SMC sierra monitor corporation Sentry Instruction Manual - Version 6
SERVICE
Page: 79
The SENSOR FAILURE me ssage for toxic gas
sensors does not indicate a specific failure of the
sensor but indicates that the sensor is not correctly
connected to the electronics. If this message
appears check that the sensor harness is correctly
installed to the connector on the bottom
electronics board.
The gas sensor which is located inside the sensor
assembly housing can be replaced without
replacement of the housing. To replace the
sensor:
1. Confirm that system power has been
removed.
2. Remove the transmitter electronics board
from the main housing and unplug the
sensor harness from the transmitter
electronics.
3. Unscrew the sensor housing from the
bottom of the enclosure
4. Hold the sensor assembly so that the
harness faces down and the sensor faces
up. Unscrew and remove the round
section of the housing from the hex
section. Be careful not to lose the spacer
washer which will be sitting on top of the
exposed sensor.
5. Carefully pull the old sensor straight up from the
socket.
6. Orient the new sensor so that the sensor pin
labeled “C” faces the socket labeled “C” which is
on the far side of the board from the vertical reed
switch. The reed switch will slide into a hole on
the side of the new sensor. Press the new sensor’s
pins into the three sockets.
7. Carefully replace the cover on the sensor
assembly including the spacer washer.
8. Install the sensor assembly into the enclosure and
tighten firmly.
9. Reconnect the sensor harness to the transmitter,
install the transmitter into the housing and restore
power.
10. Allow one hour for the sensor to stabilize prior to
recalibration.
11. Recalibrate the sensor module.
7.9.3.3 SENSOR OUTPUT ADJUSTMENT
The following procedure is to be used when, during
normal calibration of a new sensor, the span voltage
cannot be adjusted to a high enough level.
1. Adjust the span potentiometer so that it is
approximately mid range. (Twenty turns
clockwise and tens turns counter-clockwise).
2. Remove system power and remove both the
transmitter and the sensor assembly from the
enclosure. Remove the sensor assembly cover so
that the sensor is visible.
3. Plug the sensor harness into the transmitter
assembly and check that no components are
touching the enclosure. Restore power and allow
a minimum of 30 minutes for stabilization before
adjustment.
4. Connect a DVM across TP10 and GT1 on the
transmitter assembly. Figure 7-12
5. Locate the gain potentiometer which is on the
sensor electronics directly behind one of the holes
in the sensor body. The potentiometer is
accessible by inserting a jewelers screwdriver
through the hole in the sensor.
6. Determine the correct value to be read at TP10
based on the following formula:
VTP10 = 2(C/R) , where C = concentration of span gas,
and R = range of detection (100 PPM). The value
is negative for all models except 5100-06, and
5100-19.
Figure 7-12
Toxic Gas Transmitter Component Locator