![]() If you would rather work with acceptance factors, take the “V” out of the Boyle’s Law formula, and it becomes: Acceptance factor = P1/ P2 – P1/ P3. In other words, drawdown = acceptance factor x total tank volume. A word of caution: Before expanding the cut-in/cut-out differential beyond 20 psi, check with your tank manufacturer to make sure that by doing so, you will not be over-expanding the water chamber.Īnother term used in determining drawdown is “acceptance factor.” This simply is the factor by which you multiply the total tank volume to get drawdown. ![]() Therefore drawdown = 25.8 x 85 = 21.9 gallons.Ĭonsider how the drawdown changes if we widened the differential between the cut-in and cut-out pressures from 30/50 to 30/60.ĭrawdown = (28 + 14.7) / (30 + 14.7) – (28 + 14.7) / (60 + 14.7) x 85Ĭompare this with the 25.1 from Example 1. Instead of a 30/50 pressure switch, use a 40/60 pressure switch with a 38-psi pre-charge, and the numbers look like this:ĭrawdown = (38 + 14.7) divided by (40 + 14.7) minus (38 + 14.7) divided by (60 + 14.7) x 85. As we will point out later, it is possible – though not always standard practice – to pre-charge conventional tanks. Imagine a 30/50 pressure switch, but no pre-charge, as could be the case with a conventional tank with no pre-charge.ĭrawdown = (0 + 14.7) / (30 + 14.7) – (0 + 14.7) / (50 + 14.7) x 85ĭrawdown = 0.102 x 85 = 8.7 gallons, about 10 percent of the total tank capacity. Use the same tank conditions, but put the job up at Lake Tahoe in California at 6,000 feet above sea level, where the atmospheric pressure is 11.8 psi, instead of 14.7 psi. Therefore, drawdown = 0.295 x 85 = 25.1 gallons. The drawdown formula then looks like this:ĭrawdown = (28 + 14.7) divided by (30 + 14.7) minus (28 + 14.7) divided by To make the calculations easier, Boyle’s Law can be restated as: At sea level, add 14.7 psi to the gauge pressure to get absolute pressure. Remember that all pressures must be stated in terms of absolute pressure.
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