If you already own an Alicat flow meter, then I will soon be preaching to the choir. However, if you do not have an Alicat flow meter and you work in a toxics organics (TO) air laboratory, whether it be U.S. EPA Method TO-11A or TO-17, etc… then you do not know what you are missing out on. Short story: instantaneous pressure and vacuum flow measurement you can fit in your back pocket. ‘nuff said! However, if you are currently in the market for one or soon to be after that sales pitch I just laid on you, then read on…
We offer 2 different flow ranges for the Alicat: low flow (0-50 mL/min) and high flow (0-500 mL/min). Sometimes we hear the question “can I use only the high flow meter to set the flow on my Passive Air Sampling Kit at 0.5 mL/min and 250 mL/min or do I need to have the low flow meter as well? After all, the high flow meter does have a range of 0 – 500 mL/min?”
Here is my answer: Both meters will work, but there will be a lot more error associated with the high flow meter when measuring down to 0.5 mL/min. I say this, because the flow meter accuracy is defined as: ± (0.8% of Reading + 0.2% of Full Scale). The following table shows you 3 possible scenarios that could play out with the two different flow meters when trying to measure flow.
For scenario 1 we see how using a low flow (0 - 50 mL/min) meter to measure an actual flow of 1 mL/min could result in a potential reading ranging from 0.892 – 1.108 mL/min, which is perfectly acceptable for most people. However, for scenario 2 we see how the errors add up on the high flow (0 – 500 mL/min) meter when trying to measure 1 mL/min. In fact, you could be reading 2 mL/min when in reality it is 1 mL/min. That is not to say you will, as these errors/ranges are the maximum (i.e., worst case scenarios). You could also read 1 mL/min for 1 mL/min. The point is, you will not have the confidence in the low flow measurements (e.g., 1 mL/min) when using the high flow meter. However, you can be confident with the low flow meter. Scenario 3 is here to demonstrate: 1) that you may not use a low flow meter to measure 100 mL/min, as it is above the meter’s operational range and 2) to show you that the high flow meter is accurate when applied appropriately.
Sorry for the long-winded answer. Since I love to work in analogies, my shortcut/analogy response is often as follows: would you make a 1 µL injection on your 500 µL syringe or do you own several syringes? Again, ‘nuff said! So, the short answer is that most of us have to use both flow meters.