OpenEnergyMonitor Community

emonTH dew point

Hi folks,

Apologies if this has been addressed previously.

I would really like to see dew point on emoncms from my emonTH. At the moment I’m fiddling with a process to approximate it from temperature and relative humidity, but it’s a bit of a pig. I’m using

$T_d = T - \frac{100 - H}{5}$

which is an approximation, allegedly valid above 50% humidity.

Have I implemented it correctly?

Dew point is much more useful to me as a measure of the absolute water content of the air, and should stay the same as the temperature varies. This is intuitively true: heating up the air doesn’t change the amount of water in it. It’s much harder to gauge how much moisture is in the air by looking at relative humidity.

Is there a better way for me to record dew point?


Are you running node-red? if so, and alternative would be to pick up the temperature and humidity in node-red via MQTT, and pass it through a function node containing the code below. The resultant payload can be fed back by MQTT to emoncms for charting & storing.

        // Replace these 2 example readings with inputs from your emonTH
		var tem = "3";
		var hum = "10";
            // to the maths...
  		    var temp = -1.0*tem;
		    es = 6.112*Math.exp(-1.0*17.67*temp/(243.5 - temp));
		    ed = hum/100.0*es;
		    eln = Math.log(ed/6.112);
		    td = -243.5*eln/(eln - 17.67 );
		    msg.payload = td.toFixed(2);

        return msg;

PS - I’m assuming you’ve already seen the dewpoint widget in the dashboard?



I am now. Thanks, I got it working. That was surprisingly straightforward…! I’ll wire it into emoncms for graphing tomorrow.

Yes I have been using the dew point widget, but I really want to see a plot of dew point over time.


If it’s just for visualization, there is a dewpoint widget that does the calculations with temperature and humidity feeds.

See the OpenEMC - An EmonTH mod for woodworkers thread on the old forum for a similar kinda topic (although a little more bespoke).

It might prove easier to modify the emonTH sketch to calculate and send the dewpoint rather than calculate it withing emoncms or farm it out to nodered etc.

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Dear Paul,

In your function, is the temperature in °C or in °F??

I was using °C


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That Dashboard widget is quite useful and worth promoting.