Under Pressure
Dave Jesse on August 1, 2017


I couldn’t resist the opportunity to quote from Queen and Bowie. Or just show my age as I remember Under Pressure as a new song, not a golden oldie.

Actually we’re going to have a quick look at pressure measurements. Like this:


Image One
Photo: ThenAndAgain


Pressure Units

As the gauge above (I cannot bring myself to use the American spelling shown in the photo) shows, pressure is often measured in Pounds per Square Inch, or psi.

The SI metric unit is the Newton per Square Metre or Pascal. The problem is that a Newton force is about the same as the weight of an apple, and a metre is a long thing, so if you think of an apple sliced so thinly that you can spread it all across a one metre square you get a very small pressure. Sorry, M. Blaise Pascal, but your unit of measurement is really too small to be useful. OK, we can multiply by 1,000 (what else do you do with SI units?) but even so, it’s not big enough.

The more useful metric unit is the Bar, or 100,000 Pascals. This turns out to be about an atmosphere and the ISA pressure at sea level on a standard (15C) day is 1.01325 Bar. More commonly expressed as 1013.25 mBar, so a mBar is 100 Pascal. Still with me?

Pressure Datums

And here is the point of the blog. Pressure gauges are used in the air, which has a pressure of its own. You therefore need to know whether the pressure is absolute or with respect to the ambient air. Whatever units you use, there will be a value of zero like this:


Image 2
Photo: KC Tools



When the pressure is very high, the effect of the atmospheric pressure is insignificant, but if it’s a low pressure like this…


Image 3


…it makes a real difference whether zero is with respect to an absolute vacuum or the pressure of the atmosphere. ISA pressure at sea level is 14.7psi, which would be barely visible on the high pressure gauge, but would saturate the low pressure gauge.

The difference is identified by recording pressures as psia (pounds per square inch absolute) where the pressure is recorded as 14.7 psia on a standard day at sea level, and psig (pounds per square inch gauge) where the pressure is recorded as 0.0 psig under the same conditions.

Why does it matter?

Simply put, a pressure in psi is ambiguous, while values in psia or psig are not. Flight Data Services have recently completed an exercise to check all the engine pressure measurements we receive and, where the value reads about zero when the engine is not running, we gave it the units psig, and where it read about 14, we called it psia.

Of course, as those of you now familiar with flight data will have already anticipated, life is not always so simple. Airbus A320 family aircraft have a precooler inlet pressure that measures about 7psi on the ground. Now, is that a high zero or a low 14? If any engineers out there know whether precooler inlet pressure is absolute or gauge, please do tell me!