Titan Discuss Thermal Flow Meters
Hot-wire-anemometers are early examples of thermal flow meters. These were useful but delicate devices that were often used to roughly measure the air velocity in ventilation ducts.
Today modern thermal flow meters are very useful tools for measuring both large gas flows and micro-flows of liquids and gases. There are numerous thermal techniques to make these measurements, in this feature we will only cover some of them. An example of a thermal technique for metering large gas flows involves introduction of a pair of elements into the gas stream which is preferably turbulent. One element is a temperature reference and the other heated. The heat loss (or equivalent) of the second sensor is directly proportional to the mass flow of gas across the element. The only down side of these types of thermal flow measurement device is their dependency on the thermal transfer properties of the gas and therefore each meter has to be set-up for the actual gas flow. These types of practical thermal flow meters are frequently insertion devices with a low blockage ratio and therefore a low pressure drop in the system.
For measurement of small liquid flows, typical thermal flow meters & systems are very different but the fundamental principles remain very similar. In such systems the measuring device is often a capillary type tube made of glass or stainless steel against which two or three elements are placed on the outside or around the tube. The illustration below shows a three element thermal flow metering system for liquids which, because energy is being injected into the system, is capable of measuring microlitre flows.
In this device the first element measures the temperature and the second element is heated by a known amount above the reference. As a result the temperature of third element is found to be directly proportional to the mass of the flowing fluid. Again the thermal characteristics of the fluid being measured are crucial as even a small change in fluid properties will change the calculated mass flow. Extremely low flows can be metered using this thermal flow method. However using this technique there is a trade-off between device robustness and response time. Thin walled glass tube walls can make these meters relatively fragile but with careful installation and handling this need not be an issue. A robust meter with a thicker metal tube will have a correspondingly slower response time but will offer a more robust product. Accuracy claims also vary a lot so care must be taken if selecting a thermal flow meter.
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