Part IX: Tools, Calculators, and TestingChapter 14. Measure It: Testing Your Builds
Running Fair Side-by-Side Comparisons
How to test two absorbers or two glazings so the difference is the thing you changed.
- Difficulty
- Intermediate
- Cost
- Free
Two collectors tested on two different days are a weather report with extra steps. A fair comparison puts them in the same sun, at the same time, with the same airflow and the same inlet, and changes only the thing you claim to be testing. Then it swaps their places, because the left side of a bench is not the right side.
This is how to believe a difference in absorbers, glazings, or colors. The measurements themselves are the temperature log, the efficiency arithmetic, and the sun meter. Published tests in the links below were run by other people under their conditions. You cannot import their winner by copying their conclusion. You can import the habit of changing one thing.
Difficulty
- Level
- Intermediate
- Time
- One clear day for a swap, and a second day if the first was messy
- Skills
- Changing one variable and writing down the rest
Cost estimate
Free
The cost is a second box built like the first. If you only have one collector, you do not have a side-by-side test. You have a before-and-after test, which can still be fair if the sun and the inlet match and you say so.
Materials
| Item | Qty | Notes |
|---|---|---|
| Two boxes that match except for one change | 1 pair | Same aperture, same depth, same insulation, same inlet. The absorber, or the glazing, or the color, not all three. |
| The same airflow through each | 2 | Measure both. Two fans of the same model are not the same flow once the ducts differ. |
| A sun reading and a clock | 1 | So a cloud does not become an absorber result. Write the sky down even if you have a meter. |
One change
Decide the question in a sentence before you build. "Does this coating beat flat black under the same glazing?" is a question. "Is my new collector better?" is a mood. Match everything the question does not mention. Area, tilt, azimuth, insulation, leakage, and the fan all move the result as much as a paint can.
Run them together. A morning test of A and an afternoon test of B gives the morning collector a different sun angle and a different inlet temperature. If a cloud crosses, it should cross both, which is the point of simultaneous running. Log both outlets and a shared inlet if they truly share inlet air. If they do not, log both inlets. An inlet in the sun on one box is a second change you did not mean to make.
Swap sides halfway through the steady part of the day, or on the next clear day. The east box sees a different ground reflection and a different bit of wind. If the winner swaps when the boxes swap, the bench was the winner, not the absorber. If the same box still wins by more than the noise of the probes, you have a result worth keeping. Noise is the ice-bath disagreement from the logging guide, plus the wobble when a wisp of cloud goes by. A one-degree victory inside a two-degree error is a tie.
Before-and-after on a single box can answer a question if you record sun, inlet, and flow on both days and they are close. Say "two days" in the notebook. Do not say "side by side." A week of weather between the tests is a reason to be humble about a small difference.
Composite: Tuesday versus Thursday
Invented bench. Absorber A runs on a clear Tuesday. Absorber B runs on a hazy Thursday. B loses, and the notebook says the coating failed. A same-day pair, swapped at lunch, would have shown the haze and a small difference inside the probe error. The composite lesson is that the calendar is a variable. Hold it still or name it. No lab, no product, no claim that a particular coating won or lost anywhere but in this paragraph.
Write the one change
One sentence. Match aperture, insulation, tilt, and inlet. Do not "improve" a second detail while you are in the shop.
Run both at once
Same sun, measured flow in each duct, temperatures in the airstream. Log the sky.
Swap sides
If the winner moves with the seat, the seat was the effect. If it stays, and it is larger than the probe noise, keep it.
Say what you did not prove
One climate, one tilt, one flow. A winner at a gentle rise may lose when you run the collector hotter.
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.