[CAUT] Reading low humidity

[Michelle Stranges]

I *think* this is the one Don McKechnie uses. (and I know he has a PC  
for it's findings...)
I'm sure he'll post here in no time!

:)

On Mar 2, 2006, at 2:11 PM, Fred Sturm wrote:

> 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
>>
>
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