Mass flowmeters and controllers (MFMs and MFCs), sometimes called thermal mass flowmeters, are electronic devices that indicate flow by measuring the thermal properties of a given gas.
In most commonly used MFMs, the gas flows through a precision tube that has a precise sampling or bypass capillary tube running in parallel with the main flow. A sampling tube samples or bypasses the primary flow at a ratio of approximately 100 to 1. The bypass flow is passed through a controlled-heating element, which imparts a known amount of heat (i.e., energy) into the gas stream.
Knowing the temperature of the gas, before and after the heating element, together with the thermophysical properties of the gas, the main flow through the meter can be accurately calculated. Temperature effects caused by supply parameters are eliminated because the temperatures are measured within the device. The effects of pressure variations on density are also eliminated because temperature changes, measured within the device, are based only on mass flow.
Mass flow controllers are identical to mass flowmeters, with the exception of a control valve and an internal closed-loop control circuit to maintain the set point at the desired value. Upstream and downstream pressures must be defined in order to calibrate the valve for proper operation.
Mass flowmeters measure and compensate for temperature variations, have no parallax problems, and are relatively insensitive to pressure variations. MFMs and MFCs are, however, considerably more expensive than rotameters and critical orifices. However, MFCs can be controlled remotely and are ideal for intermediate and high-end thermal spray process controls.
Theoretically, rotameters, critical orifices, and mass flowmeters/controllers can all give accurate measurements, on the order of 2% error. Given the limitations and benefits described, the thermal spray application, its economics, and the need for process control drive the choice of gas flow measurement instrument.
This information is from ASM Handbooks Online, Vol. 5A: Thermal Spray Technology, Coating Structures, Properties, and Materials. To find this information (subscription required), click on the link below and scroll down to Figure 10.
Image – Critical Mass flow controller. Courtesy of: MKS Instruments
