On Wednesday 14 February 2007 15:27, [EMAIL PROTECTED] wrote: > I've been trying to figure out how to interpret the temperature data > I'm reading from the Sensirion SHT11 on the Moteiv Tmote Sky. Do I > need to calibrate each individual sensor myself? I've noticed that > different SHT11's give me different raw readings even when they're > placed in the same area. If I need to calibrate them myself, are there > any tools to help calibrate a large number of the sensors?
You should download their data sheet for more information. Some notes: The sht1x/7x are factory calibrated. Exposure to extreme temps or certain chemicals can require a 'reconditioning' procedure to return the unit to calibration. How much different are the readings? For a 14-bit reading,a difference in the raw sensor value of 100 equates to 1C. Given an accuracy of +/- 0.4C @ 25C, values varying by +/- 40 points would be "dead on". Also, the y-intercept of the temperature formula varies with voltage. Because the TMote is powered directly from the batteries (if not on USB), this could add perhaps another 10-20 points of variation. I've read that most people cannot distinguish a temperature difference of less than a couple of degrees. So, the area you think has a uniform temperature may not. I ran into this at my desk, where shifting a device housing an sht15 a few inches closer to my monitor would increase its temperature reading by ~ 1.5C. I couldn't discern the difference myself. The TMotes have a relatively large thermal mass. We had a couple here, and I recall it could take minutes for a temperature change to be fully reflected. Finally, these devices appear more prone to self heating than suggested in the data sheet. They claim the parts can be read at twice per second, but I would see self heading effects reading the part once every couple of seconds. Hope this helps, Steve FYI, we've discussed this topic before. You can find a lot of good information by searching the list archives. www.tinyos.net, or Google. _______________________________________________ Tinyos-help mailing list [email protected] https://mail.millennium.berkeley.edu/cgi-bin/mailman/listinfo/tinyos-help
