TQ Environmental Ltd.
5812M16
Issue 16: 17th Feb 2022 Page 5 of 22
SECTION 1 - DESCRIPTION
1.1 GENERAL DESCRIPTION
The TQ122 series is a range of highly reliable gas sensors designed to give extended
service under extreme conditions. The sensor is specifically manufactured and calibrated
to detect the presence of the gas indicated on the body of the sensor. The sensors are
either 24v dc operation in 4-20mA current sink mode or form part of a “Wheatstone Bridge”.
With either type, the Control Unit must be capable of supplying the operating power,
handling the signal generated by the sensor, calculating and displaying the gas/vapour
concentration as well as generating any alarms required by the application.
The TQ122 Sensors are used for diffusion sensing applications. These are applications in
which the sensor continuously monitors that part of the environment with which it is in
contact, with the gas diffusing into the sensor’s active area. This technical manual covers
the use of the TQ122 Sensor as a diffusion type sensor. A suitable device may be connected
to the sensor to allow its use in flow through applications.
1.2 METHOD OF DETECTION.
The gas being monitored diffuses through the stainless steel membrane on the head of the
unit. This then passes into a cell where the gas of interest reacts chemically with a sensing
element housed within a secure unit inside the device. The chemical reaction generates a
voltage proportional to the concentration of the gas being monitored. In sensors providing a
current output an electronic circuit within the device provides a current output within the
range 4 to 20 mA. The current is proportional to the concentration of gas being measured.
In flammable gas sensors the gas is burned catalytically in the presence of oxygen
(pellistors) generating an “out of balance” signal from a Wheatstone Bridge arrangement.
The devices have a lifetime limited by a number of factors depending upon the gas being
monitored and the type of sensing element. Many of the chemical reactions taking place
within the cell will require the presence of oxygen in the gas being monitored.
1.3 SENSOR UNIT DESCRIPTION.
The fully assembled TQ122 Sensor unit (Figure 1) has the approximate dimensions of 70
mm long (including the M20 thread used to fasten the sensor to the junction box) x 50mm
diameter. The M20 thread is approximately 15mm long. A nylon washer and M20 nut are
provided for attaching to the junction box.
Two or three wires, approximately 190mm long, protrude from the potting on the unit to
allow connection to other cabling in the junction box.
A flame arrestor / diffusion device is provided on the head of the unit to allow the gas for
measurement to enter the device.
