I have noticed my ~1.5 year old WS-1400 was recently reporting humidity often 20+% lower than most adjacent non-Ambient Weather PWSs and airports. (...)
The problem you write about is described together with my friend here_click
Thanks for the link but that appears to be a different failure mode with a different sensor. I believe I have the HR202L. I did review my station's data and found no discontinuity in humidity around rain events. This implies no water intrusion/condensation to the HR202L surface. Most suspect is the undefined operation as given in the datasheet below 0 deg C and Ambient Weather's assumptions in that operating range. I've been running the following PHP code to apply temperature driven correction and am now seeing closer humidity values to adjacent stations with the original sensor.
// - Correct Relative Humidity which drifts low at low temperatures
$humidity = $_GET['humidity'];
$tempf = $_GET['tempf'];
$dewpointf = $_GET['dewptf'];
if ( $tempf > 50.0 ) {
$humidity_adj = $humidity + 7;
} else {
$correction = round( $tempf*$tempf*0.005 + $tempf*-0.707 + $tempf/54.7 + 28.8 );
$humidity_adj = $humidity + $correction;
}
if ( $humidity_adj > 99 ) {
$humidity_adj = 99;
}
$tempc = ($tempf - 32) * 5 / 9;
$H = (log10($humidity_adj)-2)/0.4343 + (17.62*$tempc)/(243.12+$tempc);
$dewpointc_adj = 243.12*$H/(17.62-$H);
$dewpointf_adj = round((($dewpointc_adj * 9 / 5) + 32), 1);