User guide

Application Guide | Gas detection in refrigeration systems
6 DKRCI.PA.S00.A1.02 | 520H12772 © Danfoss | DCS (MWA) | 2018.05
P- Pellistor sensor Pellistors (sometimes called a bead or catalytic)
are mainly used for combustible gases including
ammonia, and are the most popular sensors for
this application at high detection levels.
The sensor functions by burning the gas at
the surface of the bead and measuring the
resultant resistance change in the bead (which is
proportional to concentration).
These are relatively low-cost, well established
and understood, and they have a good life span
(expected life time 3 to 5 years). The response
time is usually below 10 seconds.
They can be subject to poisoning in certain
applications.
Poisoning is the reduction of the reaction of the
sensor to the target gas due to the presence
(contamination) of another substance on the
surface of the catalyst, that either reacts with it or
forms a layer on top of it reducing its capacity to
react to the target gas. Most common poisoning
substances are silicon compounds.
Pellistors are used mainly with combustible gases
and are therefore suited for ammonia and the
hydrocarbon refrigerants at high concentrations.
They do sense all combustible gases, but they
respond at different rates to each, and so they
can be calibrated for particular gases.
There are ammonia specific versions.
IR - Infrared Infrared technology utilises the fact that most
gases have a characteristic absorption band in
the infrared region of the spectrum, and this
can be used to detect them. Comparison with a
reference beam allows the concentration to be
determined.
Even though they are relatively expensive in
comparison to other sensor, they have long life
time of up to 15 years, high accuracy, and low
cross sensitivity
Due to its measuring principle infrared sensors
can be subject to issues in dusty environments,
where the presence of too many particles in the
air may disturb the reading.
They are recommended and commonly used for
Carbon dioxide detection. Although technology
exists for other gases also, it is not common to
find it in commercial solutions.
Semi-conducter Electro-chemical
Pellistor
(Catalytic)
Infrared
Ammonia “low” concentration
(< 100 ppm)
–
4
– –
Ammonia “medium” concentration
(< 1000 ppm)
1
)
(4) 4
– –
Ammonia “high” concentration
(<10000 ppm)
4 (4)
2
(4)
–
Ammonia “very high” concentration
(> 10000 ppm)
– –
4
–
Carbon Dioxide
CO₂
– – –
4
HC
Hydrocarbons
– –
4
–
HCFC - HFC
Halocarbons
4
– – –
Best solution
Suitable - but less attractive Not suitable
Which sensor is suitable to a
given refrigerant?
1
) Measuring range 0-1000 ppm. Can be adjusted in the whole range.
2
) Up to 5000 ppm. For specific applications.
Suitability of different sensor technologies:
Based on the target refrigerant gas and the
actual ppm range the below table provides an
overview of the suitability of the various sensor
technologies offered by Danfoss.