Here's the thing you need to keep in mind. In the scheme of flow meters we are measuring a VERY small amount of flow, typically in cubic centimeters per minute, so getting a meter with the right scale is key. Most tanks I have seen run from about 10 cc/min to 80 cc/min depending on the size of the tank.
When you start looking up flow meters you will see lots of them with different scales, but look closely at what you are buying. For instance you will see flow meters with a SCFH scale. That stands for Standard Cubic Feet per Hour. That is wildly more than we are measuring. One SCFH unit is equal to 472 cc/min.
Just saying when people start exploring flow meters the scale is VERY important.
There are two Dwyer's that are typically used.
The Dwyer RMA-151-SSV measures from 5-50 cc/min. The Dwyer RMA-150-SSV measures from 10-100 cc/min.
Which you use depends on the size of your tank. In my 120G I use the RMA-151-SSV, and my flow rate is about 45 cc/min. In retrospect I probably should have got the RMA-150-SSV, as I am near the limit of the 151's range. But when I bought mine there was no one using these, so I had no frame of reference for how much CO2 I would be using.
And like
@Immortal1 showed above, there are lots of other brand flow meters out there as well. But again you have to be VERY careful that you understand what you are buying. Some of them are high end and are very costly, but can be found cheaper on a secondary market (ebay). With the Dwyer's they can be purchased new at a reasonable cost, the scale is known to be good, and that's why you see them more often.
I hope that helps.
