Thermal conductivity sensors
Because catalytic pellistors measure the flammability of a gas, they cannot be used to measure levels of gas above the lower explosive limit (LEL), since the reducing level of oxygen will result in a fall-off signal. However, a similar device can be used to monitor these high levels of gas, the thermal conductivity pellistor.
Thermal conductivity Pellistors (TC Pellistors) detect and measure between 0 and 100% by volume gases that have thermal conductivities significantly different to a reference gas, usually air. Examples of these gases are: hydrogen, helium and methane. This also means that some gases such as oxygen, nitrogen and carbon monoxide cannot be measured, as their thermal conductivities are too similar to that of air. This difference can vary with the gas temperature.
TC sensors works by measuring the change in heat loss (and hence temperature/resistance) of the detecting element in the presence of the target gas.
TC pellistors are available with a wide variety of working voltages and power levels. In general, the higher power devices are used in fixed systems and the lower versions in portable equipment. Low power pellistors are also available with the option of shock protection.
AEP offers a range which are designed to complement the pellistor range in terms of electrical characteristics, so that they can be used in the same Wheatstone Bridge circuits. They are supplied with a compensator bead which is sealed in an enclosure of air. This enclosure acts as the thermal conductivity reference in exactly the same way as it acts as the reference for a catalytic pellistor.
|VQ5MB||Pellistor Pair 2V/175mA, TC Closed Can Compensator||Download|
|VQ6MB||Pellistor 2V, TC, Closed Can Compensator, Solid Legs||Download|
|VQ35MB||Pellistor Pr 4.2V/55mA TC Closed Can compensator, Solid Legs||Download|
|VQ31MB||Pellistor Pr 3.5V/90mA TC C Can Solid Lg, closed can compensator||Download|
|VQ546M||VQ46 Pellistor, VQ500 Head||Download|
|VQ546MR||VQ46 Pellistor, VQ500 Head, +ve Output||Download|
|VQ500 Series||Range of Catalytic and Thermal Conductivity Pellistor certified 4 series heads||Download|
|VQ600 Series||Range of Certified Catalytic and Thermal Conductivity Pellistor Fixed Heads for Gas Detection||Download|
Working of the thermal conductivity pellistor
The thermal conductivity pellistor comprises two beads, one of which is exposed to the target gas (the detector) and the other sealed inside a chamber containing air (the compensator). Both beads are heated as before and run in the same type of circuit as the catalytic pellistor. When the detector bead is exposed to a gas whose thermal conductivity is significantly different to that of air, the rate of heat loss from the bead will change, as will its resistance. This change measured is compared with the compensator bead. The sensor is most often used to detect gases with low molecular weight, which have greater thermal conductivities than that of air, consequently giving the highest response.
The main application for methane calibrated thermal conductivity pellistors is coalmines and natural gas monitoring. Concentrations of methane well over 50% by volume can be found in fissures in coal seams.
The other main application for TC pellistors is in the monitoring of ‘’light’’ gases such as helium and hydrogen. These gases can be found in various industrial processes (for helium) and for hydrogen in fuel cell and battery charging applications.