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AC_Hacker · 07-02-09, 11:24 AM

You bring up some interesting questions...

> ...and those runs acting like a big antenna.

The data I'm collecting has a degree of 'noise'. I tried to quiet things down with tiny by-pass caps in parallel with the thermistor, but without success. I tried twisting the wire to see if that would help, not so much. I tried re-locating my setup outside my house, to an electrically quieter area, to see if that helped significantly, no love there either. At this point, I can't tell if the noise is external (stray voltage being picked up by the wires leading to the thermistors) or internal (clock noise and/or processor noise). I'm suspecting that the noise is internal. At any rate, I plan to replace the unshielded wires with 1/8 inch coax. I'd recommend the same to you.

Another approach would be to correct the noise in Excel by averaging the readings over equal intervals that are appropriate to your application. For me at this point, my tests are running about 1.5 hours, with samples every second, and the events I am measuring are temperature changes in concrete, so the change is pretty slow. If I average over every 100 samples it should be pretty good.

But even with noisy samples, the data I'm getting is quite usable.

> I would want the thermistors all over the house and would worry
> about the variable resistance of long wire runs.

You could put the thermistors in place and afterword use some reliable temperature standard, like a lab-grade thermometer or boiling water (212 at sea level) and really slushy ice water (32 at sea level) and correct your logged data to reflect the known values.

> Did you account for how long the wire is?

I used the above method.

A really big lesson for me with this project has been the massive inaccuracy of cheap digital thermometers. They can still be useful, but you need a reliable standard to compare them to.

> What is the sampling rate you are using?

As the log-o-matic ships, it will sample from about 150 samples per second (if you read all 8 inputs) to 1 sample per second. My sample rate is at 1 per second.

It would be much better if the firmware was tweaked to have the ARM processor, which has plenty of power, to average every 100 readings, and to allow much longer sampling periods, like every minute or 10 minute or so...

There are tutorials at Spark Fun that encourage firmware-tweaking. If I was more fluent in the C programming language I would take this on.

I hope this helps...

Best regards,

-AC_Hacker

07-02-09, 11:24 AM	#5
AC_Hacker Supreme EcoRenovator Join Date: Mar 2009 Location: Portland, OR Posts: 4,004 Thanks: 303 Thanked 723 Times in 534 Posts	questions on data-logger... You bring up some interesting questions... > ...and those runs acting like a big antenna. The data I'm collecting has a degree of 'noise'. I tried to quiet things down with tiny by-pass caps in parallel with the thermistor, but without success. I tried twisting the wire to see if that would help, not so much. I tried re-locating my setup outside my house, to an electrically quieter area, to see if that helped significantly, no love there either. At this point, I can't tell if the noise is external (stray voltage being picked up by the wires leading to the thermistors) or internal (clock noise and/or processor noise). I'm suspecting that the noise is internal. At any rate, I plan to replace the unshielded wires with 1/8 inch coax. I'd recommend the same to you. Another approach would be to correct the noise in Excel by averaging the readings over equal intervals that are appropriate to your application. For me at this point, my tests are running about 1.5 hours, with samples every second, and the events I am measuring are temperature changes in concrete, so the change is pretty slow. If I average over every 100 samples it should be pretty good. But even with noisy samples, the data I'm getting is quite usable. > I would want the thermistors all over the house and would worry > about the variable resistance of long wire runs. You could put the thermistors in place and afterword use some reliable temperature standard, like a lab-grade thermometer or boiling water (212 at sea level) and really slushy ice water (32 at sea level) and correct your logged data to reflect the known values. > Did you account for how long the wire is? I used the above method. A really big lesson for me with this project has been the massive inaccuracy of cheap digital thermometers. They can still be useful, but you need a reliable standard to compare them to. > What is the sampling rate you are using? As the log-o-matic ships, it will sample from about 150 samples per second (if you read all 8 inputs) to 1 sample per second. My sample rate is at 1 per second. It would be much better if the firmware was tweaked to have the ARM processor, which has plenty of power, to average every 100 readings, and to allow much longer sampling periods, like every minute or 10 minute or so... There are tutorials at Spark Fun that encourage firmware-tweaking. If I was more fluent in the C programming language I would take this on. I hope this helps... Best regards, -AC_Hacker Last edited by AC_Hacker; 07-02-09 at 11:26 AM..