Part IX: Tools, Calculators, and TestingChapter 14. Measure It: Testing Your Builds
Logging Temperatures with Cheap Sensors
A practical setup for recording collector and room temperatures you can trust a week later.
- Difficulty
- Intermediate
- Cost
- $20–80
A temperature you write down once tells you little later. A logger that writes one down every few minutes gives you a test you can still read on Thursday. Cheap sensors are accurate enough to compare an inlet and an outlet, a room and the outdoors, or a collector today and the same collector after you changed one thing. They are not a calibration certificate. The hard part is labeling the channels so the file still means something after you have slept.
Efficiency math that uses these temperatures is the next guide. A fair test between two builds is side by side. The measurement notes linked below are other writers' methods. This setup is a generic logger and two probes.
Difficulty
- Level
- Intermediate
- Time
- An hour to place and name the probes, then a day of leaving them alone
- Skills
- Writing down which wire is which before you close the box
Cost estimate
$20–80
A small battery logger or a hobby board with two digital temperature probes fits the range. A radiation shield for the outdoor probe, if you build it from plastic dishes and tape, can be nearly free. A laboratory thermometer is outside this budget and not required for a comparison.
Materials
| Item | Qty | Notes |
|---|---|---|
| A logger with a clock and enough memory | 1 | Battery powered. You should be able to open the file without the sensor still attached. |
| At least two probes | 2 | Inlet and outlet, or indoors and outdoors. One probe can only tell you that something changed, not why. |
| Labels and a notebook line | 1 | Channel, place, and the date you put it there. Tape on the wire and the same words in the file. |
Where the probes go
A collector outlet probe goes in the airstream, downstream of the absorber, not taped to the glazing in the sun. An inlet probe goes in the air you are about to heat, which might be the room or the outdoors depending on the collector. If the inlet probe sits in a sunny windowsill, you will record a fake hot inlet and congratulate yourself on a small temperature rise.
An outdoor air probe needs a shield: white, ventilated, out of the sun and off a hot wall. Otherwise you have measured a sun-warmed probe instead of the weather. A room probe goes at sitting height, away from the supply register and away from the collector outlet that would flatter the room.
Sample faster than the thing changes. A collector under moving clouds can jump in a few minutes. One sample an hour will miss the noon and invent a smooth day that did not happen. A room can be every ten or fifteen minutes. Start the logger, then let it run. Opening the box every hour to "check" is how clocks and files get interrupted.
Checking the probes and the file
Ice water is the easy check. A probe in a stirred ice bath should read about 32 F or 0 C. If two probes disagree there, believe neither difference smaller than that disagreement. Boiling water is a worse check than people think, because water boils below 212 F as you go up in elevation. Do not "correct" a probe to 212 F on a mountain and then measure a collector with a crooked zero.
Name the file with the date and the experiment. Write the units. A column called temp2 is how Thursday you swaps the inlet and the outlet and publishes a collector that cools the air. If you move a probe, write the time you moved it. The hours before that time belong to the old location.
Keep hobby electronics on batteries or a listed low-voltage supply. Do not clip a probe onto a mains conductor, into a service panel, or onto a flue as if it were a listed stack thermometer. A data logger is not an electrical meter and not a carbon monoxide alarm.
Composite: the swapped labels
Invented log. Channel A was the outlet on Saturday and the room on Sunday, and the notebook still says outlet. The Sunday peak gets averaged into the collector performance. The composite fix is a line in the log at the minute of the move, and tape on the wire that matches it. The composite names no real workshop or brand of board.
Put probes in the air you mean
Airstream for a collector, shielded shade for outdoor air, away from the sun on an inlet.
Name every channel in the file and on the wire
Date, place, units. If you move it, write the time.
Sample faster than clouds
Minutes for a collector, slower for a room. Then leave the logger closed.
Check both probes in ice water
Believe differences only when they are larger than the probes' disagreement. Do not force a boiling reading to 212 F.
Further reading
These links go to outside sources. This book cites them by link and does not copy their words or pictures.
- https://www.builditsolar.com/References/Measurements/measurments.htm
- https://www.builditsolar.com/References/Measurements/CollectorPerformance.htm
- https://www.builditsolar.com/References/Measurements/KitPyr/KitPyr.htm
- https://www.builditsolar.com/References/Calculators/Collector/ColEfic.htm
- https://www.builditsolar.com/Experimental/SunSim/SunSim.htm
- https://www.builditsolar.com/Experimental/HeatTransferCoef/HeatTransferCoef.htm
General education, not a permit or a stamped design. Follow the safety note in the guide, read the safety disclaimer, and hire a licensed trade when the work requires one.