ENMET PGD2 - Manual (Page 25)

ENMET Corporation PGD2
22
12 S ENSORS
12.1 Flammable Sensors
The PGD2 instrument can be fitted with sensors for the detection of various types of flammable gases. The sensors used are of the
catalytic oxidation type generally referred to as “pellistors”. Sensors of this type consist of a matched pair of elements known as the
detector and compensator.
Either one or two pairs of pellistors may be fitted within the instrument depending upon whether it is required to measure only up to the
Lower Explosive Limit of the gas (one pair fitted) or up to 100% volume of the gas (two pairs fitted). The warm up time for either type is
less than 15 seconds, this takes place when the instrument is switched on. No readings are displayed during this period.
The flammable sensors are non-selective that is, they will detect a range of hydrocarbon gases. However, during manufacture of the
PGD2 instrument the sensors are calibrated to methane gas and the user may then select other gases from the menu selection as
described in Section 6.11. The instrument automatically applies the appropriate calibration factor according to the gas selected so that
the correct reading is given on the display.
12.1.1 LEL Flammable Sensors
Where only one set of pellistors is fitted, to enable measurement up to the Lower Explosive Limit of the flammable gases
(approximately 5% volume), the pellistor detector and compensator are encapsulated into individual stainless steel housings behind
individual stainless steel sintered flame arrestors. The stainless steel housings are contained within in a blue coloured plastic
moulding which also houses the oxygen sensor (if fitted).
Where fitted, the flammable sensor and oxygen sensors are located behind the instruments’ right hand side gas inlet as viewed from
the front.
There are two types of LEL sensors available:-
i). Type SS10 – for general hydrocarbons e.g. Methane, Propane etc. and Hydrogen.
ii). Type VQ41 – for detection of fuels e.g. diesel, aviation fuels etc.
LEL Flammable Sensor Specification
Gases detected
- Various combustible gases and vapors
Range
- 0 - 100% LEL
Resolution
- 1% LEL
Operating Voltage
- 2 ± 0.1 Volts dc
Operating Current
- 180 mA
T
90
Response Time - < 10 Seconds
Linearity
- Linear 0-100% LEL Methane
Long Term Sensitivity
Drift
- < 5% signal / month
Long Term Zero Drift
- < 2% LEL methane / month