Finally got around to trying out the resistance temperature detector. The nominal resistance is 1K, and calculation shows the best sensitivity for an RTD/fixed resistor divider is obtained with a 1K fixed resistor, so I used that and read it out with the Arduino. I also set up a divider with a thermistor (nominal resistance 10K) and a 10K resistor.

A couple of things are evident from a comparison of their outputs immediately after applying voltage. Here “v” means the voltage read on the ADC, where 0 means 0V and 1023 means 5V. T is the calculated temperature shown in degrees:

RTD v = 530.00 T = 19.70 C thermistor v = 578.00 T = 19.13 C RTD v = 530.00 T = 19.70 C thermistor v = 577.00 T = 19.22 C RTD v = 531.00 T = 20.81 C thermistor v = 577.00 T = 19.22 C RTD v = 531.00 T = 20.81 C thermistor v = 577.00 T = 19.22 C RTD v = 531.00 T = 20.81 C thermistor v = 577.00 T = 19.22 C RTD v = 532.00 T = 21.92 C thermistor v = 577.00 T = 19.22 C RTD v = 532.00 T = 21.92 C thermistor v = 577.00 T = 19.22 C RTD v = 532.00 T = 21.92 C thermistor v = 576.00 T = 19.30 C RTD v = 533.00 T = 23.03 C thermistor v = 576.00 T = 19.30 C RTD v = 533.00 T = 23.03 C thermistor v = 576.00 T = 19.30 C RTD v = 533.00 T = 23.03 C thermistor v = 576.00 T = 19.30 C RTD v = 533.00 T = 23.03 C thermistor v = 576.00 T = 19.30 C RTD v = 533.00 T = 23.03 C thermistor v = 576.00 T = 19.30 C RTD v = 533.00 T = 23.03 C thermistor v = 576.00 T = 19.30 C RTD v = 533.00 T = 23.03 C thermistor v = 576.00 T = 19.30 C RTD v = 533.00 T = 23.03 C thermistor v = 576.00 T = 19.30 C RTD v = 533.00 T = 23.03 C thermistor v = 576.00 T = 19.30 C

First, notice that at room-ish temperature, a change of 1 in the signal (like 530 to 531 on the RTD, or 578 to 577 on the thermistor) makes a 1.1°C change in the calculated temperature for the RTD, versus a 0.1°C change for the thermistor. Precision is awful, in other words! But that’s at room-ish temperature. At balloon altitude the RTD precision is somewhat better, while for the thermistor it’s much worse.

The other thing to notice is the fact that the RTD temperature rises 2.3 degrees (in 10 seconds), while the thermistor temperature rises only 0.2 degrees. This presumably is due to self heating… current through the resistive element produces heat. A couple of factors come into play here. The RTD is much smaller in size than the thermistor. More significantly, I think, the RTD is nominally 1K ohms while the thermistor is 10K, and with 1K and 10K fixed resistors both have about 2.5 V across them. Power dissipated in a resistor is V squared over R, so the thermistor dissipates a tenth the power of the RTD.

Raising the fixed resistor in the RTD divider by a factor of 10 would drop the voltage across the RTD and hence the power dissipated, but it would also reduce the output voltage range — that 1.1°C per ADC channel would become more like 5°C per ADC channel! So that’s no good. Dropping the reference voltage at the top of the divider by a factor of 3 would also reduce the power dissipation by a factor of 9, but again the precision would take a hit, though not as big a one. (In fact the uLog’s default reference voltage *is *lower, 3.3V.)

There’s a nice discussion of similar issues at http://openenergymonitor.org/emon/buildingblocks/rtd-temperature-sensing. Unfortunately, first of all, I’m using a uLog, not an Arduino, and I don’t know if the uLog’s reference voltage can be lowered. And second… this is going inside a ping pong ball. The amplifier stage shown there adds six resistors, a trimmer pot, and an op amp to what would need to go inside. Per temperature input! (I was considering two inputs.) Might be easy enough with surface mount components on a custom PC board. I’m barely competent to do through-hole components on a perfboard.

Well, now what? Need to think…

Also the LM324 used in that openenergymonitor circuit is specced down to… 0°C. Sigh. There’s the similar LM2902, specced to –40°C. Might be usable.

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OK, I think I can stack the battery (without holder) and uLog in the bottom half (plus a little) of the ping pong ball, put all the components of the amplifier stage (and the fixed resistor of the RTD divider) on a 1″ circular perfboard and stack that on top, maybe wrap all the above in a layer of Thinsulate, and still have room for a second 1″ circular perfboard on which the RTD and power switch are mounted. Hole in ping pong ball allows access to switch and exposure for RTD. Maybe a simple RTD+fixed resistor divider, unamplified, could provide a crude monitor of temperature near the uLog.

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