[CAUT] Reading low humidity

[Roger Wheelock]

Hi Fred,

With Dickson units we see a fairly repeatable 1% RH drift per year downward 
when compared to a US secondary standard hygrometer we use to calibrate our 
humidistat switches.  I am not sure of the direction of drift for all 
sensors.  My understanding is that airborne pollutants coat the plates in 
the sensors, giving a variation in the reading with time.  I believe most 
sensors measure capacitance of the air between two small closely-spaced 
plates.  I think there are also some that measure resistance.

Roger

----- Original Message ----- 
From: "Fred Sturm" <fssturm@unm.edu>
To: "College and University Technicians" <caut@ptg.org>
Sent: Thursday, March 02, 2006 2:11 PM
Subject: Re: [CAUT] Reading low humidity


> 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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