Honeywell has updated and simplified their datasheets. I use the formula
straight from the datasheet:
http://sensing.honeywell.com/index.cfm/ci_id/142534/la_id/1/document/1/re_id/0
Voltage output (1st order curve fit) VOUT=(VSUPPLY)(0.0062(sensor RH) +
0.16), typical at 25 ºC
Temperature compensation True RH = (Sensor RH)/(1.0546 – 0.00216T), T in ºC
Earlier versions also had a complicated quadratic temperature compensation
(2nd order curve fit), which I used for a while (and documented in that web
page). Since they don't publish the parameters for the 2nd order curve fit
any more, and the datasheet graphs look pretty straight to me, I've gone
with these.
One issue is the offset voltage (0.16V) in the formula. Willy Robinson
tested the (earlier) HIH3600 and stated that the offset should be scaled to
supply voltage as well, rather than being a fixed offset as in this formula.
We do that for the HIH3600, but I have no reason to change the Honeywell's
specifications for the HIH4000 without some actual measurements. The Datanab
people also use a fixed offset in their HIH4000 calculations.
Paul Alfille
One thing I did find odd in the code but is unrelated is that the
> temperature compensation for the HIH4000 seems to be using the same
> formula as for the HIH3600 instead of its different one as presented
> here:
>
> http://owfs.org/index.php?page=temperature-compensation
>
> From ow_2438.c
> FS_Humid_3600:
> humidity_uncompensated = (VAD / VDD - (0.8 / VDD)) / .0062 ;
> temperature_compensation = 1.0546 - 0.00216 * T ;
> OWQ_F(owq) = humidity_uncompensated / temperature_compensation ;
>
> FS_Humid_4000:
> humidity_uncompensated = ((VAD/VDD) - 0.16) / 0.0062 ;
> temperature_compensation = 1.0546 - 0.00216 * T ;
> OWQ_F(owq) = humidity_uncompensated / temperature_compensation ;
>
> Whereas from the owfs.org page
>
> HIH3600:
> RH = (Raw RH) / ( 1.0546 - .00216 T )
>
> HIH4000:
> RH = (Raw RH) / ( 1.0305 + .000044 T - .0000011 T^2 )
>
> Copy/Paste error? The difference is significant, of the order of 2
> percentage points of humidity, much larger than the actual temperature
> compensation tends to be.
>
> Pedro
>
>
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