Learning to trust the instruments…

We had this discussion several years ago, I could probably find it if I cared to search long enough – every now and then the water supply system fails, it has found several ways to do it, and while I accept that Murphy will occasionally stick his finger in my soup I was very dissatisfied with the fact that my first clue was always the tank running completely dry. I wanted a warning. There’s 2500 gallons of water in that tank and the idea of a warning just didn’t seem unreasonable to me. But I couldn’t think of one on my own.

Which is why, when the float switch on the pump circuit failed a few years ago so that the pump wouldn’t automatically turn off, I considered it a good thing: It forced me to stop on my way past the tank a couple of times a week, climb up a ladder, and physically look inside the tank at the water level. My driveway goes right past it, it’s not out of my way. And when the level was low enough, I would walk to the pump house, not far, and manually turn the pump on. This worked for me, I’m already out and about a lot.

There was, as I said, blog discussion on the topic of better more elegant ways to determine the tank level than climbing a ladder, spinning the lid off, and looking inside. Maybe four years ago Big Brother sent me a gigantic pressure gauge on the theory that water level in the tank would affect pipe pressure slightly but significantly enough that it would show on a big enough gauge, a truly steampunk solution that appealed to me. It meant tearing out the undersink plumbing so I didn’t get around to installing it until I had to replace the faucet anyway, but it went into place in April 2019. And a little experimentation confirmed that the theory was sound in practice: The gauge does react – slightly but noticeably – to changes in tank level. This past summer I learned what the gauge would read when the tank was almost completely empty, as it was necessary to empty the tank for Ian’s water pump project. Good to know.

And last winter I found an unexpected add-on benefit of having the gauge so handy, since it will of course also react to freeze-related breaks in the Lair’s plumbing, something to which the Lair is prone. But for its primary purpose it wasn’t really all that useful because I was still in the habit of stopping at the top of the ridge, looking inside the tank, and manually switching the pump on or off as needed.

UNTIL this summer, when I learned to my surprise that the plumber who (sort of, long story) installed Ian’s water pump was apparently offended by my non-functional float switch and fixed it without telling me. It took me a while to relax to its sudden reappearance; I hadn’t asked for it and didn’t consider it a plus because I liked the “verify” part of trust but verify.

Which is a longwinded way of saying I’m kind of thrown back on accepting that I can verify a full tank by simply glancing at that big goofy gauge on my sink.

Sorry about the photo quality but max pressure is in the morning and the light is all wrong.


For some reason I never understood, the pressure is slightly temperature-sensitive and is highest first thing in the morning when everything is cool. It drops about a pound over the course of the day. But if it reads 19 1/2 pounds first thing in the morning or 18 1/2 later in the day, the tank is definitely full – and that’s all I need to know. I just have to relax and rely on physics, and I can stop climbing that damned shaky ladder every other day.

About Joel

You shouldn't ask these questions of a paranoid recluse, you know.
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10 Responses to Learning to trust the instruments…

  1. The Neon Madman says:

    If the tank is vented to atmosphere, I would expect the pressure to read slightly higher in the morning when the air is cooler and thus slightly more dense as opposed to the hotter times of the day.

  2. Mark Matis says:

    cool water is slightly more dense than warm water, and therefor the pressure is greater when the water is cooler. Until, of course, when it freezes and becomes LESS dense…

  3. Robert says:

    What Neon Madman and Mark Matris said. Or something. Dalton and Boyle would chime in, too, if I weren’t lazy. All I remember from school is PV=nRT which is for gas, not liquid, Not helping, am I?

    A big round of applause for Joel for typing “affect pipe pressure”, vs. “impact”. I’m being a grammar Nazi, but it grinds my gears to hear “impactful”. Gah! Hey, journalists! If yer “impacted”, yer efffin’ constipated, not “affected”.

  4. H. Greeley says:

    Analogy is slipperier than real logic, but this one occurred to me:
    Think of your water system as a great big upside-down thermometer, with a big bulb on top partly full.
    Cold water contracts, but the water level in the “bulb” changes only slightly, so the denser water that is sucked down into the plumbing weighs more, and the gauge detects more pressure. It’s the same column height (almost), but the water in the column is heavier.
    Or it could be that you have a really tall barometer, and all you are detecting is air pressure differences.
    I’m sure that a real physicist would barf all over my analogy, but I thought I’d put it out there.

  5. Mark Matis says:

    It would indeed be a barometer if the tank at the top was properly sealed against air leaking in or out. But most commercially made large tanks do not have that capability.

  6. Malatrope says:

    Mark, if the tank were sealed it would form a vacuum as the water went down. Your water flow would in the best case slow down, and in the worst case collapse the tank in on itself. You don’t want to “airlock” your tank.

    The vent does need to have a filter to keep out things like tarantula hawks, though…

  7. John in Philly says:

    Some sort of float, a piece of small diameter PVC pipe to act as a rod, and a disk big enough to see from afar at the top of PVC piping.
    This would stick out of a sloppy fit gland atop the tank hatch.
    Unless the water freezes, this is sorta immune to temperature changes.
    And you can probably make it from stuff on hand.
    If you already tried this, then I apologize.

  8. Mark Matis says:

    But if you want a “barometer”, Malatrope, it must be sealed at the top!

  9. Malatrope says:

    Correct, Mark, but I was concentrating on the other part of your hypothesis, that you could make a barometer out of a large tank without breaking it 😉

    Should have been more clear, but I’ll admit to writing a fly-by…

  10. B says:

    A piece of clear tubing tee’d into the bottom of the tank (kept vertical), Simple level indicator. IF you use a piece of 2 inch tube, you can put a bright colored ping pong ball in and have a level indicator you should be able to see from the cabin.

To the stake with the heretic!