[QUOTE=Kennebec Captain;187622]A few years back *we got a new fire main pressure gauge in MPa. What the? * Looking it up; *1 MPa = 10 bar.[/QUOTE]

True.

That had me scratching my head, *till I went to log the barometric pressure which is measured in millibars (mb). Atmospheric pressure varies of course *but is about 1000 millibars or 1 bar. I know atmospheric pressure at sea level is about 14.5 psi so that gives a number to use for rough conversions.

True. 1 atm=14.696 psi, 1 bar=14.504 psi

1000 mb = 1 bar. (about atmospheric pressure)

*1000 mb is about 14.5 psi.

So 1 bar is about 14.5 psi

Yes, almost exactly 14.5 psi

Bar it is is not an official SI unit but it is *based on the metric system so *bar can be converted exactly to the *pascal which is the official SI unit of pressure. Pascal (Pa) is a whimpy European unit so it takes a lot to amount to anything, 100,000 Pa just to get to 1 bar, so 10 bar is *1,000,000 pascals or 1 MPa, which is the units the new gauge uses.

All true.

So;

*

Atmospheric pressure is about 1000 millibars or 1 bar.(from the barometer)

Atmospheric pressure is about 14.5 psi. (from grade school science class).*

10 bar=1 MPa (had to look it up)*

So if 1 bar (0.1 MPa) *is about atmospheric pressure *or 14.5 psi than:

*[U]1 MPa is about 145 psi.[/U]

Yes. 1 MPa is 145.038 psi

So on the fire main if the gauge reads *0.5 MPa thatâs about 70 psi, *100 psi is going to be about 0.7 MPa etc.

True: 0.5 MPa=72.519 psi, 100 psi=0.689 MPa

This gauge is in kPa, 10,000 kPa (10 mPa) *should be about 1450 psi.

True: 10â000 kPa=1450.377 psi

So your math is all good. Itâs important to know that pressure is measured in different ways, which make certain readings not directly comparable to others. âGauge pressureâ is pressure relative to the current atmospheric pressure. If I have a pressurized tank, and a low pressure weather system moves in, the gauge pressure inside the tank will be higher, even though nothing inside the tank has changed. If my gauge pressure is âzeroâ it means that my tank is equal to the current atmosphere, not that it has a vacuum.

The second type of reading is âAbsolute pressure.â When this reading is zero, my tank is experiencing hard vacuum. When my tank is 14.5 psi, then itâs about the same as atmosphere, and if it reads more than that, it is under pressure.

The third type of reading is âDifferential pressure.â If you have a gauge in parallel with a pump or an orifice, it is measuring the difference between the inlet and outlet pressure. If the inlet pressure on my pump is Absolute 14.5 psi, and the outlet pressure is absolute 24.5 psi, the differential gauge will show 10 psi.

The fourth type of reading is called âSealed pressure.â Iâve never seen this one used. Apparently it reads the pressure relative to some chosen fixed value. Letâs imagine that we are making a CO2 extinguisher to use in a hyperbaric chamber. We might want the gauge to read relative to some number higher than atmosphericâŚ But why wouldnât you just use a differential pressure gauge? I donât know why we have this, but we do.