Detcon TP-424C - Manual (Page 11)

3.8 C ALIBRATION
Note 1: Detcon solid state MOS hydrogen sulfide sensors give erroneous readings in oxygen depleted envi-
ronments. Calibration gas mixtures of H2S in air are required. (Do not use span gas cylinders with nitro-
gen and H2S mixtures.)
Material Requirements
Detcon P/N 6038 Threaded Calibration Adapter
Span Gas: 25 ppm H2S in air at a controlled flow rate between 200 and 500 cc/min.
Detcon P/N 985-241100-321 In-Line Humidifying Tube
NOTE: Span gas bottles contain 0% humidify and this ultra-low condition will cause inaccurate read-
ings when used to calibrate a sensor. To prevent this error, Detcon prescribes the use of a 24” flexible
In-Line Humidifying Tube, which adds the relative humidity to the span gas. The humidifying tube is
not necessary when using a gas generating calibration device that consists of pumped ambient air and
an H2S generating source.
NOTE: Span gas mixtures in the range 20 to 100 ppm are acceptable but 25 ppm is recommended to
provide the best signal linearity. Span gas mixtures less than 20 ppm are not recommended as these
will reduce the accuracy of signal linearity and are known to lose H2S concentration over time, espe-
cially at higher temperatures.
Caution: Calibration procedures require that the sensor junction box cover be removed and, therefore,
negate the electrical classification. De-classify the area before proceeding.
1) Declassify the area around the sensor.
2) Remove the junction box cover.
3) Measure the voltage between heater test points (see figure #5). A nominal reading of 5.25 ±0.15VDC
should be observed; minimum 4.8 VDC; maximum 5.5 VDC. See section 3.10 “Heater Voltage
Adjustment” for additional information.
4) Measure the voltage between signal test points. A zero gas reading of 39 to 40 mV DC is expected in
clean air.
5) Remove the rain shield or splashguard and attach the threaded calibration adapter.
6) At a flow rate of between 200 cc/min to 500 cc/min (200cc/min is the recommended flow), use the
span gas humidifier and apply a humidified gas standard of 25 or 50 ppm. Wait 3-5 minutes for signal
stability.
7) Using the span potentiometers (coarse and fine), adjust the signal to a reading equal to the level as
reflected in the calibration table below.
8) Remove the gas standard and calibration adapter. Observe that the signal decreases to 39-40 mvdc
within 2-3 minutes.
Calibration Table
Each 1 ppm increase in H2S gas will increase the signal by 1.6 millivolts from a baseline of 40 millivolts.
Below are a sample of gas concentrations and the correct signal output they will provide.
PPM mV DC
0 39-40
10 56
25 80
30 88
40 104
50 120
60 136
70 152
80 168
90 184
100 200
Detcon Model TP-424C Hydrogen Sulfide Sensor PG.11