ABSTRACT
Thermal mass flow measurement infers measurement from the thermal properties of the flowing medium (such as specific heat and thermal conductivity) and provides measurements that are proportional to the mass of the medium.
The two most commonly used methods are measuring the rate of heat loss from a heated body in the flow stream and measuring the rise in temperature of the flowing medium when it is heated.
In a capillary tube thermal mass flowmeter the medium divides into two paths – one through bypassing a defined portion of the flow in order to maintain a constant ratio of the flow through the bypass and the sensor.
This method is largely independent of the flow profile and the medium viscosity and pressure. The flow calibration for any gas can be obtained by multiplying the flow calibration for a convenient reference gas by a constant K-factor. K-factors are now available for over 300 gases, giving capillary tube meters almost universal applicability.
Although the main application of the thermal mass flowmeter lies with gases, the same technology can also be applied to the measurement of very low liquid flows for example down to 30 g/h.
