At 06:45 PM 6/22/2011, Jed Rothwell wrote:
Joshua Cude wrote:
If the meter is giving mass per unit volume of the output, you need
to know the *volume* of the output to get the mass of the steam.
Ah. Here is what you overlooked. It also says that it gives mass of
water per unit of mass. That is "degree of humidity (g/kg), partial
pressure in water vapour in mbar/hPa."
and: "Ethalpy kcal/kg" (Interesting that they use kcal.)
See:
http://www.testo.com/online/embedded/Sites/INT/SharedDocuments/ProductBrochures/0563_6501_en_01.pdf
It also measures "Absolute humidity g/m^3" which is what you had in mind.
- Jed
You do understand, Jed, that this is the Testo meter used by Essen
and Kullander, not the other meter used by Galantini, right?
The absolute humidity is calculated from relative humidity and
temperature and pressure. These are displayed values, not measured
values. Basically, the device does some math for you, based on
certain assumptions. Unfortunately, the assumptions are the very issue here!
Among other things you must notice are the specifications of the instrument:
Notice that calibration certificates are not available for over 95%
RH, nor at any temperature over about 80 C.
One page 6, the list of humidity probes begins. The "robust" probe,
part number 0628 0021, is rated to 180 C. The measurement range
extends from 0 to 100% RH. However, the accuracy is not rated above
98%. Basically, the accuracy is 2%, from 2 to 98% RH.
Technically, wet steam would have a "relative humidity" above 100%,
that is, the water content of the mixture would exceed that of pure
water vapor, defined, as I'm correct, as 100% humidity at the boiling
point. If I've got this right!
But the meter has no capacity to measure that excess water, it would
simply peg at 100%, it seems.
I see no sign, anywhere, of any expert opinion that RH meters have
any application to the measurement of steam quality. Instead, as Cude
and others have pointed out, there are complex methods described to
measure steam quality. If you could do it with an RH meter -- and the
devices described are merely fancy RH meters that will do some math
for you based on temperature and pressure and RH data -- all these
other methods would be unnecessary, and I'd expect the meter
manufacturers to advertise the function, and even provide a direct
calculation mode.
