Det-Tronics 505 Combustible Gas Detector Transmitter with Combustible Gas Sensor CGS - Manual (Page 13)

10 95-8472 3.1
5. Calibrate the Model 505 Transmitter using the
Calibration Procedure described below.
CAUTION
Calibration of the Model 505 Transmitter requires
removal of the junction box cover with power
applied. Therefore, the hazardous area must be
de-classified.
6. After calibration is completed, restore the system
output loads to ready condition.
CALIBRATION
The transmitter must be calibrated upon startup and
periodically thereafter to ensure proper performance
and gas measurement accuracy. Various factors affect
the time interval between periodic calibrations (typically
30, 60, or 90 day intervals, depending on the ambient
conditions). Since each application is different, the
length of time between regularly scheduled calibrations
can vary from one installation to the next. In general,
the more frequently a system is checked, the greater the
reliability. The transmitter must be calibrated:
– Before a new system is initially put into service
– If the sensor is replaced
– If the Model 505 transmitter circuit board is
replaced.
For maximum calibration accuracy, allow a new sensor to
operate under power for several hours to ensure a stable
output before performing calibration. For the highest
degree of accuracy, perform a second calibration after
24 hours.
For a new system startup, or when gas hazard conditions
change within an existing protected area, calibration
should be checked frequently, approximately twice in
the first month. The calibration data can then be used
to determine the optimum interval between periodic
calibrations.
Before performing calibration, the operator should ensure
that the sensor opening is pointing down, and examine
the sintered metal cover of the sensor to be sure that
it is not missing or damaged. If the cover is defective
or missing, the sensor must not be operated, since the
exposed sensing element can act as an ignition source.
A dirty sintered sensor cover can significantly reduce the
sensitivity of the sensor. Dust covers, splash guards, or
other sensor accessories may be desirable if ambient
conditions result in sensor inlet fouling or blockage.
CALIBRATION PROCEDURE
See Table 2. Refer to Figure 8 to locate the
potentiometers and test points on the transmitter circuit
board. Always calibrate at startup, and after replacing
either the sensor or transmitter circuit board.
A digital voltmeter with probes, a screwdriver for adjusting
potentiometers, and calibration gas are required for
performing a standard calibration.
It is important to maintain a log book for all system
calibrations performed, and to record the zero, span,
adjustment, and sensor sensitivity data before and after
each calibration is performed.
NOTE
If sensor response to 50% LFL methane is less
than 15 millivolts, the sensor should be replaced.
Refer to Table 2, “Calibration Procedure” for
information regarding calibration and sensor
sensitivity.
CALIBRATION USING K-FACTORS
The Span (gain) control adjusts the Model 505 signal
output (% LFL reading) to match the LFL percentage of
the applied calibration gas. The span setting will vary
for different combustible gases. Therefore, calibration
should always be performed using the same gas as
the one expected to be detected. If the gas or vapor
to be detected is not available from Det-Tronics in a
compressed calibration gas kit, then a “K-factor”
must be used to ensure proper sensitivity to the gas
hazard. Refer to Det-Tronics Technical Note 76-1017
for information and guidance in using K-factors.
To calibrate the Model 505 in an application that requires
the use of K-factors:
1. Determine the calibration span output level using the
formula:
C x K = S
C — Concentration of calibration gas in % LFL
K — K-factor from Table 1 in 76-1017
S — span output level
2. Multiply the span output level (S) by 0.0067, then
add 0.17 to obtain the new calibration span output
level (S1).
(S x 0.0067) + 0.17 = S1
3. Calibrate the Model 505 using the new calibration
span output level (S1) when adjusting the span
potentiometer.
Example:
Calibration gas being used — 50% LFL Methane
Gas to be detected — Acetone