Whenever you buy a Mass Flow Meter or Controller the sales folks always ask “What flow rate are you wanting?” What?! You say, “If I knew that I probably would not have to buy a Meter! You folks ought to understand my flow rate! You are in the business of selling Mass Flow measuring gadgets!” I always hated when my teachers explained to me to look up how to spell words in the dictionary! It never stops. First the teacher and today the silly Flow Meter Company! Circular reference, what came first? The chicken or the egg? The reality is the Mass Flow Meter company doesn’t know your flow rate and it is really not their job to be familiar with YOUR flow rate.
The GREAT news is you CAN estimate the mass flow rate if you have nothing more than a stop watch and an appropriately sized plastic bag. Let’s say you want to estimate the flow rate for a process you are working on where you are using a small fish pond air pump that is delivering air to your process. You can see that the outcome of the process is HIGHLY sensitive to the air flow rate that you manually set with a needle valve.
It would be VERY handy to have a technique to accurately measure and control that flow precisely. You have the idea that you can buy a Mass Flow Controller and precisely control the flow so that your process will work with the efficiency that you need. You further discover that when the PH value is EXACTLY at 5.7 the process is even more efficient. It turns out that you realize you could further adjust a Mass Flow Controller to “close loop” on the PH reading and get the desired efficiency.
To estimate the flow you simply start by preparing a plastic bag that is modified to allow you to direct the gas flow into it. This bag should be of such a size that will allow at least 30 seconds to fill it fully. You simply time how long it takes to fill the bag. Evacuate the bag and repeat a few times to see that it takes about the same time to fill the bag. When you are done with the bag you then simply fill the bag to the same level but you will use water to fill it. Once full dump the water into a good graduated beaker or accurate measuring cup to measure how many liters of water you used.
Let’s say you fill the bag fully in 30 seconds and the volume of the bag is 0.5 liters. This would equate to 1 liter per minute of a gas at around 70 Deg F and 1 Atmosphere. In Flow Meter parlance this would be equivalent to 1 SLPM where the “S” refers to the “standard” conditions of 70 Degrees F and 1 Atmosphere… In short, it simply defines the standard temperature and pressure. Mass Flow Meter Companies will call this STP. Therefore, your PERFECT flow is 1 SLPM!
However, in order to cover the complete “range” you will most likely require it is generally best to order a flow meter where this 1 SLPM represents about 63% of your full-scale flow of the device. The math is Full Scale = X SLPM/ 0.63 or 1 SLPM/.063 = 1.58 SLPM. This would have you ordering a 1.58 SLPM Flow Controller. Since we all LOVE whole numbers you would be advised to buy a 2 SLPM Mass Flow Controller for air with the STP conditions of 70 Degrees F and 1 Atmosphere. What you need now is a great low-cost from stock Mass Flow Controller. See the links below to see some samples!