Flow range: the speed limit
Alicat mass flow devices offer different available flow ranges depending on your application. Knowing the correct flow range for your application will help you determine the best meter or controller for your system. This applies to all gas and liquid units, but also pressure controllers for proper valve sizing.
Ideally, the device’s full scale should match the largest flow rate that you need to be able to measure. Alicat offers custom flow ranging when you purchase your instrument, to give you the best possible accuracy and control over the device’s full range. If a device has a smaller or larger flow range than your system requires, it will suffer from reduced accuracy and will not provide the best measurement.
In some applications the flow range is too wide for one device to handle. This is less frequently an issue with Alicat devices, thanks to our turndown ratio of 200:1. This is 4 times larger than industry standard devices at 50:1.
As an example, imagine you require a 50 SLPM (Standard Liter per Minute) mass flow meter to measure your highest flow rates. But your application also requires you to measure a flow rate of 0.3 SLPM (300 SCCM) on the low end. This would mean that the device would need to measure a flow rate that is 150 times smaller than its full scale range. Below (or above) the specified flow range, your device may still detect flow, but since it is outside of specification, there is no scientific certainty to the numbers.
For the 50:1 turndown of typical devices, you’d have to buy two flow meters in this application. A 200:1 Alicat can handle both flow rates and remain within specification. The standard accuracy specification for an Alicat gas flow device is ± (0.8% of reading + 0.2% of full scale). A high accuracy calibration is also available. Both accuracy ranges offer 200:1 turn down capability, saving set-up time and money.
If you need an even higher turn-down ratio, such as 1000:1, two or three Alicat devices can be daisy-chained to fully cover the range of flows you are measuring.
DETERMINING FLOW RATE
What happens if you do not know the flow rate of your system?
Use the Ideal Gas Law:
You can derive this equation to calculate the volumetric flow rate from a pressure decay test:
Q = Volumetric Flow Rate
V = Volume of the pressurized system
Pdifferential = Pressure difference in PSI
t = time
Patm = Standard atmospheric pressure [14.696 PSIA]
To perform this test, you will pressurize your system to your test pressure, and observe how long it takes to drop in pressure by a given amount. Ensure that your pressure readings are in the same units, and your volume and time units are the same as the volumetric flow rate you are expecting.