At 08:19 PM 2/20/2003 +1000, you wrote:
>I wonder if any one has come across a quick and dirty way of
>calculating dew point.
>
>  All the meteorological websites I've been to, offer formulae with
>high precision resolution, something I don't think I really need. I'm
>also trying not to use floating point maths. The main sticking point
>is the log10 function all the other constants I can convert to whole
>numbers.
>
>I'm using the SHT11 sensor from Sensirion and have solved the
>temperature and RH maths, just the dew point is causing me problems.
>
>Sensirion's and others offer the following formula
>
>logEx = 0.66077+(7.5 * t_C) / (237.3 + t_C) + (log (RH) -2)
>
>DewPoint = (logEx - 0.66077) * 237.3 / (0.66077 + 7.5 - logEx)
>
>Where t_C = temperature in celsius
>              RH = true relative humidity
>
>I'm hoping to make this into a greenhouse controller, so I assume
>that the frost point would need to be known - gardening ain't my
>thing :)

If you have a program such as MATLAB, plot the 2-D function in 3-D
so you can get some insight into the nonlinearity compared to where
the dew point number is important, and the range of each variable- such
as logEx. You may be able to use a 2-D LUT with interpolation
to give you DewPoint(t_C, RH).

Implementing log10 isn't that hard either- one general approach is to
transform to reduce the range, then use a series or polynomial to
evaluate the log and correct the result.

I don't know why the frost point would be important in a greenhouse.

Dew point is a good indication of the absolute dryness of the air, since
it is independent of the temperature of a given sample of air , whereas
RH varies with the temperature of that sample.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com

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