The flow or pressure device’s full scale should match the largest flow rate that you need to be able to measure. If your device has a higher or lower range than is required by your system, you will be unable to acquire a proper 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 a 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. However, your application also requires you to measure a flow rate of 0.30 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 meters can be daisy-chained to fully cover the range of flows you are measuring.