|
OK, this is a nit, but it will make owfs a little
more accurate (admittedly, it’s not very significant difference)… The calculation given by Sensor_RH = (VAD/VDD) -0.16 / 0.0062 And Honeywell gives the following: VAD = VDD (0.0062(sensor_RH) + 0.16), but they
specify that VDD = 5V dc, at 25 deg C. Which is exactly what we have in owfs code (solved
for Humidity, of course). Honeywell’s documentation explains that the
HIH-3600 series humidity sensors produce a liner voltage response to humidity
that is in the range of 0.8 Vdc to 3.8 Vdc (typical) and is proportional to the
input voltage. So, the error is, their listed calculations don’t
correctly adjust for varying input voltage. The .16 constant is 1/5 of 0.8 – the minimum
voltage produced. When adjusting for voltage (such as (VAD/VDD) portion),
this constant should also be divided by the input voltage, not by 5 (the
calibrated input voltage), as shown in their documentation. So, their documentation is a little wrong. The level of error this produces would be
proportional to how far from 5 Vdc your input voltage is. In my case, I
seem to have a constant 4.93 Vdc input, so it doesn’t have a great effect
(about .25 degrees RH) The corrected calculation is below. Regards, Vince PS: Honeywell also admits that there are variations
in response by the sensors and will ship a calibration printout (according to
their documentation) with any of these sensors, if asked. SO, shouldn’t
we be checking that our calculated humidity isn’t >100 or <0, since
they vary, and may be off? ----------------------------------------cut
me----------------------- ---
module/owlib/src/c/ow_2438.c.old 2006-06-07
20:29:26.858959153 -0400 +++
module/owlib/src/c/ow_2438.c
2006-06-07 20:29:59.129448997 -0400 @@ -133,7 +133,7 @@ static int FS_Humid(FLOAT * H , const struct
parsedname * pn) { FLOAT T,VAD,VDD ; if ( OW_volts( &VDD , 1
, pn ) || OW_volts( &VAD , 0 , pn ) || OW_temp( &T , pn ) ) return
-EINVAL ; - *H =
(VAD/VDD-.16)/(.0062*(1.0546-.00216*T)) ; + *H =
(VAD/VDD-(0.8/VDD))/(.0062*(1.0546-.00216*T)) ; return 0 ; } |
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