Using Psychrometric Charts in Labview to calculate Humidity?

[ryan_cork]

I'm constructing a chilled mirror hygrometer for a college project and controlling it through labview. I will have the dew point temperature and dry dry bulb temperature I need to calculate the humidity through an algorithm or a look up table for psychrometric charts. can anyone help please.

[Zac Fisher - NGC · Accepted solution]

Equations necessary to calculate relative humidity from pressure and dry and wet bulb temperatures can be found here:
http://www.srh.noaa.gov/elp/wxcalc/formulas/rhTdFromWetBulb.html
With the two temperatures and the pressure coming in as variables, I would break the calculation down just as they have on the webpage linked, by first calculating the saturated and wet bulb vapor pressures ==> then calculating the actual vapor pressure ==> then the relative humidity. All the operators you need for these calculations should be under the math operators function palette. You could call out all the individual operations including the exponentials, or just use a few formula nodes.
Of course, just dump everything into a while loop (with a time delay), to make the system constantly monitor and update the humidity.

[ryan_cork]

Thanks for that problem solved

[DanThomas]

Here is a VI that calculates RH using wet & dry bulb temps. I don't remember where I got it. Maybe it'll help.

This reply didn't post the first time, so I'm trying it again.

[M._Arins]

Please put the sub VI. Thanks.

[HVAC_master]

Calculating wet bulb from dew point and ambient temperature is hard.  It requires iterative algorithms.  It is easy to calculate RH, absolute humity and enthalpy.

I have attached a paper I find usefull, "Numerical calculations of psychrometric properties on a calculator" by Singh, et al.  

If I find an algoritm for wet bulb  or figure out how to  implement Singh's I will upload my VI.  

[taak12]

For calculations with relative humidity the trial version on www.psychrometricchart.net is also very usefull.

[mohitsoni95]

Can anyone please help with how we could use lookup tables for calculating dew point temperature or other parameters? 

Basically, I want to know how we could use lookup tables.

[altenbach]

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@mohitsoni95 wrote:

Can anyone please help with how we could use lookup tables for calculating dew point temperature or other parameters? 

Basically, I want to know how we could use lookup tables.

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If you have a lookup able, you don't need to calculate anything, you just need to look up values. Do you also want to interpolate?

What "other parameters" are you interested? You need to be more specific, we cannot read your mind!

What have you tried? Do you have the relevant tables?

[HVAC_master]

A few years back I made an excel plugin to do Psych calcs.  Someone after me translated it into a python script which if works you might be able to integrate into labview, or you can use the excel plugin to generate look up table perhaps.  
http://wcec.ucdavis.edu/resources/software-resource-applications/

The chapter of the ASHREA book is posted as a PDF in the zip file.  

If you just need to do one implementation you may be able just to use the equations from the book.