Humidity calculation using wet and dry bulb

[citizen423]

I am having trouble believing how complicated this is. I'm using CLX and I wish I could find some sample code or easier formulation to calculate RH from a wet bulb and dry bulb thermocouple. Any one ran across this before?

Thanks a bunch!!!

 

[Andybr]

Unfortunately it really is that complicated. In order to determine RH from wet and dry bulb temperatures you need to plot these on a psychrometric chart. Converting this into some sort of look up table in a PLC would require a substantial amount of memory and is probably not worth the effort. You would be better off using a purpose made RH sensor.

 

[danw]

Are you sure there's no function block for wet/dry bulb RH?

The Honeywell has an RH function block (below) and most of the function blocks are in CLX.

 

[BobB]

Does the CLX have a linear approximation function? If so it is very easy to plot fixed values in memory and then use the function to extrapolate the values along the curve.

 

[BeeRing]

I just month ago has same problem, and some googling gave me this link:
http://joseph-bartlo.net/articles/070297.htm
which i translated into my machine math.
I can say that it works well, as i verified simulated results with tables and now in real action too.

 

[shooter]

you can also find this calc in the text version at www.oscat.de
wetbulb is different from dewpoint.
if needed more look at joseph-bartlo.net
or write here a next question, like what type of plc you are using.

 

[lostcontrol]

Hi,
I know this is an old thread, but I am finding myself in the same position as the OP. I the Dry & Wet Bulb temperatures already & can probably safely estimate the pressure.

I have a bunch of intense formula's (for a PLC anyway) that I am working through in Excel (from VBA samples) so that I can implement in the PLC, but am also wondering if there is an easier way.?
The examples I have start with conversion to Kelvin, Saturation Vapour Pressure, Humidity Ratio, all to get Dew Point.
With the DewPoint, the Humidity can be calculated.

 

[Tom Jenkins]

This is a major problem, and there isn't a good answer. If you look at a psychrometric chart it is a very non-linear function.

There are some empirical equations available on the ASHRAE site. They are quite complex and probably more accurate than you need. The best approach is to use an empirical method. You can tabulate the RH for various wet and dry bulb temperatures and use linear interpolation to get non-tabulated values. Or, you can use Excel to create a chart ad use the built in create trend functions to develop empirical equations.

Or, as suggested above, you can get a sensor that reads RH directly. I have had decent luck with the ones from Omega Engineering.

 

[BeeRing]

If you have RH sensor, this topic is not for you. As once I had dry and wet temperature sensors, for me it was much easier to use this math rather than tables:

Lv:= 2500800.0-2370.0*Tw;
Ew:= 6.112*EXP_REAL(17.67*Tw/(243.5+Tw));
Rw:= 0.62197*Ew/(P-Ew);
Ri:= Rw/2.0;
R:= Rw+(1006.3+Ri*1850.0)/Lv*(Tw-T);
Ri:= (R+Rw)/2.0;
R:= Rw+(1006.3+Ri*1850.0)/Lv*(Tw-T);
Ri:= (R+Rw)/2.0;
R:= Rw+(1006.3+Ri*1850.0)/Lv*(Tw-T);
Ri:= (R+Rw)/2.0;
R:= Rw+(1006.3+Ri*1850.0)/Lv*(Tw-T);
E:= R*P/(0.62197+R);
X:= LN_REAL(E/6.112);
Td:= 243.5*X/(17.67-X);
Es:= 6.112*EXP_REAL(17.67*T/(243.5+T));
rH:= 100.0*E/Es;

- you put in T, Tw and P (temperature, wet temperature, pressure(mbar))
- you get out Td, rH (dew point temperature, relative humidity(%))
- this is for Celsius as I work in Europe
- as I dont had pressure sensor, I used it as constant 1013.0 mbar (near sea level)
- as you see, this all is real type or floating point math
- this piece is written in "structured text" in Unity Pro

 

[lostcontrol]

From where did you get your formulas?
They look a lot simpler than what I have previously found, & look easier to implement in a PLC, so will give it a crack in the CLX..

 

[BeeRing]

If you look at my post from year 2011, you see link to source, that no longer exists and I have no copy. Of course, not easy to explain how they work. But you can calculate results and compare with tables to check it. Also you see iterration with Ri, and there you can put in additional iterration, if it makes any sense of precision for you.

 

[lostcontrol]

If you look at my post from year 2011, you see link to source, that no longer exists and I have no copy. Of course, not easy to explain how they work. But you can calculate results and compare with tables to check it. Also you see iterration with Ri, and there you can put in additional iterration, if it makes any sense of precision for you.

Thanks BeeRing, :beerchug:I have done a quick test & compared it with an online calculator & it is not out by too much, so will try it is a CLX ST routine like yours to see to check the result, but I think this may be acceptable.

 

[jonb42]

The original article can be found on The Internet Archive, https://web.archive.org/web/20040918074926/http://joseph-bartlo.net/articles/070297.htm

Paraphrased here:

During a wet-bulb process, air and water vapor coexist. Some of the vapor is condensed, or water vapor is evaporated in air, saturating it. During these isobaric processes, water vapor content changes, and air volume adjusts accordingly. Vaporization latent heat is responsible for all temperature change (no ice crystals assumed). The governing equation for this process is :
(Md Cpd + Mv Cpv) dT = - Lv dMv [1]

Md : dry air mass
Mv : water vapor mass
Cpd : dry air specific heat, constant pressure
Cpv : water vapor specific heat, constant pressure
Lv : vaporization latent heat
T : temperature
R : water vapor mixing ratio
Rw : saturation mixing ratio for Tw

Calculating dew point and relative humidity using (1) involves the following procedures:
1. Calculate Lv using Tw & [7]
Lv = 2500800 - 2370 T~

2. Calculate Rw using [8] & [10] with Tw & Ew
(= Es with T = Tw)
Es = 6.112 e(17.67 T/(243.5 + T))
R = z E / (P - E)

3. Calculate R~ using [6] (use R~ = Rw/2 initially)
R~ = (R + Rw)/2 T~ = (T + Tw)/2

4. Estimate R using [5]
R = Rw + ((Cpd + R~ Cpv)/Lv)(Tw - T)

5. Repeat 3 & 4 until R converges (should only require 2 or 3 iterations)

6. Calculate E using [11]:
E = R * P / (z + R)

7. Calculate Td using [9]
X = ln(Es/6.112) [9a]
T = 243.5 X/(17.67 - X) [9b]

8. Calculate H using [12]:
H = E / Es

________________________________________

Example : Suppose T = 25.0 °C (77.0 °F), Tw = 16.0 °C (60.8 °F), and P = 1000.0 mb
1) Lv = 2500800 - 2370(16.0) = 2462880
(last 2 digits insignificant)

2) Ew = 6.112 e(17.67 (16.0)/(243.5 + 16.0)) = 18.169
Rw = (.62197)(18.169)/(1000.0 - 18.169) = .011510

3) R~ = .011510/2 = .005755

4) R = .011510 + ((1006.3 + (.005755)(1850))/2462880)
(16.0 - 25.0) = .007794

5) R~ = (.007794 + .011510)/2 = .009652
R = .007767
R~ = .009639
R = .007768
R~ = .009639
R = .007768 (converged)

6) E = (.007768)(1000.0)/(.62197 + .007768) = 12.335

7) X = ln(12.335/6.112) = .70215
Td = (243.5)(.70215)/(17.67 - .70215) = 10.076

8) Es = 6.112 e(17.67 (25.0)/(243.5 + 25.0)) = 31.674
H = E / Es = 12.335/31.674 = .38942

(Credit Joseph Bartlo)

 

[cardosocea]

I've made an AOI to calculate this and uploaded it to Rockwell's code library (hopefully it's available for download). If not, JeremyM here has a copy that was supposed to go on his library (which you'll be able to find on Github).



I used the Stull formula and look up tables to reduce the calculation on the humidity terms. Pretty proud of that. LOL

 

[JeremyM]

^ your version is on the Git.