- What part of my plant is using the most natural gas?
- Where could my process save money on compressed air usage?
- I need to increase combustion efficiency but how can I optimize my air-to-fuel ratios?
These are questions that can be answered by a flow meter, but which flow meter would be the best for air/gas in a pipe or duct?
Most gas flow meters measure actual cubic feet per minute (ACFM) which is the flow rate at the actual operating conditions. What is more important, however, is the flow rate corrected for a particular pressure and temperature. This flow rate is commonly referred to as standard pressure and temperature (STP) and often measured in units such as standard cubic feet per minute (SCFM). This is why flow rate measured by most gas flow meters require pressure and temperature correction to convert the flow rate from operating conditions (ACFM) to standard conditions (SCFM). Thus to get the mass flow rate of gas in standard conditions using the actual flow rate, they must know the operating pressure and temperature.
For many, the answer has been to install an orifice plate, measure the differential pressure to get a volumetric flow rate along with the temperature, then put those values through a graduate level polynomial so you compensate for the changing flow conditions to get to a mass flow rate. If you did that, it took some doing but you made it!
What if you could skip all that and read the mass flow without having to worry about compensation or your lack of understanding Fourier series? Here’s where an insertion thermal mass flow meter can be a winner for you.
Insertion Thermal Mass Flow Meters
A thermal mass flow meter is a precision instrument that measures gas flow entirely differently. They measure the heat transfer as the gas flows past a heated surface. The gas molecules create the heat transfer so the greater the number of gas molecules in contact with the heated surface, the greater the heat transfer. Thus, this method of flow measurement is dependent only on the number of gas molecules and is independent of the gas pressure and temperature.
Here’s a run through of how the meter’s magic happens. Figure 1 is an illustration of a thermal mass sensor. It consists of two temperature sensors. The temperature sensor is the reference and also provides a measurement of the gas temperature. The flow sensor is heated to maintain a slight temperature difference above the temperature sensor. As gas flows past the heated flow sensor, heat transfer occurs. The instrument measures the amount of power required to maintain the temperature differential which is proportional to the mass flow rate. The amount of power in the form of heat is very low and therefore is safe technology for natural gas, hydrogen, and other flammable gases.
Figure 1: Thermal Mass Flow Sensor
Thermal mass flow measurement is an inferred flow measurement. These meters measure heat transfer and then relate the heat transfer to the mass flow rate from the NIST traceable calibration. While calibrating the flow meter, a known amount of gas is run past the sensor and the signal is measured. This is performed multiple times over the operating range of the meter. A calibration curve as shown in Figure 2. below for signal vs flow rate is created for each instrument.
Figure 2: Signal vs Flow Rate Calibration Curve
The benefits of thermal mass flow meters include:
- Mass flow measurement without the need for pressure and temperature correction.
- Excellent sensitivity at low flow rates.
- High turndown capabilities.
- Easy installation.
Since gas is compressible, its volume changes under pressure or when heated or cooled. Thermal mass flow measurement is based on heat transfer and measures mass flow instead of volumetric flow and thus does not require temperature or pressure correction. So next time you need to measure gas flow, consider a thermal mass flow meter.
How to Buy a Flow Meter
Flow meters come in different styles, technologies, abilities, etc. No one meter that can do everything - but they all have their purpose. Before you can buy with confidence, learn what questions must be answered!
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