Many experimenters will plan their build around how much HHO they THINK it will produce. This is a huge mistake. It is true that experienced builders can estimate gas output by simply looking at a hose in a bucket, or looking at the size of the cell. Don't make this a habit. Test your cell and know how much HHO it produces. You must understand that every cell configuration is different. It's output is subject to voltage, amperage, electrolyte, plate material, temperature and (guess who?) the operator! Every cell configuration is different, because everyone is different. You may have mixed your electrolyte differently, used thinner or thicker gaskets, or even implemented your own voltage and amperage scheme.
When you have your cell complete, you should perform two LPM tests. One should be performed while the cell is cool. Another should be performed after the cell has been conditioned for a few hours or days. DO NOT rely on traditional Flow Meters that are normally calibrated for a single gas like Hydrogen or Oxygen. For instance, a Flow Meter calibrated for Oxygen won't give an accurate measurement for HHO because you are also introducing two additional parts Hydrogen. An accurate flow test can be performed cheaply, and easily. You will need a five gallon bucket, drinking bottle and a stopwatch or clock. You can use anything from 500mL to 2 or 3 Liters. Push the bottle into the bucket of water and allow it to fill completely. Put the cap on and take it out. Turn it upside down and insert it back into the water with the cap down. Remove the cap. Insert your output hose into the neck of the open bottle and turn your electrolyzer on. Time how long the bottle takes to fill completely. This is your volume-per-minute test.
For example: If your bottle is 2 Liters in size and takes 2 minutes to fill, your cell has an output of 1 Liter Per Minute. If it fills in 20 seconds, your cell has an output of 6 Liters Per Minute. Now, without question, you have an accurate measurement. Now that you have an accurate measurement of the output of your cell, you can implement appropriate safety measures.