Dear Matthew and David, (01)
DL> Hence we have two different instances of kind of quantity, for
> which the same unit is used. Pressure is a quantity for which zero
> is well defined, and there is no problem in regarding the bar, psi
> or Pa as firstly a magnitude of absolute pressure. If it is assumed
> that there is an addition operator for the kind of quantity absolute
> pressure, then the same unit can be used for the kind of quantity
> gauge pressure. This assumption is usually unstated but I believe
> that it is necessary for the use of the same unit for both kinds
> of quantity. (02)
If you have a scale with an absolute zero, such as temperature
or mass, you can still talk about intervals, such as adding
an extra 10 degrees of temperature or 10 grams of mass. (03)
But for interval measures, such as time and space, there is
no absolute zero, and only intervals are meaningful. The
choice of "zero" is an arbitrary convenience. (04)
John (05)
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