OK - here are my results for 3” Hg and 4” Hg and 5” Hg:
Keep in mind this is testing just 1 of my gauges - the one attached to my brass brake bleeder (commonly used here):
3” Hg on the gauge, 50” water, 23% higher than 40.8” water
4” Hg on the gauge, 64” water, 18% higher than 54.4” water
5” Hg on the gauge, 75” water, 10% higher than 68” water
It appears that the gauge is even less accurate at lower vacuums (lower on the scale).
The good news is I repeated the tests multiple times with the same results. The bad news is the gauge is even less accurate than I thought, particularly at 3” Hg. However, in actual usage the problem may not be significant.
Another way of looking at the results:
3” Hg on the gauge, 50” water, 3.7” Hg actual
4” Hg on the gauge, 64” water, 4.7” Hg actual
5” Hg on the gauge, 75” water, 5.5” Hg actual
So in the operating range of 3”-5” Hg the gauge is high by 0.7 to 0.5 respectively. In reading my gauge I cannot distinguish between 0.5 and 0.7 anyway so it is about the same to me…
I think I am going to go look for a better gauge and…. take all that tape and tubing off my wall.
Cheers!