Today I think I got it figured out so we can fairly accurately determine RVP at home (HVP) as suggested in yesterday’s entry.
Test 1–First, I refrigerated the complete test bomb for an hour, bringing its surface temperature down to 34 degrees F (measured with my $29 Harbor Freight infrared gun). Then I took the fuel sample chamber and filled it with cold water and immediately connected it to the cold air chamber. Then I submerged it in the circulating 100 F water bath and left it until the air pressure in the bomb quit rising. Result: expanding air pressure in the upper chamber EA 2.25 psi
Test 2–Next I removed the water–filled sample chamber, emptied and dried it, then put it back in the refigerator to chill it down to 34 F (this is to ensure that front ends are not lost from the fuel sample prior to testing) while the warm air chamber was left in the 100 F bath. Once it was chilled I filled I filled the sample chamber with a mixture of stale race gas and isopentane then immediately attached it to the 100 F and conducted an actual RVP test. Result: actual RVP 7.0 psi
Test 3–Finally, I removed the bomb from the bath, removed and emptied and dried the fuel sample chamber and once again chilled it and the upper air chamber down to 34 F, and did a test. A fresh sample of my volatile mixture was poured into that sample chamber and I immediately attached it to the air chamber. Then it was immersed in the 100 F bath and was agitated and heated to 100 F. So this time we had a combination of boiling off front ends and expanding air in the upper chamber. Result: test pressure 8.75 psi
I initially expected the HVP to be RVP + EA, but it was .50 psi lower. Mmm. We can surmise that the expanding air in the top air chamber was acting like nitrogen pressure on the sample–perhaps prevent some loss of front ends compared to an actual RVP test which has zero Expanding Air!
So it looks like the math for HVP testing is starting with a refrigerated homemade bomb 80% cold air and 20% fuel sample is as follows: fuel at as close to 100 F as possible and shaken to agitate = test pressure 8.75 psi – AE (air expansion pressure) of 1.75 psi = HVP 7.0 psi.
Now this is not perfect–for example if your fuel is completely dead, you should come up with a test pressure of 2.25psi so the AE will diminish some as actual RVP increases. But surely close enough to know whether your fuel is volatile or dead. And, if you build your own HVP test bomb, you should do the test once with 20% water to see what your AE pressure is on your gauge which is what counts. Then you can do the math based on your own instrument.