[CAUT] Reading low humidity

[Fred Sturm]

Hi Roger,
	How much annual drift do you see on the Dickson? Is it always in a  
particular direction? Does that apply to any electronic sensor  
(direction of drift)? Is drift caused by "accumulation of  
crud" (sensor getting dirty, picking up air pollution, etc)? Or is  
there something else involved? I've got a bunch of other questions  
bubbling to the surface as well (how does an electronic sensor work  
in physical/electrical/chemical terms . . .), but any enlightenment  
you can provide along these lines would be most appreciated.
Regards,
Fred Sturm
University of New Mexico
fssturm@unm.edu
On Mar 2, 2006, at 11:28 AM, Roger Wheelock wrote:
> Hi Fred and Debbie,
>
> While this device is quite affordable, the spec. sheet lists the  
> relative humidity accuracy between 20-80% RH as +/- 3.5%, rising to  
> +/- 5% at the extremes.  They also list annual drift at 1% and  
> offer no means (that I can see) for recalibration.  The added cost  
> over their temperature-only data logger is $20.  This cost  
> differential means there must be a low-end sensor for humidity,  
> similar to a $25 hygrometer.
>
> By comparison a Dickson data logger is +/- 2% from 0-95% RH.  These  
> drift downward 1% per year.  They can be recalibrated.  The bad  
> news is they cost about $270.
>
> Our experience is that performance improves with increasing price  
> for most electronic hygrometers and data loggers.  We have yet to  
> see one that shouldn't be recalibrated annually.  However, even  
> when a unit drifts, it can give accurate differences between two  
> humidities.  This means that the reading will become less accurate  
> over time but the difference between a max. and min. reading will  
> stay fairly constant.
>
> Roger
> Dampp-Chaser Corp