Gas flow is a much-disputed topic in many fields; thermal spray is no different. Issues with which the thermal spray industry wrestles include the type of measurement instrument, its calibration, accuracy, and others. This section describes critical orifices, one of the three basic types of thermal spray control devices used to measure gas flow, and describes its strengths and weaknesses compared to rotameters.
A critical orifice, sometimes called a sonic nozzle, is a disk-shaped element with an axial hole. The orifice is usually inserted into a fitting, tube, or pipe through which flows the gas to be controlled. As the upstream or supply pressure (P1) increases, the velocity through the orifice reaches sonic velocity. Above sonic velocity, the flow through the hole becomes a linear function of increasing supply pressure. As a rule of thumb, these conditions exist at P1-to-P2 (upstream/downstream pressure) ratios higher than 2 to 1.
Where the downstream process pressures are higher than 1 atm, the upstream pressure must be correspondingly higher. For example, in HVOF processes, gun back-pressures may be 690 kPa (100 psi). In this case, to achieve critical flow, the minimum P1 pressure will need to be 1380 kPa (200 psi), so the supply pressure to P1 must be even higher to have a control range.
Critical orifices have two major advantages over rotameters: they exhibit no parallax problems, and, provided the 2-to-1 pressure ratio is maintained, downstream pressure has no effect on flow. The control point for an orifice is either an analog pressure gage or an electronic transducer. Even with an analog pressure gage, parallax error is minimal. The basic accuracy of an orifice is determined by the precision of the hole and the accuracy of the P1 pressure gage, usually ±2%.
Another advantage is lack of wear, particularly in jeweled orifices. Critical orifices are smaller and less expensive than rotameters. They are not subject to excessive wear and are relatively insensitive to process pressure variations. However, orifices have the same sensitivity to temperature as rotameters.
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 9.
