[dsg-svt] Dew point calculation

[Peter Bonneau]

For the hardware interlock system, we use the magnus formula.

However to have a sensible number, RH must always be a positive number.

The formula fails with a negative number. 
<https://www.calcunation.com/calculator/dew-point.php>

The majority of the EPICS sensors are showing slightly negative values.

In the hardware interlock system, we limit the lowest possible RH to 
.01% for the calculation.

Also, we currently display the dew point temperature differential on the 
screens.  Showing the actual dew point rather than the differential 
would make it more clear for the users.






On 5/21/2018 3:12 PM, Brian Eng wrote:
> This is only for the software (VME) interlocks...
>
> The simple one should really only be used when RH is >50%
> So they should probably all be changed to magnus.
> The formula looks like it's calculating the dew point correctly on the expert screen.
>
> I'm not so sure about the dew point interlock though, that seems wrong.
>
> As an example for SB6 H1
> RH ~ 2%
> T ~ -10C
> Dp ~ -35C (magnus, simple says -5C)
> DewTempDiff ~ 50C  (shouldn't it be 25C?)
>
> ----- Original Message -----
>> From: "Yuri Gotra" <gotra at jlab.org>
>> To: "dsg-svt" <dsg-svt at jlab.org>
>> Sent: Wednesday, May 16, 2018 9:18:52 AM
>> Subject: [dsg-svt] Dew point calculation
>> Hello Peter,
>>
>> Could you please check the dew point calculation.
>>
>> In the CSS for T=7 C and RH=5% the dew point difference is 19 C which
>> means the dew point is -12 C.
>>
>> These calculators give the dew point at -30 C.
>>
>> http://hyperphysics.phy-astr.gsu.edu/hbase/Kinetic/relhum.html#c4
>>
>> http://www.dpcalc.org/
>>
>> Thank you
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