Well, mmHg (or cmHg) is an old pressure unit almost equivalent to the (also dated) Torr:

https://en.wikipedia.org/wiki/Millimetre_of_mercury

Atmospheric pressure is 760 mmHg (or 76 cmHg), so your actual pressure is (76-Display)*1333 to get the result in Pa which you need to divide by 105 in order to get to bar.

Dear Dr. Abul Hossion,

through the different conversions, we can obtain:

-30 inHg = -78 cmHg = -1015,92 mbar = - 1 atm (about - 1 bar).

As a logic, I think that the minus sign (-) in front of thirty is not to be considered ... in this way we pass from a value of about 1 atm (as the pointer is positioned in the photo) to subsequently lower values.

Best regards, Pierluigi Traverso

The conversions are well-explained in the previous answers. Please disregard the minus sign.

Dear all,

The pressure is lower than the atmospheric pressure. And it is lower by 30inHg. But I need the value of the vacuum pressure. Vacuum pressure can be given in terms of 1bar, 0.1bar, 0.01bar, 0.001bar....So there is no meaning of -1bar.

See my calculation,

-76cmHg = 1/76=0.013cmHg=0.013*0.013=0.00017bar=0.173mbar.

Do you think the calculation is correct??

No, that is definitely wrong, there is absolutely no indication that this is a reciprocal scale. Read my first posting. -76cmHg probably means it is 76 cmHg below atmospheric pressure and since 76 cmHg is atmospheric pressure, -76 cmHg is the formal 0 bar point of absolute pressure. For your orientation:

-76 cmHg Display = 0 bar absolute

-38 cmHg Display = 0,5 bar absolute

0 cmHg Display = 1 bar absolute.

For determining the pressure in the range where you need to go logarithmic this pressure gauge is not suitable. This one can only be trusted with a ca. 5mbar accuracy.