# Confused Reading of Pressure on Vacuum Gauge [closed]

I have a gauge used to measure vacuum but I'm a bit confused as the measurements state "Hg" (I would have supposed inches of mercury except it goes to -30) and "Bar" going to minus 1.

I need the pressure in Torr, how can I properly convert my readings?

I made a google conversion but it gives me a negative result in Torr which cannot be right.

• A picture of the set up would be helpful. – Zhe Jul 21 '20 at 18:15
• Vacuum is negative with respect to 1 atmosphere. – Jon Custer Jul 21 '20 at 18:23
• See Pressure measurement. The table there will be helpful. – Ed V Jul 21 '20 at 18:46
• This already in Torr - millimetres of Hg. – Mithoron Jul 21 '20 at 18:59

OP's question: I need the pressure in $$\pu{Torr}$$, how can I properly convert my readings?

$$\pu{1 Torr} = \pu{1 mmHg}$$ and $$\pu{1 inch} = \pu{25.4 mm}$$.

Thus, any reading in $$\pu{inchHg}$$ can be multiplied by $$25.4$$ to convert it to $$\pu{Torr}$$. For example, if your reading is $$\pu{-20.4 inchHg}$$:

$$\pu{-20.4 inchHg} = \pu{-20.4 inchHg} \times \frac{\pu{25.4 mmHg}}{\pu{1 inchHg}} \approx \pu{-518 mmHg} = \pu{-518 Torr}$$

If the barometric pressure is $$\pu{760 Torr}$$ at where you live, then the reduced pressure in your container is $$\pu{(760-518) Torr} = \pu{242 Torr}$$.

Similarly, $$\pu{1 Torr} = \pu{0.001333 Bar}$$. If your reading is $$\pu{-0.46 Bar}$$:

$$\pu{-0.46 Bar} = \pu{-0.46 Bar} \times \frac{\pu{1 Torr}}{\pu{0.001333 Bar}} \approx \pu{-345 Torr}$$

Pressure gauges typically measure pressure relative to ambient, so "0" on a gauge means equal to ambient (~ 1 bar absolute pressure depending on where you are). In some cases, the units have a "g" at the end, for example "psig" to indicate that the measure is "gauge pressure" rather than absolute, but that isn't always the case.

• Thanks for your answer Andrew! How do I make sense of it? How do I convert the reading into Torr? – Hans Jul 21 '20 at 18:38
• @Andrew: This is useful information, but did not give answer to OP's question. – Mathew Mahindaratne Jul 21 '20 at 19:01
• Sorry. Thought it was obvious how to convert once you know absolute pressure is gauge + ambient. – Andrew Jul 21 '20 at 19:06
• I'd think so too, but OP's question is specific. – Mathew Mahindaratne Jul 21 '20 at 19:14