Better temperature readings using an aspirated thermometer

Unlike regular thermometers that can get incorrect readings because of the sun’s heat, shading, and airflow, aspirated thermometers isolate the temperature sensor from precipitation and the sun, while providing constant air circulation. Take ten 1-wire T2SS boards and combine them with DS18B20s and you’ve got yourself the start of an aspirated thermometer. A foot of PVC pipe, fans, and the above mentioned parts and you’ll have accurate temperature readings in no time.

[Dave] made his to control a natural gas boiler, pumps, and 11 gas-fired unit heaters for a combined output of 5.3 million BTUs per hour – keeping his greenhouse nice and toasty.

Update: Thanks Firetech for pointing out our silly typo.

15 thoughts on “Better temperature readings using an aspirated thermometer

  1. Take ten 1-write T2SS boards…

    Don’t you mean… Take ten 1-wire T2SS boards?

    The DS18B20′s are easy to work with, and are great for temperature readings.

    I’m working on a home built mini-weather station… this is a great article!

  2. I don’t get what the T2SS boards are for .. He doesn’t even mention them in the PDF. (and did I read correctly the $66 price tag?)

  3. janin-
    It appears that the pdf is a writeup on the construction on a single aspirated thermometer and that multiple (10?) units were needed for multiple greenhouses. The T2SS boards were probably used to tie them all together at the control point. It’s a bit cryptic.

  4. Actually, the write-up is just about the aspirated thermometer design, I haven’t gotten around to writing up the overall system. The T2SS boards mount in or on the unit heaters (depending on the type) to turn them on/off using the thermostat inputs. They are mostly 2 stage and around 220K BTUs/hr. All SSRs are OA5 type (http://www.embeddeddatasystems.com/1781-IO-Modules_c_52.html). The main boiler and circulator pumps are hooked up to an Opto-22 board hanging off the printer port of the controlling PC (runs FreeBSD). User interface is with a web browser.

  5. > UR DOING IT RONG

    Last night it dropped below 15 degrees Fahrenheit here. Glass has a U value (heat loss) of about 1.13. So enough glass to cover 22K+ ft^2 maintained at 62 degrees F inside needed about 1.7M BTUs/hr at that point. The heat shut off shortly after sunrise.

  6. Thanks for giving feedback, Dave. As a side note, since you know electronics, why didn’t you also replace the expensive T2SS + OA5 by a simple DS2406 + a relay ? Maybe cost isn’t an issue to you, but you could save at least $50 per board.

  7. > why didn’t you also replace the expensive T2SS + OA5 by a simple DS2406 + a relay

    I felt that I didn’t really have time to design and build any hardware that I could buy pre-made for a reasonable price. A T2SS board and two OA5S modules came to $86 to allow control of a $1500 heater (e.g. Reznor UDAP 225 2-stage). By contrast, commercially available aspirated thermometers are in the $400 range (and not 1-wire). I had to sell the project to management, and needed basic operation by the middle of September. As of July 1, I had a fair amount of software to write, and although I have written code for lots of first article hardware over the years I have never designed and built a PC board from scratch by myself. The lead time on a small quantity of DS2406s is also far longer than on DS18B20s. And I feel really comfortable with the OA5 switching characteristics (no back-EMF from coil discharge, no sparking contacts, etc).

    As it is, the software is functional but not yet complete.

  8. if you also want to read humidity, add another thermometer that is kept wet (wet bulb thermometer). the temperature difference between a wet and dry thermometer gives relative humidity (and dew point). http://en.wikipedia.org/wiki/Wet-bulb_temperature

    since this thermometer already has a fan, all one would need is a wick and a water container:

    http://www.tifton.uga.edu/tobacco/wetbulb.htm

    http://support.radioshack.com/support_tutorials/weather/weather-18.htm (also contains link to calculation table)

    if both thermometers are in the same housing, the wet one should probably be after the dry one in the air flow.

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