First, thanks for another great project. I've been using the myFP2ESP for quite a while now. Works great. This looks like another winner.
In the documentation you've commented that the HYU21D has high accuracy. I've found that mine has a tendency for self heating. At first I thought maybe it was my enclosure for the sensor but I've tried it in various setups. Both enclosed, in the open air and in various orientations.
Next question to ask, is it just my sample? Maybe, but I did look on the internet and there is discussion with many experiencing the same thing. From power up the initial ambient temperature reading will slowly climb and then settle off. I've used other measuring devices and the initial reading seems to be the correct reading with my particular sample showing a 3 degree Celsius offset after about 30 mins.
You do provide an ambient offset value but this causes the humidity to read low. All this rambling is to suggest that humidity values should likely be recalculated if the ambient offset is not zero. The HTU21D calculates a relative humidity based on what it thinks the temperature may be. If its self heating the humidity reading will be low. I've seen a few percentage points difference in the humidity which also causes a few degrees difference in the calculated dewpoint.
Using the equations from http://hyperphysics.phy-astr.gsu.edu/hbase/Kinetic/relhum.html I back calculated the vapor density and maximum vapor density that the HTU21D values imply. Then calculated an adjusted maximum vapor density at the adjusted ambient temperature. Then assuming the vapor density from the HTU21D is correct I used the adjusted maximum vapor density to get an adjusted relative humidity.
I tweaked the myDCP4ESP web interface to display the adjusted and raw humidty just to show the difference. I left the dewpoint alone but if calculated against the raw values it would be around -6.9 C for the values in the attached image.
Robert,
First, thanks for another great project. I've been using the myFP2ESP for quite a while now. Works great. This looks like another winner.
In the documentation you've commented that the HYU21D has high accuracy. I've found that mine has a tendency for self heating. At first I thought maybe it was my enclosure for the sensor but I've tried it in various setups. Both enclosed, in the open air and in various orientations.
Next question to ask, is it just my sample? Maybe, but I did look on the internet and there is discussion with many experiencing the same thing. From power up the initial ambient temperature reading will slowly climb and then settle off. I've used other measuring devices and the initial reading seems to be the correct reading with my particular sample showing a 3 degree Celsius offset after about 30 mins.
You do provide an ambient offset value but this causes the humidity to read low. All this rambling is to suggest that humidity values should likely be recalculated if the ambient offset is not zero. The HTU21D calculates a relative humidity based on what it thinks the temperature may be. If its self heating the humidity reading will be low. I've seen a few percentage points difference in the humidity which also causes a few degrees difference in the calculated dewpoint.
Using the equations from http://hyperphysics.phy-astr.gsu.edu/hbase/Kinetic/relhum.html I back calculated the vapor density and maximum vapor density that the HTU21D values imply. Then calculated an adjusted maximum vapor density at the adjusted ambient temperature. Then assuming the vapor density from the HTU21D is correct I used the adjusted maximum vapor density to get an adjusted relative humidity.
I tweaked the myDCP4ESP web interface to display the adjusted and raw humidty just to show the difference. I left the dewpoint alone but if calculated against the raw values it would be around -6.9 C for the values in the attached image.
Thanks again.