Det-Tronics R8471NH34001,2,3 Single Channel Gas Controller, NH3 - Manual (Page 20)

95-8529 1.1 18
Figure 16 shows the response of a typical sensor
to repeated exposures to gas. Upon repeated
exposures to gas, the sensor quickly responds to
the same level as the previous exposure, and then
continues to produce an increase in response at a
much slower rate. Note that the shape of the sensor
response curve in Figure 16 would be very similar to
that of Figure 15 if the exposures to clean air were
eliminated.
Because of this inherent characteristic of the sensors,
it is necessary to modify the standard operating
practices as well as the calibration procedure to
ensure optimum sensor performance.
Frequency of Calibration — To ensure correct
calibration, the sensor must be free of gas for at least
30 days prior to calibration. Unlike other sensors,
over-calibration results in a longer response time.
Calibration Gas Concentration — Unlike other
sensors, the calibration gas concentration is not field
selectable. A gas concentration equal to 50% full
scale must be used for the span calibration:
5 ppm for Cl
2
sensors
50 ppm for SO
2
sensors
10 ppm for NO
2
sensors.
Calibration Time — Unlike other sensors, the length
of time that the calibration gas is applied to the sensor
is field selectable.
The length of time that the calibration gas (50% full
scale) is applied to the sensor during calibration
determines the speed of response of the sensor. A
calibration time of 5 minutes is recommended and
will produce satisfactory results in most applications.
Applying the calibration gas for a shorter period
of time shortens the response time. Applying the
calibration gas for a longer period of time typically
results in a slower speed of response.
Alarm Condition — If an alarm condition occurs, the
sensor must not be calibrated immediately after the
alarm. The sensor must be free of gas for 30 days
before a proper calibration can be performed. The
operator must choose one of the following options:
• Operate the sensor for 30 days and then calibrate.
• Test the sensor to verify correct operation (without
calibrating) by applying gas to the sensor and
checking for the proper controller response.
• Replace the sensor cell with one that has been free of
gas for at least 30 days, then calibrate.
Failed Calibration Attempt (Assuming sensor is
not defective) — If the calibration is not performed
successfully on the first attempt, do not attempt a
second calibration. A successful calibration can be
performed
only if the sensor has not seen gas for
at least 30 days. As an alternative, the sensor can
be removed from service and replaced by a sensor
that has not seen gas for at least 30 days. Perform
a normal calibration on the replacement sensor. The
original sensor can be returned to service after 30
days.
Summary
The following guidelines are important and must
be adhered to carefully to ensure that the sensor
operates properly and that safety is not compromised.
• Electrochemical Cl
2
, SO
2
and NO
2
sensors should
be used in leak detection applications, i.e. “on/off” or
“gas present/not present.” Use of these sensors in
applications that require a high degree of accuracy or
stability is not recommended.
• The sensor must be free of gas for 30 days before a
proper calibration can be performed.
• A 50% full scale gas concentration must be used for
calibration.
• Sensor response time is determined by the length
of time that gas is applied to the sensor. (A span
calibration time of 5 minutes is recommended.)
• If a calibration is not successful, do not attempt to
repeat the calibration. Either wait 30 days before
calibrating, or replace the sensor cell and then
calibrate.
• Do not calibrate after an alarm condition. The sensor
must be free of gas for 30 days to ensure a proper
calibration.
5 Min. 5 Min. 5 Min. 5 Min. 5 Min.
Figure 16—Sensor Response to Repeated Exposures to Gas