Detcon FP-424C - Manual (Page 12)

ment and will, in most cases, not require significant adjustment on start up. However, Detcon recommends
that a complete calibration test and adjustment be performed within 24 hours of installation. Refer to cali-
bration instructions in later text.
7.0 C ALIBRATION P ROCEDURE
7.1 Material Requirements
*
Digital volt meter
* Jewelers type screwdriver
* Detcon PN 943-000006-132 Threaded Calibration Adapter
* Span Gas 50% LEL methane in air at a controlled flow rate between 200 ml/min.
7.2 Calibration
1) Declassify the area around the sensor.
2) Remove the junction box cover.
3) Measure the voltage between the bridge test points: The bridge voltage is factory set at 2.7V. Use the
temp potentiometer to make adjustments if necessary.
4) Measure the voltage between the signal test points: zero gas conditions should provide a reading of
between 39 and 40 mV DC. If necessary, adjust the zero potentiometer to achieve this reading.
5) If applicable, remove the rain shield or splash guard from the sensor housing.
6) Attach the calibration adapter to the threaded sensor housing. Apply the test gas at a controlled flow rate of
200 ml/m. Allow 1-2 minutes for signal stability.
7) Measure the voltage between the signal test points. Use the coarse potentiometer to adjust the signal to
a reading of 120 mV DC. Use the fine potentiometer to make finer adjustments as needed.
8) Remove the test gas and observe that the signal decreases to between 39 and 40 mV DC .
Note: Because there is interaction between the span and zero functions, it may be necessary to repeat steps 4 through 8.
Calibration is complete. Replace the rain shield or splash guard and junction box cover.
7.3 Calibration Notes
Detcon Model FP-424C series sensors provide a signal output of 4-20 mA DC which corresponds to 0-100%
LEL. This signal is reflected by a 40-200 mV DC voltage across the signal test points. Therefore the signal
output will move by .16 mA (or 1.6 mV across the signal test points) for each 1% of movement.
If calibration is being done with a gas concentration other than 50% LEL, you can determine the proper sig-
nal voltage by dividing 160 mV by 100 and then multiplying that figure by the concentration of gas. For
example, if you are calibrating a sensor with a gas standard that contains 10% LEL, you would divide 160
(mV) by 100 (%) and then multiply that figure by 10 (%). Then add the base line of 40 (mV) to arrive at the
proper signal voltage of 56 mV. Refer to the chart below for some sample signal outputs with different gas
concentrations applied.
% LEL Signal Reading
0 40 mV
10 56 mV
25 80 mV
50 120 mV
75 160 mV
100 200 mV
7.4 Calibration Frequency
In most applications, monthly to quarterly calibration intervals will assure reliable detection. However,
industrial environments differ. Upon initial installation and commissioning, close frequency tests should
be performed, weekly to monthly. Records should be kept. Less frequent test schedules should be imple-
mented based on analysis of tests prior to adjustment.
Detcon Model FP-424C Combustible Gas Sensor PG.12