Saturday, September 10, 2011

HOBO with a thermometer

I have been preparing to do some comparative tests of different DIY solar collector designs. Accurate temperature measurement is key. Recording a series of measurements over time is also a requirement.

Gary Reysa of very kindly offered to loan me a data logger. Of course I accepted. Gary has an excellent section on his website about all types of measurments of interest to DIY solar designer/builders.

I had expected it to be bigger I guess. (click on any picture here to enlarge it) When the HOBO made by Onset arrived from Gary I was amazed at it's size. In the palm of my hand was a self-contained four input data recorder with a USB interface. Gary also sent along four temperature sensors with 20 foot leads, the USB link cable and the software. I have been having fun learning about HOBO. Thank you Gary!

The software (HOBOware) was easy to install and the manual is excellent. The software identified the HOBO and installed the drivers. It then offered to check for a newer version online. I am now running version 3.2.1 which is licensed software but it offers to run for 30 days as a trial. It nags me each time I run it to enter a key or to continue as a free trial but that's ok. It is great software.

HOBO can operate without the USB connection but is linked to a computer to set it up for recording and then to download the recorded results. The software also produces graphs and allows for data export (to a spread sheet for example) and well as doing various things to the data like truncating or filtering.

To start, I taped the sensors together with masking tape and set HOBO to record for a short interval. After downloading the data from HOBO, the software then offered a graph which was auto zoomed appropriate to the data recorded.

Each line corresponds to one of the sensors. While it appears that there are only three lines, two of the sensors were virtually the same. The bottom line is sensors 2 and 3 while the top line is sensor 1 and the second line down is sensor 4.

From this first effort, I could see that the temperature resolution (the smallest measurement increment) of HOBO was 0.05 degree F. Those are the steps in the graph. It was early in the morning and the room air was slowly warming. The sensors weren't all giving the same reading even though I had them taped together and had allowed them to stabilize without touching them for about a half hour. I had expected there to be some +/- tolerance between the sensors and between the four inputs. Also, temperature measurments in air are notoriously difficult to do accurately because air is not a great heat conductor and stratification and movement of the air could also cause differences between the sensors even though they were taped together.

Gary had advised to check the HOBO with ice water and boiling water so I set about to do that.

I took off the masking tape and bound the sensors together with a tie wrap thinking that this would provide tighter thermal coupling while tolerating the boiling and freezing water better than the masking tape.

I then prepared an ice water bath on the kitchen counter and a pot of boiling water on the stove. Our ice crusher normally used for making margaritas came in handy.

I had read that when doing these tests it is important that there be an excess of ice and much surface area (the small chips) so that lots of ice is always melting, that the water should be stirred vigorously and that the sensors not be allowed to touch the countainer so I tried to do all those things. The towel and the double bowls for the ice water was to provide a bit of extra insulation.

I set the HOBO to recording, disconnected it from the computer and carried it up to the kitchen. I first dipped it in the ice water thinking that this was closer to room temperature. If I did the hot test first, the ice would have to cool the sensors from boiling.

Of course I couldn't see what HOBO was recording but I kept the sensors moving in each bath for several minutes. The actual graph of the test is in the first screen shot above.

One of the nice things about HOBOware is that the graphs are interactive. You can click and zoom in on a particular area of interest or you can select one of the sensors to highlight it to make it stand out from the others. You can even click a particular data value in the table in the top part of the screen and it's location in the graph is highlighted.

Here is the area of the graph in the cold bath where the temperature stabilizes. Sensor 1 and 4 are the two at the top of the graph, 2 and 3 cluster at the bottom. I am not using distilled water so I am not sure what the actual melting temperature of ice made from our mineral rich tap water is supposed to be (probably a bit higher than 32F) but the important thing for me might be that the four sensors are within 0.15 F degree of one another which is remarkably good I think.

Here I have zoomed in on the hot part of the graph. Interesting that the temperature of three of the sensors (top to bottom - 1,4 and 3) is falling yet one (2) is rising. Although the temperature indicated is over 2 F degrees lower than the 212F that might have been expected (not pure water?), the scatter between the sensors is low, they are within 0.35 F degrees of one another.

So I am now somewhat familiar with HOBO and HOBOware and I am looking forward to applying it to the measurments that I will make. As I will be primarily concerned about temperature differences, the absolute accuracy of any one measurement will not be as important as the differences between pairs of measurements.

I may repeat the hot water - cold water bath tests in future but with distilled water. But first I would have to make some distilled water ice cubes.

Many thanks, Gary!

PS - The title of this post is a play on a recent Canadian low budget thriller Hobo with a shotgun about a homeless vigilante who blows away crooked cops, pedophile Santas, and other scumbags with his trusty pump-action shotgun. The HOBO about which I am writing above is a much gentler sort of beast. Sorry - I couldn't resist!

Index - Comparing concentrator to flat plate solar collector

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