DEW formula

siggibald · Feb 18, 2015

hihi

i'm programing micrologix 1100 plc for drying room and i have to use this formula for calculate dew

is that possibel

Aabeck · Feb 18, 2015

If each step of the calculation can be done individually with the step result saved to a floating point number, then do the next step & save that, then continue until you have the final result

IE:
17.62 * T = F8:1
243.12 + T = F8:2
F8:1 / F8:2 = F8:3
RH / 100 = F8:4

etc...

siggibald · Feb 18, 2015

Aabeck said:
If each step of the calculation can be done individually with the step result saved to a floating point number, then do the next step & save that, then continue until you have the final result

IE:
17.62 * T = F8:1
243.12 + T = F8:2
F8:1 / F8:2 = F8:3
RH / 100 = F8:4

etc...

okey bud how do you use the ln in the formula ??

Bob O · Feb 18, 2015

Check to see if the Natural Log [ln x] is in the math tool set in the ML1100.

siggibald · Feb 18, 2015

Bob O said:
Check to see if the Natural Log [ln x] is in the math tool set in the ML1100.

no it is only aloud for 1400

Bering C Sparky · Feb 18, 2015

I have never tried it but this is supposedly a simple formula that will give you a approximate Dewpoint (Td).

Td = T - ((100 - RH)/5.)
This would be very easy to calculate in a PLC, if it is a close enough approximation to work in your application.

Here is a link that you can read more on this formula.

http://iridl.ldeo.columbia.edu/dochelp/QA/Basic/dewpoint.html
BCS

Tom Jenkins · Feb 19, 2015

I used my handy dandy Excel trick to get a polynomial that will likely give you results as accurate as your RH measurement.

DTan · Feb 20, 2015

siggibald said:
okey bud how do you use the ln in the formula ??

Remember that "Ln(X)" is the same as "Log(X) base e". "Log(X)" is just a shorthand for "Log(X) base 10". You can calculate the value of Log(X) base Y by using the change of base formula. See http://www.purplemath.com/modules/logrules5.htm for an example.

In this case, you'd wind up with the image attached:

Timbert · Feb 20, 2015

Bering C Sparky said:
I have never tried it but this is supposedly a simple formula that will give you a approximate Dewpoint (Td).

Td = T - ((100 - RH)/5.)

That formula works okay if RH>50%.

Here in the desert where you get low RH it is worthless.

For example: At 20C and 50% RH, the difference is less than a degree.

At 40C and 20% RH, the more accurate formula is 12.78C while this simple formula is 24C, over an 11 degree difference.

So, I'm assuming the drying room is more like the desert than the Bering Sea.

siggibald · Feb 22, 2015

thanxs !! The system works like mother ****er now

DEW formula

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