Monday, May 7, 2018

By The Numbers

In The Van I live on solar power, and since my 12-volt compressor fridge is by far the single largest user of that power, squeezing the best possible ‘mileage’ out of it has a big impact on the overall livability.

But, like most fridges, RV or not, the so-called thermostat control is just a knob with meaningless numbers around it. (I’ve yet to see food safety guidelines that say “Keep Refrigerated To At Least Setting 3”!) I suppose by now there are some high-end fridges out there with app-controls that can be used to set specific temperatures, and one-day those might migrate to RV’s, but in the meantime it’s pretty much a guessing game.

I suppose I can stick my hand inside and fondle the shredded cheese to check the temperature but that’s highly prone to miss-interpretation. On a hot day the cheese might feel nicely chilled, even if it’s not. And on a cold day it may feel too warm, even if it’s not.  In this case about the only sure fired indicators are those too-late warnings such as frozen lettuce or slimy sliced turkey.

And besides ruined food, a too-cold fridge is sucking down power like a kid that’s found an unguarded jug of Kool-Aid!

OK an aside here: Notice that I keep saying fridge and don’t refer to the freezer at all. That’s because, though my fridge technically has a freezer section it’s a bit of a joke since there is no thermal separation between freezer section and fridge. This means that, in practice, if I get the freezer section down to around 30 degrees, the delicates such as lettuce and other vegetables down there in the fridge section are frozen, and it’s no fun eating frozen lettuce! And if I get the freezer cold enough to be a real freezer, down to 10 degrees or less, you know, ice-cream and ice-cubes ready, then everything in the fridge, except maybe the vodka, is frozen too. So I use my unit as a two-compartment fridge. Things I want extra cold, such as meats and fruit-cups (mummm, nothing like a frosty fruit-cup after a long hike!) go in the upper (freezer) section and everything else in the lower (fridge) section.

Of course the only way this works out consistently is if I know the actual temperature inside there, and in my case I use these wireless sending units. One in the upper section and one in the lower.

By the way, my fridge is not a frost-free unit and I learned the hard way that these sending units need to be protected from the condensation coming off the refrigerant coils if they are going to survive long-term.

The receiver sits outside the fridge and though it’s still a manual process of checking the receiver unit and tweaking the fridge-knob, it’s much more accurate than the ol’ hand-in-the-fridge method.

And by the way, there is no single setting of the knob that corresponds to the proper temperature so this is not a set-once-and-forget prospect, because the temp maintained by any one setting is affected by ambient temperatures. The warmer the days the higher I need to turn the knob to maintain my target temp.

In the interests of conserving power (Which is kind of where this post started) I shoot for keeping the lower section between 45 and 50 degrees which, as you can see, keeps the temperature in upper section cold enough to safely refrigerate things like meat and fish.

And proper temperature control has a big effect on my power consumption. If I try to keep the lower section in the 35 to 40 degree range, during the overnight hours (No incoming power from the solar panel) the fridge will suck 22 to 25 amp-hours of power out of the batteries. By keeping just the upper section that cold and letting the lower section run warmer, but still within food-safety guidelines for many products, I average more like 13 amp-hours consumed overnight.

Big difference!

Which means that the holy-grail for everybody that lives off solar, full batteries, is achieved well before noon on sunny days and usually by late afternoon on less than sunny days. 

I’ve tried out several wireless thermometers over the years and have found this Acurite model to be the most rugged and reliable. 

Though designed as a fridge-freezer thermometer I use a second one in The Van as an indoor/outdoor thermometer. One of the sending units is inside The Van and the other is wrapped in a heavy-duty sandwich bag and zip-tied to a wire-harness under the left-rear corner of The Van where it’s as far as I can get it from drive-train and exhaust components but still in the shade.

Fun fact, at least I thought it was interesting, that low temp of 44 occurred the morning of my Spicewood Springs hike and the high that day was 77 for a temperature gain of 33. A day of matched  pairs.

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