Vacuum presssure to absolute pressure 1

[cougarfan]

How does one convert vacuum pressure reading to absolute pressure reading. I have a manomenter reading 25 " Hg - what is that in absolute pressure

 

[phex]

if it's a direct read from manometer, this is probably absolute pressure. to convert to known scale use e.g.:

750 mm Hg equals 1 bar

which means the absolute pressure in your system is about 866 mbar.

 

[joerd]

It really depends on your local atmospheric pressure. As a standard, we usually take 1.0135 bar abs at sea level, which converts to 760 mmHg or 29.9 inches Hg.
In your case, you would get 25 + 29.9 = 54.9" Hg.
I hope you are not up in the Rockies or something... ;-)

Cheers,

Joerd

 

[joerd]

Sorry, disregard previous post, I agree with phex that the manometer probably already shows absolute pressure, so 866 mbar is correct.

Cheers,

Joerd

 

[25362]

Could it be 846.7 mbar ?

 

[phex]

possibly... i did the calcs in head and pretty fast...

yes, you're right... where do i have my brains nowadays...

 

[hacksaw]

A 25" Hg vacuum reading is gage pressure by convention i.e. 25" below ambient. this is 5" HgA or 169 mBarA.

25" HgA pressure reading is 844 mBarA.

 

[25362]

Paraphrasing David Saletan in his "Creative troubleshooting in the chemical process industries": certain totems of the engineering profession block rather than enhance our understanding of mechanisms, such as the bureaucratic SI system of units. Atmospheres and cyles per second, as an example, promote our physical feel for problems; pascals and hertz do not. The large number of measuring units is indeed confusing.

Nowadays, by convention, atmospheric pressure is 1 atm abs = 760 mm Hg @ 0^oC = 760 torr = 14.696 psi = 29.92 in. Hg @ 0^oC = 1013.25 mB = 101.325 kPa = 101,325 N/m². We see that 1 bar = 100,000 Pa = 1,000,000 dynes/cm² = 100,000 newtons per square meter or N/m².

The right interpretation of the present thread is whether dideyjohn speaks of a manometer, and means a barometer thus its reading is absolute pressure:

(25 in x 25.4 mm/in) /750 mm/bar = 0.8467 bar = 846.7 mB abs.

Otherwise, if it is a vacuum reading, then it is {[760-(25x25.4)]/750}1000 = 166.7 mB vacuum.

The more accepted SI unit nowadays is the Pa = 1 N/m² = 10 dyn/cm² = 1 kg/(m.s²). Thus 1 atm = 0.101325 Mpa = 101.325 kPa.

Units as mm Hg, or in. Hg, or in. H₂O must be given in reference to temperature. 1 in. H₂O @ 60^oF = 248.8 Pa.
1 in. Hg @ 60^oF = 3,377 Pa.

It is always advisable to add the words "absolute pressure" or "vacuum", or their abbreviations, as befits the case in hand.

 

[hacksaw]

i thnk i am beginning to see why nature abhors a vacuum, more or less

 

[cougarfan]

I appreciate all your answers - let me explain my reason for asking. I am looking to solve an equation in reference to the process we run and the two variables I need are:

Initial Vacuum pressure - we run a vacuum of 25 " Hg on a monometer at sea level - so I would say this value is 25 " Hg

Atmospheric pressure - 29.92 " Hg -

Would this be correct or where am I off - If I saw a value of 25 " Hg, how do I differtiate it from being vacuum or positve pressre and if vacuum, I do I show it on the same scale as the atmosheric pressure

 

[hacksaw]

HgA refers to absolute pressure at some mercury temperature

Hg by it self, is usually referenced to local ambient(i.e. a u-tube or inclined manometer), at least that is my usage.

there is a lot of ambiguous usage, i am guilty of it in routine conversations as much as any one, it is all context driven and satisfys a need to simplify the information being communicated.

differential manometry is far mor common than abs. meas, but both are important.