Instruction and operating guideline CO-detector
Type:
VCD2 CO-Detector
8
Span Gas not within ±5% of Full Range
Note : A current source is required for this procedure.
1.
Apply a gas of known concentration to the sensor for 2 to 5 minutes.
2.
Measure the current from the sensor. This can then be used to calculate
the sensitivity (in
μΑ/
ppm) for the specific sensor being calibrated (see
step 3).
3.
Calculate the expected current when the sensor is exposed to the full
scale of target gas.
Example
Required 4-20 mA range is 0-500 ppm
Available span gas is 300 ppm
Current from sensor when exposed to span gas = 33
μΑ (
from step 2)
Therefore, sensitivity = (33
μΑ
/ 300 ppm) = 0.11
μΑ/
ppm
Expected signal at 500 ppm = (0.11
μΑ
/ppm x 500 ppm) = 55
μΑ
4.
Connect the current loop power to the transmitter
5.
Connect the current source to the transmitter.
or Oxidising Sensors (CO, H
2
S, SO
2
, NO ...) Connect the negative to 'SEN'
and the positive to 'CNT'. Connect 'CNT' and 'REF' together. For Reducing
Sensors (CL
2
, NO
2
...). Connect the negative to 'CNT' and the positive to
'SEN'. Connect 'CNT' and 'REF' together.
6.
Set the current source to zero.
7.
Hold down the program button until the LED flashes at approx. 1 Hz (4 mA
level now set in RAM).
8.
Set the current source to the value calculated for the full scale of target
gas (from step 3).
9.
Press and release the program button (20 mA level now set in RAM).
10.
The LED will flash at approximately 8 Hz for 8 Seconds. This is the timeout
period.
11.
Calibration is complete. The LED will clear and the transmitter will return
to normal mode.
Important Notes
Recalibration is only possible if the output of the sensor at full scale is greater
than 50% of the original factory calibration. Failure can occur if:
1. Attempt is made to recalibrate to a range less than 50% of the original
calibrated range.
2. The output of the sensor has fallen by more than 50%. In this case, the
sensor must be replaced.
3. Incorrect span gas is used.
4. Insufficient time is allowed for the output to settle after exposing the sensor
to span gas. Span gas should be applied for 2-5 minutes before setting the 40
mA level.
