Atmospheric pressure versus boiler pressure reading

November 25th, 2013, 8:46 am

A bar of pressure is equal to atmospheric pressure at sea level. When the boiler on my Cremina reaches the point it's ready to brew (ie. the pressurestat turns off the boiler), my manometer will reach ~1 bar (1.1 to be precise). Shouldn't it start at 1 bar given the first statement?

#2: Post by HB » November 25th, 2013, 8:57 am

Gauge pressure is relative to atmospheric pressure, i.e., it isn't compared to absolute zero but to atmospheric pressure at sea level. The excerpt below from pressure measurement elaborates on this point:

Pressure measurement (Wikipedia) wrote:Everyday pressure measurements, such as for tire pressure, are usually made relative to ambient air pressure. In other cases measurements are made relative to a vacuum or to some other specific reference. When distinguishing between these zero references, the following terms are used:

Absolute pressure is zero-referenced against a perfect vacuum, so it is equal to gauge pressure plus atmospheric pressure.

Gauge pressure is zero-referenced against ambient air pressure, so it is equal to absolute pressure minus atmospheric pressure. Negative signs are usually omitted. To distinguish a negative pressure, the value may be appended with the word "vacuum" or the gauge may be labeled a "vacuum gauge."

Differential pressure is the difference in pressure between two points.
The zero reference in use is usually implied by context, and these words are added only when clarification is needed. Tire pressure and blood pressure are gauge pressures by convention, while atmospheric pressures, deep vacuum pressures, and altimeter pressures must be absolute.

November 25th, 2013, 10:04 am

Thanks, Dan. That makes sense.

#4: Post by **jonr** » November 25th, 2013, 10:29 am

The zero reference in use is usually implied by context, and these words are added only when clarification is needed.

Which apparently it usually is when talking about espresso boilers. Best to always state absolute or gauge.

Note that the difference isn't exactly 1 bar - it varies with the weather and altitude and this creates a small temperature error where a pressure stat is used.