I just took Arhammon's advice and built an external circuit using the LM35DT and a PP3 9v battery. The DT is a TO-220 package so I can clamp it to items, such as my radiator etc. The only slight down side, is it's reaction to temperature changes is slow, probably due to the mass of the device. By contrast, my K-type bead probes is lightning quick.
I did consider using a PT100 or PT1000 as these are great accuracy and a linear output, but there were two problems for me; The output would not be human readable at 0.384 ohms per degree C and secondly my 22000 count UT61E is only 22000 count on voltage so I couldn't make an accurate reading.
The LM35 supplies 10mV/ degree C and requires a negative power supply for negative temperatures, which almost put me off, but a couple of diodes in series with the ground pin sorts that out so not too painful. Attached is a pic of the diagram from the data sheet which is here:
http://www.ti.com/lit/ds/symlink/lm35.pdfAt 10mv per degree C, all you need to do is multiply the display output by 100. Even someone with my maths ability can do this. If you look at the pic of the DMM, the reading is 0.2077 or 20.77 degrees C.
Accuracy and resolution
The LM35 has a better accuracy than the TMP3x range and is 0.25 degree at 25 degree C and a maximum of 1 degree C across the whole range of -50 to 150 C which is good enough for me.
So what's the point of having a resolution of 0.01 C when the accuracy is only 1 C over the whole range? We I found it very useful as a check to see if something is heating or cooling. On our radiator, when it cools, the temperature drops by .01 C every second so you can tell where the cycle is.
Data logging
I took a look at UltaDMM and on my Windows 7 64 bit laptop, it comes up with exceptions, particularly when you choose Gauges/Add. However, if you just choose Continue it doesn't seem to mind and still works OK. This is a very nice program, but I don't think it's supported any more which is a shame.
http://www.ultradmm.com/I also played with the UNI-T data logger and it's pretty basic, but probably good enough for my use. A slight issue is it only supports COM1-4 and my 4 port USB/RS232 device installed itself as COM6,7,8,9 but a bit of uninstalling and reinstalling offered me COM1 to try.
I did have some connection problems and suspected the Quad RS232 device and opened it up and it was rusty inside so I gave it a clean with lighter fluid and now it seems to work fine.
While I was playing around fault finding, I sniffed the serial port with my favourite program TeraTerm but got no output. The settings were correct at 19200, 7,O,1 but nothing. Then I found a great page on reverse engineering the protocol here:
http://www.starlino.com/uni-t-ut61e-multimiter-serial-protocol-reverse-engineering.htmlHere is says you must set DTR high and they use a serial program called Termite where you can click the DTR led and toggle it. Termite is here:
http://www.compuphase.com/software_termite.htmThere is no installation program and you also need to download the plugin "Status LEDs filter, including source code" and save it in the same folder as Termite. Then go to Settings and enable the plugin. Next connect to the UT61 at the above baud rate etc and click the DTR led and you should see the meter communicating.
Piccies
Once built, I did a few tests and have included the graphs below:
UT61E_20.77DegC shows the meter with a temp of 20.77 C
BoilingPan shows a pan of water heating up on our induction hob. Notice the wiggle at the start
BoilingPanCycling zooms into the graph above and shows the initial on/off cycling
LighterGasFreeze tests the sensor below 0 by spraying it with lighter gas. Best put your cigarette out first! You can see from the graph I had to spray it twice
RadiatorPipe shows the temperature of a radiator flow with the LM35 clamped to the copper flow pipe. You can see the heating and cooling cycles easily.
The boiling pan test is a method used to calibrate thermometers and I was pleased to see the peak temperature measured was 99.81 or only 0.19 C error at 100 C.
All in all I'm very please with the device and the current drain measured from my UT61E was only 20uA. I'm guessing this isn't too accurate so as soon as I can find my uCurrent I'll test it properly. The battery is rated at 550mAH so I think the shelf life will kill the battery before it runs out!
That's all for now, I hope you've enjoyed reading this.
Kind Regards
Steve