All thermal spray devices use some type of gas. This section discusses the practical application of gas flow measurement techniques as they relate to thermal spray processes. The three basic types of devices used in the thermal spray controls are rotameters, critical orifices, and thermal mass flow meters. Each of these devices has strengths and weaknesses.
Rotameters, often called variable-area flow-meters or flow tubes, consist of a vertical glass or polymeric tube having a tapered or conical gas passage (hence, variable area) over the length of the tube. As shown in the figure, a float located in the gas passage is used to indicate the gas flow.
While rotameters are inexpensive and compact, they are sensitive to pressure and temperature variations and parallax errors. Because rotameters are inherently inaccurate, measurements taken with them require interpretation. In decreasing order of significance, pressure, temperature, and parallax have the greatest effect on accuracy. Other factors, such as wear of the float and tube, are longer-term problems.
Rotameters are calibrated at specific gas pressures and temperatures. Generally, they are referenced to room temperature and atmospheric pressure. Any variation from the calibration conditions results in an error.
All factors combined, under calibrated conditions, rotameter accuracy and resolution (the ability to read scale divisions accurately) vary as an inverse function of length. Scale types and location also affect accuracy.
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 8.
Image – Rotameter gas flow measurement gauge.
