7
SD2-NH3
Operation
Start-Up
Sensor can be response tested and/or span calibrated immediately after
power up. Allow 60 seconds for power up time delay of sensor to end (green
power LED will flash during power up).
Once the unit is powered up, the SD2-NH3 starts the 10-minute sampling
rate, and the sample-draw cycle is as follows:
• 2-minute sample tube drying cycle.
•
2-minute sample time from the room being monitored. At the end of the
two minutes, the sample data is stored and held for the next 10 minutes
also while being transmitted via 4-20 mA output to the controller or PLC.
•
2-minute fresh air purge from outside fresh air source.
•
4-minute pump shut-off phase.
LED Functions
(see Figure 3 on page 8)
Calibration
The SD2-NH3 sensor comes factory calibrated and should require only
minimal adjustment after installation. There are two pots on the preamp
that are used for calibration (see Figure 3).
Note:
Never measure sensor output in mA. Always use mVDC or VDC
voltmeter settings.
Bottom back board calibration mode:
Since the sample data is stored and held for 10 minutes during the sample-
draw cycle, the real-time 4-20mA signal is only accessible on the upper
testpoints. Follow this procedure to output the real-time 4-20mA signal on
the lower testpoints.
1.
Locate zero adjustment pot on sensor backboard. It can be found on
the left side below the pump and status LEDs. (see Figure 3)
2.
Turn pot 1/4 turn clockwise and back again. Successful entry into
calibration mode will be indicated by the Power (green) LED blinking
twice per second and the real-time signal is now active and can be
viewed on connected monitoring equipment or voltmeter connected to
the lower testpoints. Real-time mode will time out after 4 minutes.
Zero Calibration:
After the sensor is installed and has been powered up for
a minimum of 8 hours, the unit can be zero calibrated by the following:
Note:
Place bottom back board in calibration mode and connect voltmeter
to lower testpoints (see Figure 3).
1.
Remove the ammonia cell from the aluminum housing. Wiggle cell back
and forth while pulling upwards (see Figure 3).
2. Be sure the unit is in clean air.
3. Adjust the zero pot on the bottom back board until the sensor outputs
40 mV from Test [-] to Test [+] (see Figure 3).
Note:
To zero the sensor
immediately after power up or cell replacement, unplug cell from the
transmitter and adjust signal to 40 mV.
Span Calibration
: DO NOT ADJUST THE SPAN POT WITHOUT CERTIFIED
CALIBRATION GAS! If span adjustment is required, use this procedure:
Note:
Place bottom back board in calibration mode and connect voltmeter
to lower testpoints (see Figure 3).
1.
Remove the ammonia cell from the aluminum housing. Wiggle cell back
and forth while pulling upwards (see Figure 3).
2. Apply span gas at 0.5 to 0.8 L/min (span gas must be in air, not nitrogen
or other carrier).
3. Sensor should react to gas within 15 seconds.
4. Once the output signal has peaked (or 2 minutes maximum) adjust the
Span pot on the top sensor board until the correct output is achieved
(see Figure 3). With full-scale span gas, the calculated span value is 200
mV. ((span gas / sensor range * 16 + 4) (
mA output)
)
5. Reinsert ammonia cell back into its aluminum housing. Press
downwards firmly until it snaps into place.
Note: Below are a few response characteristics which may be an indication
that the gas sensor is at or near the end of its useful life. If any of these
characteristics are observed, the cell should be replaced.
•
Slow response to / recovery from calibration gas.
•
Failure of the output to reach 50% of the calibration gas value prior to
span adjustment.
• Unable to achieve correct output during span.
