Thermal-mass flowmeters are also used by many heat treaters. In most industrial-grade devices, gas enters the flow body and divides into two flow paths. Most of it goes through the laminar-flow bypass, creating a pressure drop that forces a known fraction of the flow through the sensor tube (Fig. 2). A power supply is used to direct a constant amount of heat into the gas stream. Resistance temperature detector (RTD) coils are placed around the bypass sensor tube at its upstream and downstream ends. Heat is transferred to the molecules of the flowing gas, independent of pressure or temperature fluctuations.
The gas flow carries heat from the upstream coil to the downstream coil. The downstream coil, therefore, has a higher temperature and more resistance than the upstream coil. The coils are legs of a bridge circuit with the resultant output voltage proportional to the difference between the coils’ resistance, which in turn is proportional to the mass-flow rate. The two other parameters — heat input and coefficient of specific heat — are constant.
Another type of mass flowmeter uses one flow channel with a temperature sensor located in the path of the flow. This technology is simpler, but often less accurate, and is limited to higher flow rates.
Flowmeter Accuracy and Repeatability
Thermal-mass flowmeters are gas-specific devices and must be calibrated with either the actual gas or a reference gas. This “inconvenience” has led to the development of many “fixes” and is driving the development of smarter devices. For now, however, primary calibration with the actual gas or a gas of similar molecular characteristics is the only way to ensure accuracy.
The accuracy of mass flowmeters and mass-flow controllers is determined by two factors: flow calibration and repeatability. Proper instrument calibration ensures starting-point accuracy. Repeatability is the measure of continuous performance to specification over the lifetime of the device. Most mass flowmeters and mass-flow controllers have an accuracy of ±1% of full scale and a repeatability of ±0.25% of full scale.
Several factors affect repeatability. To compensate, highly stable materials and electronic components, as well as precise internal voltage and current regulation, are used. Sensor and bypass design also play a major role in preventing errors caused by contamination and clogging. For example, U-type sensor tubes exhibit residual stresses from bending, which can cause long-term strains and unraveling of sensor coils. These sensors are also more likely to develop drift due to contaminant deposits.
Consideration should also be given to the bypass element. Accuracy can be degraded by changes in temperature if the bypass is an orifice (or venturi) as opposed to a pure laminar-flow element. With an orifice bypass, the pressure drop is proportional to the square of the bypass flow. In this case, the ratio of bypass flow to sensed flow is not a constant but instead is a complex nonlinear function having temperature-dependent terms such as gas viscosity. Both the nonlinearity and temperature dependence of the orifice bypass can seriously degrade the accuracy of a mass-flow controller.
Mass Flowmeters for Vacuum Applications
A heat-processing application of thermal-mass flowmeters and mass-flow controllers is maintaining a specified gas-flow rate into a vacuum chamber when the process requires a partial pressure of additive gas. Typically, a throttle valve or an orifice-limiting device is used to control the output of a vacuum pump. This is an extremely pressure-sensitive method and can result in inefficient gas delivery and poor product quality. Mass-flow controllers automatically compensate for changes in system pressure caused by vacuum-pump fluctuations and deliver a precisely controlled gas-flow rate to the chamber.
References provided previously.
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