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. Gas flow is a much-disputed topic in many fields; thermal spray is no different. Issues with which industry wrestles include the type of measurement instrument, its calibration, accuracy, and so on. The three basic types of devices used in the thermal spray controls are rotameters, critical orifices, and thermal mass flowmeters. Each of these devices has strengths and weaknesses.
Rotameters, often called variable-area flowmeters 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 image, a float located in the gas passage is used to indicate the gas flow.
Floats are either spherical or machined and are made from a variety of materials having different densities (specific gravities.) In order of increasing density, float materials are typically nylon, glass, sapphire, stainless steel, WC/Co, or tantalum. The choice of float material is made primarily based on the required gas flow.
For a given tube diameter and length, increasing the float density increases the flow for a given position or reading on a scale. Readings are taken the widest point of the float. The widest point of a spherical float is the equator; for a machined float it is usually a knife-edge near the top.
Scales located on, or next to, the rotameter tube are used to read the float position. Scale units are given in one of three ways:
- 0 to 100 for rotameters that read as a percentage of full scale
- 0 to X, where X equals the length of the tube in centimeters or millimeters, for example, 0 to 150 for a 150 mm tube
- 0 to X, where X equals the calibrated flow, for example, 0 to 1500 scfh or 0 to 750 sLm
Because rotameters are inherently inaccurate, measurements taken with them require some interpretation. In decreasing order of significance, pressure, temperature, and parallax (see the following) 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 (21 °C, or 70 °F) and atmospheric pressure (100 kPa, 1 bar, or 14.5 psi absolute.) 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. Depending on the rotameter manufacturer and materials, accuracies range from ±2% at 250 mm to ±20% for 50 mm long tubes. It has been observed in the field that errors of 2 to 30% exist in calibrated systems. Scale types and location also affect accuracy. While rotameters are inexpensive and compact, they are sensitive to pressure and temperature variations and parallax errors.
Image – Rotameter gas flow measurement gage.
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.
https://dl.asminternational.org/handbooks/edited-volume/12/chapter/133823/Feedstock-Material-Considerations-for-Thermal
