Thermal mass flow meters principle

The most common high precision laboratory instruments are the thermal mass flow meters. (They are among the most expensive and can cost over 2000. ) The principle is based on an energy balance either the entire flow or a slipstream is heated at a constant rate and the temperature rise is recorded. The temperature (or temperature difference) of the fluid is measured and amplified it is proportional to the mass flow rate ... 

Thermal mass-flow meters directly measure the mass-flow rate utilizing the thermal transfer process between a heated element and the flowing mass. According to the thermal transfer mechanism, they can be classified as calorimetric- or boundary-layer-type flow meters. Calorimetric flow meters utilize the simple principle that as the mass flows through the device, power has to maintain the temperature of the incoming mass at a known level. By measuring the supplied power P and the temperature above ambient, AT, the mass-flow rate M can be calculated as the expression... 

Figure 3.10 shows the working principle of the mass flow meter. The gas is forced to flow through a tube with a small diameter and the thermal components are placed on the outside of the tube. The temperature profile is symmetric if there is no gas flowing through the tube. The peak of the temperature profile will shift downward if the gas flows inside the tube. The difference between the temperature profile peaks is proportional to the mass flow rate and the flow rate and hence can be detected indirectly by readings from the temperature sensors. 

For metering air mass various physical principles can be used. The first automotive application in 1972 used a vane type air-flow meter [1], The signal from a vane meter has to be corrected for air density variations, so the thermal measurement principle was chosen for the next generation of air-flow meters. 

A typical thermal mass meter is shown in Figure 4.20. The operation of these meters is based on the principle that the rate of heat absorbed by a flowing fluid is directly proportional to the mass flow rate [23]. In general there are three types of thermal mass meters constant temperature, constant power, and energy balance. 

In principle, differential thermal analysis, DTA is a technique which combines the ease of measurement of the cooling and heating curves discussed in Chapter 3 with the quantitative features of calorimetry which are treated in Chapter 5. Temperature is measured continuously and a differential technique is used in an effort to compensate for heat gains and losses. In the case of DTA as also in calorimetry, the actual heat measurement does not rely on a direct measurement of the heat content. A heat meter, as such, does not exist. In volume or mass determinations (see Chapters 6 and 7, respectively), the total quantity of interest can be established with one simple measurement. In the determination of heat content, in contrast, one must start at zero kelvin and measure all heat increments and add them up to the temperature of interest. In DTA one derives the flow of heat, AQ/dt, from a measurement of the temperature difference between a reference material and the sample. ... 

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