The most common gas detection measurement methods used in instruments are described below. Please note that it is not only measurement methods that must be taken into account, but also the placement of fixed detectors so that the most representative gas concentration is found and in which environment to measure.
Katalytisk förbränning (brännbara gaser)
This measuring method for gas detection is based on burning the combustible gas on a heated wire that is specially treated. When the gas is burned, the wire gets even hotter and the resistance changes. The change in resistance is detected in an electrical circuit and displayed on a scale.
Characteristics
- Reacts to all flammable gases.
- Only gives the "correct" reading for a specific combustible gas.
- Only usable up to 100%LEL.
Elektrokemisk cell (oxygen och många giftiga gaser)
The electrochemical oxygen cell consists of an anode and a cathode, located in an electrolytic bath. An electrical voltage is generated as long as oxygen is present. The voltage from the sensor is proportional to the oxygen content.
Characteristics
- The measuring cell is made selective to measure only a particular gas, by choosing the right electrolyte and electrode material.
- Has a specific lifespan (compared to a battery).
NDIR (Infrared Measurement)
Denna mätmetod för gasdetektering (IR) bygger på absorptionen av infraröd strålning vid specifika våglängder när den passerar genom en mängd gas. Vanligtvis mäter två infraröda ljuskällor och en IR-ljus detektor intensiteten vid två olika våglängder, en vid absorption och en utanför absorption.
Om en gas finns mellan källan och detektorn, minskas mängden strålning som faller på detektorn. Koncentrationen bestäms genom att jämföra de relativa värdena mellan de två våglängderna. Detta kallas dual beam IR detektor.
Characteristics
- Long service life.
- Works without oxygen being present.
- Higher purchase cost.
LEL and UEL
LEL = undre explosionsgräns, det vill säga den koncentration av en brandfarlig gas i luft under vilken gasblandningen inte kan antändas.
UEL = övre explosionsgräns, det vill säga den koncentration av en brandfarlig gas i luft över vilken gasblandningen inte kan antändas.