[Hai-users] Temp sensors accuracy again.

[Ken Schumm]

We used to use an Analog Devices current source temperature sensor. They sold in a wide range of accuracies, from IIRC +/- 5 degrees to +/- 0.1 degrees. I forget the exact numbers but the accurate ones were like 100 times more expensive than the cheaper ones. Then, to get the accuracy you had to use high precision parts in the entire analog circuit. For example if you had an 8 bit A/D and were measuring an indoor temperature range of 50 degrees (total range of 50-100 degrees) a single bit error would throw you off 2 tenths of a degree, so we had to use a 10 bit A/D. And to eliminate the need to calibrate we needed to eliminate slop caused by resistors so we had to buy 1% precision instead of the 5-10% popcorn parts. So the end product ended up costing something like 300% more. Our system was already at the high end of the price curve so it was deemed impractical.

The $20 Radio Shack thermometer you mention has an advantage over any field wired sensor. The length of the wire is known and it can be calibrated in a factory. The use of any of the cheap voltage style sensors can be made accurate if the total system, including wire length, is known. In a custom home the wire length cannot be known. Longer wires cause more voltage drop which can greatly affect accuracy. In this type of system a different type of sensor has advantages, like a current source (current varies with temperature) or frequency (frequency varies with temperature). Those types of sensors are more expensive but relatively immune to different lengths of wire used to hook them up.
  ----- Original Message ----- 
  From: Brad 
  To: 'Ken Schumm' ; hai-users at tssi.com 
  Sent: Friday, January 13, 2006 1:58 PM
  Subject: RE: [Hai-users] Temp sensors accuracy again.


  Ken, not sure what you mean by expensive parts, but I sure don't consider what the temp/humid sensor HAI sells as cheap.

  It's sad, when Radio Shack can sell $20 temp/humid displays that all hit the correct temps and humid % almost dead on.

   


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  From: hai-users-bounces at tssi.com [mailto:hai-users-bounces at tssi.com] On Behalf Of Ken Schumm
  Sent: Friday, January 13, 2006 2:24 PM
  To: hai-users at tssi.com
  Subject: Re: [Hai-users] Temp sensors accuracy again.

   

  I used to work for a company that manufactured a proprietary commercial building control system. We tried for years to come up with an inexpensive and reliable way to accurately measure temperature to +/- 0.1 degree that didn't require field calibration. The only way we found to solve that problem was to use expensive components (sensors, precision resistors, etc) that significantly raised the price of the systems. Marketing balked at the price and it never did happen.

   

  Not a solution to your problem but if HAI can't solve the problem maybe they need to come up with an easy and supported way to field calibrate the parts.

   

    ----- Original Message ----- 

    From: Brad 

    To: hai-users at tssi.com 

    Sent: Friday, January 13, 2006 10:19 AM

    Subject: [Hai-users] Temp sensors accuracy again.

     

     Just installed 3 HAI temp/humidity sensors. Also installed 3 HAI OmniStats with remote temp sensors. The Thermostat remote sensors and the Temp/Humidity sensors are mounted on inside walls within 6 inches of each other, each set in a different room. Temp/Humidity sensors show temps being 3-5 degrees higher than the thermostats show.

    Apparently HAI STILL has not resolved the issue of the Temp/Humid sensor's being inaccurate.

    The Humidity is reading low on them as well. Very frustrating.

    Anyone else seeing this?


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