ENMET EN Series - Manual (Page 6)

Sensor/Transmitter, Toxic Gas & Oxygen ENMET Corporation
4
3.0 Initial Calibration
The S/T is pre-calibrated at the factory before shipment, and initial field calibration should result in only fine-tuning of the
circuit. In all cases allow the S/Ts to stabilize; this may take as much as an hour depending on the type of sensor and the length
of time it has been inactive. The S/T output is monitored with a voltmeter by means of the test points on the S/T circuit board
see figure 2; the transfer function is:
4mA = 100 mv 20ma = 500 mv
tolerance: + or - 5 mv
On Toxic sensor/transmitters the LED also indicates current flow; it is dim at 4mA and bright at 20mA. When the sensor is
stable and is in air uncontaminated by the target gas, adjust zero to
4mA = 100mv with the zero potentiometer, see Figure 2.
If a calibration kit was purchased and field calibration is desired, apply the span gas to the sensor. The regulator of the
calibration adapter screws into the top of the gas cylinder; the cup of the adapter slips into the hub of the sensor/transmitter, see
Figure 3. Allow the gas to flow for the time indicated in Table 1 in Section 4.0 and observe that the reading is stable. Adjust
the gain potentiometer on the S/T board to the appropriate output while monitoring the voltmeter. The voltage readings for
standard ranges are given in Table 1 in Section 4.0.
If the sensor/transmitter has a non-standard range, and/or a non-standard calibration gas is used do a linear interpolation to
determine the voltage reading at the calibration point.
Example: toxic, if the maximum range is 100ppm = 20ma, and a 25ppm gas is used, adjust the gain potentiometer so that the
display reads 25 or the voltmeter reads 200mv.
(25/100)(500-100) + 100 = 200mv
Example: oxygen, the maximum range is 30.0% = 20mA, and a 20.9 gas is used, adjust the gain potentiometer so the display
reads 20.9 or the voltmeter reads 379mv.
(20.9/30)(500-100) + 100 = 379mv
Remove the span gas and replace the S/T cover.
Figure 3: Calibration Setup, 4 – 20mA Sensor/Transmitter
CALIBRATION
ADAPTER
REGULATOR
GAS CYLNDER
CALIBRATION
ADAPTER
TB1
Red LED
Circuit Board
POTs: Gain and Zero
• Note: No Zero pot in oxygen S/T
Test Points
Red LED
TB1
Test Points
POT’s: Gain Zero
• Note: No Zero pot in Oxygen S/T