Saturday, 3 November 2012

Mini Project: Temperature Measurement & Logging

Ability. My electronic multimeter does not have the ability to record temperature. Or does it? Long story short, I love beer, and have been thinking about constructing my own home brew system in a spare bedroom cupboard for a while. However, that room gets a lot of light, and may be too hot for brewing. So I needed to get an idea of how the temperature varies in there over time. Unfortunately my multimeter is an el cheapo Dick Smith variety (before Dick Smith submitted to Jaycar as supreme overlord of electrical components), and only has the basic functions. So I thought about whether I could use some sort of variable resistor to get a number that represented temperature, which I could calculate from there. Enter the $1.35 thermistor.

So the idea is that this is hooked up to the ports of your multimeter, and set to resistance (20KΩ). The reading you get can then be compared against a table of known values for this type of thermistor, or put through an exponential equation for more accuracy. 

Old ATX PSU 12V plug, some speaker wire, and a bit of 
heat shrink to make the probe
Utilising the furtherest apart pins only, so the standard 
multimeter probes can be used
And after a few minutes of soldering, we have the finished product:
New temperature probe, compatible with pin-type multimeter probes

From here, I had to let it sit for a while to check that it actually worked, and reasonably accurately. Of course, "accurately" is subjective when I've got nothing to compare it against, so I watched it and decided whether it was giving a sensible reading or not. After about 20 minutes sitting on my desk, not being disturbed or coming into contact with anything (other than the atmosphere), it seemed to stabilise at 14.60KΩ, indicating a temperature between 16-17°C - which is about what it felt at the time.

Part way through testing the probe on my desk, indicating a temperature of 20-21°C

The next step was to put it in the cupboard and record the values over a 24 hour period. The best way I have of doing that is by using my camera (Canon A650IS) with CHDK firmware, running an intervalometer script. I didn't want to have hundreds of photos to go through and get the values out of, and the temperature wasn't going to vary noticeably every 15 seconds. So I set the script to operate with a 5 minute delay (to stabilise the temperature once I closed the doors), and then take a photo every 30 minutes, for 24 hours consecutively (total 48 photos).

Measuring the temperature. Probe is dangling in front of the box
So now we play the waiting game. Unfortunately I've misplaced (see: stolen by a parallel universe being) my external power cable for the camera, and wasn't able to knock one up (without enormous amounts of swearing). With any luck, the batteries will hold for the 24 hours. It's running without the screen or flash (backlit LCD on the multimeter, fancy), so power consumption is somewhat minimised. Ideally, I want the temperatures somwhere in the range of 15-25°C. I'll graph the results for the hell of it when it's finished. 

If all goes well, I'll go hunting for a 25L water cube I've got somewhere (one of those ones that has a threaded bung for fitting a tap to it), and look into a heating pad, small ~80L secondhand bar fridge, and something like an STM 1000 temperature controller (hot on/cold off, hot off/cold on).

Stay tuned.

Edit: Camera made it from 1500 to 0100, so I got some data. Temperature rose until around 9pm, where it began to decline. The range is only 2°C with 22-24°C, so my plan should work fine.

Temperature and shit

So, now to start collecting bottles, get a fermenter, old fridge, heat pad, etc, etc, etc.....

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