# Re: [Vo]:HP474AC probe measures Relative Humidity, not steam quality

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On 01/22/2011 10:13 PM, mix...@bigpond.com wrote:
> In reply to  Jeff Driscoll's message of Sat, 22 Jan 2011 21:24:36 -0500:
> Hi,
> [snip]
>
>> This probe does not measure the amount of liquid water droplets in the
>> "steam" (ie. mass fraction of water vapor to to total water).  It measures
>> Relative Humidity (Relative Humidity measures how saturated the air is for a
>> given temperature).
>> What we want is a a device that measures "quality" of the steam.  For
>> reference, 100% quality = 100% vapor.
>>
> Consider that droplets can't form unless the RH is 100%. Anything less than
> that
> and no droplets form.
> In short if they measure an RH < 100% then the steam must be dry.
>   ```
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If it's pure steam (no entrained air) at 100 C, then the RH must be
100%, a priori, since the vapor pressure of water at 100 C is 1
atmosphere.  And at 101 C, pure steam will have a RH very close to 100%,
since it's only 1 degree above its dew point.  So it's not clear to me
what a RH meter is going to tell you that you don't already know,
particularly given that the meter is +/- 3.5 % at 99% RH.  (That assumes
they're in Bologna, which is only 54 meters above sea level, according
to some website or other.  If they were up in the mountains, the RH of
101 C steam would be lower, since the boiling point of water would be
lower, and we might expect 101 C steam to be drier, in general.)

Furthermore, I'm not sure it's correct to say <100% RH implies there
can't be entrained droplets.  The drops don't form in the steam, they're
blown/broken/pulverized off the surface of the water and carried off
with the steam.  They can be carried off with a stream of air at < 100%
RH just as easily as they can be carried off by a stream of pure steam.
If it's <100% RH at the point where you measure it, then some way
downstream the steam may be expected to cool as the droplets evaporate,
what you think it does.

> Regards,
>
> Robin van Spaandonk
>
> http://rvanspaa.freehostia.com/Project.html
>
>
>

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