Measuring the Garden
"A man with one thermometer knows what the temperature is. A man with two thermometers is never sure."
Since I live where the population density is very low, I can't just copy my neighbors' gardening practices and have to figure things out on my own. Thus understanding how soil temperature at different depths correlates to air temperature and how much difference there is compared to the greenhouse is a lot of work, so automating the data collection as much as possible seems worthwhile.
Searching online for suitable thermometers led me to a company called Ambient Weather, where I found this wireless 8-channel console (
https://www.ambientweather.com/amws09c.html) and these wireless wet temperature probes (
https://www.ambientweather.com/amf007tp.html). I opted for 4 probes, with 2 to go inside the greenhouse (soil & air temps) and 2 to go outdoors (soil & air temps). Ideally, the unit would also report to my computer and/or the cloud, but, alas, the unit they list as doing that (
https://www.ambientweather.com/amws8482.html) never seems to be in stock -- not to mention it doesn't seem to have wet probes. Having a current/hi/lo temperature display with independent reset for each probe is better than nothing!
Reading the specifications, one notices that the accuracy is listed as +/- 2F. Which, frankly, suggests these units are worthless crap. So what can we do about that? For starters, understanding that the sensors are thermistors powered by batteries tells us that they should be fairly repeatable in a narrow range, specificaly the narrow range of our normal climate, provided the battery voltage holds stable. To keep the battery voltage stable over both the temperature range and the battery lifespan, then lithium batteries are the obvious choice. Alkaline batteries are fine only if you can keep the temperature above freezing... below freezing makes readings read significantly low compared to a mercury thermometer; lithium batteries do not have this drawback until the temperature is tens of degrees below freezing.
The second thing to notice about the console is that you can adjust the display readings, which means you can calibrate the probes. Doesn't change the display built into each remote sender, but you're going to read the main console display from inside the comfort of the house rather than walk around to each probe location, right?
Calibration then becomes a matter of locating all the probes next to each other, comparing to a trusted thermometer, and adjusting the display to read correctly for each probe. Ideally, I'd probably drop the probes into an icewater bath with the sending units kept just above freezing on a cold night. Instead, being eager to use the units, I set them outside in a rain-sheltered location with the probes next to each other on a cool, rainy, overcast day to eliminate drift in the air temperature.
After calibration, the console display shows the temperature spread among the probes to be no more than 0.6F, usually within 0.3F of each other, and they're tracking my trusty outdoor wall thermometer within 1F. I can live with that!
Since the senders aren't weatherproof, they'll need shelter. My quick solution is an upside down container (flower pots from the dollar store) attached to a stake pounded into the ground. The stake keeps the units from flying around in stormy weather yet allows me to move them around.
Positives
- Wireless range is adequate for my house and garden, tested good for 150 ft
- Repeatability is sufficient once calibrated and using lithium batteries
- Remote units are low cost
Drawbacks
- Uploading data to a computer or internet connection is not available
- Maximum temperature of 140F is too low for use as a compost thermometer, where 180F is desired
- Each time you add a remote probe, you have to remove/replace the batteries from the console and recalibrate