ENMET PGD2 - Manual (Page 29)

ENMET Corporation PGD2
26
12.3 Oxygen Sensors
WARNING: The instrument is not designed for use in oxygen enriched atmospheres i.e. greater than 21% oxygen.
The PGD2 instrument can be fitted with either of the oxygen sensors detailed in the following section 12.3.5 to allow measurement of
oxygen deficiency.
With the exception of instruments fitted with 0-100% volume flammable sensors, the oxygen sensor is contained within a blue colored
plastic molding normally located behind the instruments’ right hand side gas inlet as viewed from the front. On instruments fitted with
0-100% volume flammable sensors, the oxygen sensor (type C/2 only) is mounted behind the black colored flammable sensor
housing.
12.3.1 Cross Sensitivity
In general, toxic gases at the levels of concentration experienced in normal use have no cross sensitivity effect on the oxygen sensor.
However, acid gases such as CO
2
and SO
2
give a small enhancement (e.g. about 0.3% of signal per 1% CO
2
) to the oxygen reading.
12.3.2 Temperature
The oxygen sensors themselves have a characteristic that varies with temperature. However, this is compensated in the PGD2
instrument software and is therefore insignificant to the user.
12.3.3 Pressure Transients
Oxygen sensors are sensitive to sudden changes in pressure as may be experienced when travelling in a lift shaft or through doors
underground where there is a significant ventilation pressure differential between the two sides of the door(s). Under such
circumstances the instrument may react to the pressure transient, possibly resulting in a temporary alarm condition being given. This
transient will normally fade away after approximately 10 seconds, whereupon the alarm can be reset.
12.3.4 Humidity
Because the humidity in the air affects the volume % concentration of oxygen this will be reflected in the oxygen sensor reading. This
also significantly varies according to temperature as shown by the following tables of approximate figures: -
At 0 degrees C
Relative
Humidity % Volume of
Oxygen in Air
0% 20.94
50% 20.9
100% 20.84
At 20 degrees C
Relative
Humidity % Volume of
Oxygen in Air
0% 20.94
50% 20.7
100% 20.45
At 40 degrees C
Relative
Humidity % Volume of
Oxygen in Air
0% 20.94
50% 20.15
100% 19.42
Therefore if an instrument is calibrated in a laboratory at 20
°
C with ‘dry’ compressed air, the reading will fall when the instrument is
placed into use in an atmosphere having humidity. This can be ‘compensated’ by use of the Sensor Zero function (where enabled) as
described in section 6.17 to give a 20.9% reading if required.
12.3.5 Oxygen Sensor Types and Characteristics
Type No’s 2C / 2 4OX-2
Nominal Range 0-25% Oxygen 0-25% Oxygen
Expected Operating Life Two Years in Air Two Years in Air
Response Time <10 secs <15 secs
Temperature Range -20 to +50 degrees C -20 to +50 degrees C
Pressure Range Atmospheric ± 10% Atmospheric ± 10%
Operating Humidity 0 to 99% non condensing 0 to 99% non condensing
Storage Life 6 months in container provided 6 months in container provided
Storage Temperature 0-20 degrees C 0-20 degrees C
(The above figures are based on conditions at 20 degrees C, 50% RH and 1013 mBar).