Why does my vacuum gauge show only 27 inches of mercury when running my pump?

There could be several reasons for this. You need to turn the pump off and make sure the vacuum former maintains the vacuum for at least a few minutes. Assuming there are no leaks in your vacuum connections, there are likely other things at play that affect the reading on your gauge. First, these gauges are not very expensive. Quality vacuum gauges are very expensive. The gauge we use on the VF-1 does fairly well for what it costs. Most vacuum gauges are going to have +/- 2 inches of mercury accuracy. If your gauge is not going to 29.5 inches of mercury, your gauge could be slightly off. Another possibility is that your vacuum pump does not actually pull 29.5 inches of mercury. Many, many vacuum pumps do not perform at the level they are rated and that is most likely the reason. We have purchased two identical pumps and tested them side by side and had differences of up to two inches of mercury between them.

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