be made. Table 7-4 provides the pin out for the sensor assembly. PIN FUNCTION WIRE COLOR 1 DETECTOR WHITE 2 COMPENSATOR BROWN 3 CALIB. SW. RED 4 DETECTOR BLACK 5 DRAIN BARE 6 CALIB. SW. GREEN Table 7-5 Combustible Gas Sensor Wiring Pin Out 1. Detector element resistance should be between 1.5 to 2 ohms. 2. Compensator element resistance should be between 1.5 to 2 ohms. 3. Calibration switch should be open in normal operation and should close when the calibration magnet is applied. If the sensor fails any of these tests it should be replaced. If the sensor passes these tests the electronics are suspect and BOTH the sensor and electronics should be replaced. (The electronics and sensor which are removed should be returned to the factory for evaluation). When a new sensor is installed the following actions must be taken: • Make the bridge voltage adjustment immediately to avoid over-voltage damage to the sensor. • Use diagnostic code 0020 to establish nominal value calibration. • Allow 24 hours for full stabilization of the sensor, recheck the zero and span adjustments and calibrate the s...