User guide

Application Guide | Gas detection in refrigeration systems
© Danfoss | DCS (MWA) | 2018.05 DKRCI.PA.S00.A1.02 | 520H12772 5
EC - Electrochemical sensor
(continued)
Time
Tolerance rang
e
Sensitivity
max.
min.
Max. operating time
before calibration
Important!
Sensor must be calibrated or new
sensor must be installed.
If the sensitivity of the sensor falls
below 30%; install new sensor
“Substantial”
gas leak
30% sensitivity
Gas 1 Gas 2 Gas 3
Target Gas
Gas 4 Gas 5
Gas specification
Sensitivity
“Broad” sensitivity
spectrum
– Semiconductor
– Pellistor
“Narrow” sensitivity
spectrum
– Electrochemical
– Infrared
SC - Semiconductor sensor
(solid state)
The semi-conductor sensor functions by
measuring the resistance change (proportional to
the concentration), as gas is absorbed on to the
surface of a semi-conductor, which is normally
made from metal oxides.
These can be used for a wide range of gases
including combustible, toxic and refrigerant
gases.
It is claimed that they perform better than the
catalytic type in the detection of combustible
gases at low concentrations, up to 1.000 ppm.
These are low-cost, long life, sensitive and can be
used to detect a large range of gases including
all the HCFC, HFC refrigerants, ammonia and
hydrocarbons.
However, they are not selective, and are not
suitable for detecting a single gas in a mixture, or
for use where high concentrations of interfering
gases are likely to be present (fig. 3).
Interference from short term sources (e.g.
exhaust gas from a truck), creating false alarms,
can be overcome by enabling a delay of the
alarm.
Semi-conductors for halocarbons can be used
to detect more than one gas or a mixture
simultaneously. This is particularly useful in
monitoring a plant room with several different
refrigerants.
Fig. 2: Large ammonia exposure shortens the lifetime of electrochemical sensors.
Fig. 3: Sensitivity spectrum of various sensor technologies