Recently I was involved in test of the suction pressure of pump, that faulty because cavitations. It was an interesting experience and I noted that not everyone understand requirements for pump’s inlet pressure from manufacturers. I made a diagram above to add more clarity for everyone and added below my answers for most popular questions about vacuum pressure.
So first of all, there are two types of pressure we use in our practice:
– Absolute pressure.
This is a pressure that is relative to absolute vacuum. This is why the absolute pressure can not be negative!!!
– Relative pressure.
This is a pressure that is relative to the sea level. This pressure can be negative, but is limited to the absolute vacuum value.
I think this is logical and should be clear, if not – take a look to diagram above.
The relative pressure we use in pressure gauges dials. In hydraulic system the positive pressure measured in PSI, bar, kPa, etc. units. But the negative pressure dial scale is mostly in inches of mercury (inHg) – a unit of atmospheric pressure, most common used in the North America (inHg is a non-SI unit).
Here is a good example of near zero pressure gauge with PSI scale for the positive pressure and inHg scale for the negative pressure measurements:
PSI is a general term for pressure measurement (Pounds per Square Inch), while PSIG specifically refers to gauge pressure (Pounds per Square Inch Gauge)
Like I said, the atmospheric pressure is always absolute and relative to the absolute vacuum; we measure the values in ‘atm’, ‘inHg’ (inches of mercury) or ‘PSIA’ (Pounds per Square Inch Absolute).
0 PSIA = 0 atm = 0 inHg – it is the Absolute Zero Pressure
1 atm = 29.92 inHg = 14.7 PSIA
In our calculations of hyd systems we operate with units such as BAR, PSI, MPa with known dependencies:
1 bar = 14.5038 PSI = 0.986923 atm = 29.53 inHg = 0.1 MPa = 100 kPa
You are free to use our Unit Convertor for a quick and an accurate calculations.
Ok, now it’s time for exercise!
Parker P1 pumps have a minimum inlet pressure 0.8 bar absolute (see below the table from Parker catalog). What number you can find on pressure gauge (image above, inHg units) to match this value?
Leave your answer in comments!!!
