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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.

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    $\begingroup$ A picture of the set up would be helpful. $\endgroup$ – Zhe Jul 21 '20 at 18:15
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    $\begingroup$ Vacuum is negative with respect to 1 atmosphere. $\endgroup$ – Jon Custer Jul 21 '20 at 18:23
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    $\begingroup$ See Pressure measurement. The table there will be helpful. $\endgroup$ – Ed V Jul 21 '20 at 18:46
  • $\begingroup$ This already in Torr - millimetres of Hg. $\endgroup$ – Mithoron Jul 21 '20 at 18:59
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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 $\pu{-0.46 Bar}$:

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

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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.

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

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