Methods of measurement
The most common measuring systems for gas detection are Semiconductive, Electrochemical, Catalytic and Infra-red absorption. The systems have different advantages and disadvantages and vary a great deal in costs.
Advantages and disadvantages:
| Measuring system |
Advantages |
Disadvantages |
Ammonia
(NH3) |
HFC
(HCFC, CFC)
and blends |
Flammable
(HC) |
Carbon dioxid
(CO2) |
|
Semi-conductive/
chemosorption
|
- long service life and
long-term stability
- low price and low
maintenance cost
- suitable down to -30oC
|
- limited selectivity
- Limited measuring range
|
Yes |
Yes |
Yes |
|
|
Electrochemical
|
- medium price
- high selectivity
|
- short service life (normally 1-3 years)
- Needs recalibration after exposure to high concentrations
- limited measuring
range
- high maintenance
cost
- not suitable at sub-zero temperatures,
unless heated |
Yes |
|
Yes |
|
|
Catalytic
|
- medium price
- high selectivity
- can accurately register
and record high concentrations
|
- not suitable for low
concentrations
- needs recalibration after exposure to high concentrations
- not suitable if there are large fluctuations
in temperature and/or
humidity |
Yes |
|
Yes |
|
|
Infra-red absorption
|
- high selectivity
- long service life
- wide measuring range
- great accuracy
|
- high price and high
maintenance cost
- large size
- not suitable at sub-zero temperatures,
unless heated
|
Yes |
Yes |
Yes |
Yes |
|
Charge carrier
injection
|
- medium price
- suitable for use in harsh
conditions
- high selectivity
- wide measuring range
|
- short service life (approx. 2 years)
|
Yes |
|
|
|
"False Alarms"
False alarms occur rarely. If a detector is alarming it often means that something is effecting the detector e.g.:
-A low concentration of gas in the room
-Incorrect calibration of the equipment
-The equipment has not been inspected and maintained
-Other gases in the room e.g.
· solvent/detergent
· exhaust gases
· smoke (cigarette smoke)
-Other, like e.g. compressor oil after drawing during maintenance etc.
False alarms will occur in all measuring systems. The system based on IR-technique is the most selective system and therefore is it less risk of false alarms.
