Portable meters (like Alicat’s new rechargeable portable mass flow meters) cause some people to question the effects of changes in orientation on the accuracy of these instruments. We have been making battery-powered portable meters for many years, and these devices are regularly used at awkward angles as a technician attempts to validate the flow rate inside a cramped instrumentation enclosure. This post will explain the basis for Alicat’s ability to achieve orientation independence.
Orientation and Accuracy in Thermal Mass Flow Meters
Invariably, the question of orientation comes to Alicat from a customer who has used thermal mass flow meters. Operating on the principle of thermal dispersion, thermal mass flow meters measure the amount of heat that dissipates from a heated probe inserted into the gas stream. Because hot gases rise, heat may disproportionately gather away from or towards the flow sensor if the meter’s orientation changes significantly. As a result, a loss in accuracy may occur when the meter falls onto its back or is inverted, depending upon the architecture of the internal flow channels. For this reason, the calibration of thermal mass flow meters is dependent upon the meter’s intended orientation, and users must request non-standard orientations in advance when they are anticipated.
Orientation and Accuracy in Differential Pressure-Based Mass Flow Meters
In contrast to the above situations, the laminar differential pressure-based mass flow meters that Alicat manufactures do not suffer from orientation-induced losses of accuracy. Differential pressure generates flow, and these devices do not introduce other active elements (like thermal dispersion) into the flow equation. We measure the absolute and differential pressures at two points within the laminar flow channels, and we measure the temperature of the gas stream as a whole, all of which are passive processes. Whether a gas flows horizontally, vertically or diagonally, the pressure differential between the two measurement points remains the same because both gravity and the force of the gas stream act upon these points equally. Furthermore, pressure within a tube acts upon the tube walls equally in all directions, so no matter the orientation of the sensor in relation to the tube, the measurement will always be the same.
Because of these fluid properties, all Alicat pressure-based meters can operate in any orientation without any loss of accuracy or repeatability. Many of our mass flow meters, as well as our mass flow controllers, are installed in vertical, lay-flat or upside-down orientations with no concern for altering their calibrations. This quality makes our portable mass flow meters ideal for field technicians who may be validating flow rates in cramped spaces or high above their heads. It also means that our portable meters can be operated in hand without concern for maintaining a specific orientation. This also applies to the oil-filled 316L stainless steel sensors that we use in our MS and MCS series for aggressive gases, as long as the device is tared once in its new orientation.
The next time you accidentally knock your Alicat on its back, don’t worry. Its flow is just as accurate as it was before.