If you’re measuring gas flow, mass techniques will simplify your task. This tutorial and video demonstration explains the concepts.
Measuring gas flow is more fraught with problems traditionally than measuring liquid flow, primarily because gas can be compressed. Changing gas pressure and temperature has a direct effect on its volume, which is why a gas volume measurement has to be qualified with pressure and temperature assumptions. In the U.S., we use standard cubic feet (SCF) and the corresponding metric measurement is normal cubic meters (Nm3). Both have underlying conditions specified. If you understand those specifications, you can convert the volumetric value to other conditions more relevant to your needs, or a mass value. In any case, when dealing with a gas flow, you have to be clear what you’re talking about. For example, an air compressor’s output can be rated in SCFM, or CFM at a specific pressure and temperature such as 100 psi and 70 °F.
Measuring gas flow using a volume-based approach such as a rotameter or variable area flowmeter is tricky if you don’t understand how it works. Such units have a direct reading scale, but it only gives you a correct value under very specific operating conditions. If you are operating at a different pressure for example, you can correct the value, but you have to know or measure all the relevant data. If you need a reading from the device in a situation where the conditions don’t change, most rotameter manufacturers can provide you with a unit scaled specifically for those conditions. However, you can’t use that device for anything else without corrections.
The best way to avoid all this is to use a mass-based approach. A pound of gas is still a pound at any pressure or temperature. For many applications, a Coriolis flowmeter is best for gas, but these are typically expensive. There are alternatives, and one of those is thermal mass technology.
Thermal mass flowmeters are based on the understanding that a given mass of fluid will remove a known amount of heat from a given body. In basic terms, the sensor uses a heating element and at least two temperature sensors. The first measures the temperature of the process fluid stream. The second measures the heating element. There are two approaches for using the measurements: one feeds a specific amount of current into the heating element and calculates flow by measuring how much lower the actual temperature is than it should be for the amount of current. The other heats the element to a specific temperature, and calculates flow by measuring how much current it takes to maintain that temperature. Both compare the heater temperature to the incoming fluid. The current levels and temperature differences provide the data to calculate mass flow.
When designing flowmeters, the technology can be used in two ways. Inline flowmeters wrap the heater around a section of pipe and heat the wall from the outside. These are typically used in smaller designs. Insertion sensors put the heater and temperature sensor on a probe and place it in the gas stream. These are more suitable for larger pipe sizes where a probe won’t cause too much of an internal flow obstruction.
The video demonstration uses an insertion approach, attaching a thermocouple to a 25 W soldering iron. When air moves through the duct, it carries away heat from the soldering iron. Since the wattage of the heating element is fixed, it simply gets cooler as more air moves through the pipe. In a more sophisticated application, the heater temperature would be compared to the incoming gas. The demonstration doesn’t calibrate the reading, however, it does use a simultaneous differential pressure reading to confirm a change in flow.
If you’re evaluating an actual installation, there are some practical considerations. The effects of these vary between manufacturers so check the specifics with your potential suppliers.
Since the design uses a heating element, it operates continuously (or at least with long on periods) at high power levels to keep the temperature stable, so battery powering probably isn’t a practical option.
Probes can be made in many sizes and for many pipe diameters, so they are scalable and can minimize duct obstruction.
Using a probe for measurement means that it may only be reading a small section of the gas stream. The normal practice is to place the sensing point in the center of the pipe and the final flow calculation is based on characteristic velocity profiles for that pipe size. This means that the probe length has to be adjustable or fixed for a specific pipe size. It also means that turbulence has to be minimal, which calls for flow stabilizers or long sections of straight pipe up and downstream.
For large ducts or where turbulence is unavoidable, some probes have multiple sensing points across the full diameter, reading the flow profile to correct for poor gas distribution.
Dirty and corrosive gas streams can leave deposits on critical surfaces and interfere with measurements or damage fragile sensing points.
Consult with your instrumentation suppliers to work though these variables. The end result can be an economical and practical way to measure gas flow without having to work around complex corrections.
—Peter Welander, [email protected]
Control Engineering
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