"Karl Denninger" wrote in message
...

You and I both know that's not correct - that their "zero" is really
14.7psi, but nonetheless, their "zero" is calibrated at 14.7psi.

No, their zero isn't calibrated at any pressure. "Gauge" pressure is always
a differential measurement, the difference between what the pressure
"outside" the gauge is and what the gauge is measuring.

The 14.7 value is only applicable at sea level. Go up into the mountains
and the difference between absolute and gauge will be significantly less.
Dunk the gauge underwater and it will be significantly more.

When working with physical characteristics of materials it is necessary to
work with absolute measurements: absolute pressure and absolute
temperature. On these scales, there is no negative. The lowest temperature
is zero, the lowest pressure is 0.

At least with temperature the convention is to use negative numbers when
referencing values below the datum on the "non-zero" scales. With pressure
and acceleration we have the unfortunate habit of giving a different name to
the portion of the axis that lies below the datum. The magnitude is usually
the same, however. A "zero" amount of "deceleration" is the same as a zero
amount of acceleration, that is, the velocity isn't changing. A zero amount
of vacuum should be the same as a zero amount of pressure (gauge).

"Absolute" or "total" vacuum are terms that are often used to represent as
low as you can go, or zero on the absolute pressure scale. What that
represents on the "negative gauge" scale is totally dependant on what the
pressure is that you are referencing as "zero".


Rod McInnis