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Thread: Humidity gauge

  1. #1
    Registered User PT66's Avatar
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    Default Humidity gauge

    I have two inexpensive humidity gauges. One on the main floor of our house and one in the basement. My wife has been saying that the basement was damp but the gauge read 50%. So I took the one from the main floor and put it next to the one downstairs. After 24 hours they read 10% different. These are identical gauges! Now I don’t know which one is right. Can anyone recommend a better gauge? I don’t want to pay for some expensive one but moderately priced would be nice.
    Dave Schneider

  2. #2
    Adrian Minarovic
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    Default Re: Humidity gauge

    SImple analog hygrometers are often very imprecise. Digital are a bit better but still there are differences between electronic elements. You can do the salt test with both of your hygrometers to see which one is closer to reality but sometimes they show same numbers at one humudity and then split when it drops or rises...
    Caliber III or Caliber IV hygrometers are considered very good. I have one and it works well and is within 2 percents (at least for my test values). I bought my for $25 or so. They can be calibrated at one value (I did the salt tests).
    BTW, judging humidity without proper measuring device is hard, or impossible, often folks mistake cool with moist.
    My shop is in basement that sometimes has leaksthrough concrete from bottom and without adding humidity ATM I have 29% RH @ 26°C because central heating just makes it drop that much while the air outside is still cool.
    Adrian

  3. #3
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    Default Re: Humidity gauge

    RH sensors are inherently not that accurate. It's easy to measure temperature accurately, not so much molecules of water in the air. I used them at work and I needed very accurate sensors for our humidity chambers, and they were hundreds of dollars. I was looking for +/- 2% accuracy and I could only find that at $250 or above. Those units were geared towards scientific applications, and you can probably find consumer grade units with that accuracy for around $200 or so.

    So, the bottom line is that +/- 5% is probably what you're going to get unless you do spend a little cash. If you see a RH meter advertising better than +/- 5% accuracy at less than $50, I'd be skeptical. It's possible you can two hygrometers to agree to less than 2%, but they could still be off from the calibrated value. A low-cost option might be to go buy 4 or 5 cheap humidity sensors at $10-20 a pop and take the average reading. Again, no guarantee there, but it's a bit better. Try to find capacitive sensors, they're a bit better than resistive ones.

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  5. #4
    Registered User TheMandoKit's Avatar
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    Default Re: Humidity gauge

    I've been using a ThermaPro temp/humidity gauge for several years and have been pretty pleased with it. It tends to stay within +/- 5% points or so as compared to a sling psychrometer.

    ThermoWorks (different company from ThermaPro) has a couple small temp/RH gauges under $50 that they claim have +/- 3% accuracy. I don't have one, but I do have other products of theirs (best cooking thermometers out there), and they are terrific. If I needed one, or if mine dies, I would give them a try.

    As always. YMMV. NFI with either company other than as a customer.
    Kit
    Guitars, Mandos, Violins, Dulcimers, Cats

  6. #5
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    Default Re: Humidity gauge

    "I have two inexpensive humidity gauges". as the man had said, "never measure anything twice". you need 3 (or 5) gauges with majority voting in case one or more sensors are faulty. remember the 737 MAX MCAS debacle. while (1) { if (read_sensor() > some_value) fly_down_now(); }, the missing sleep(1) does not save the day.

  7. #6
    Adrian Minarovic
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    Default Re: Humidity gauge

    For instrument work all you need is check your device at 33% with magnesium chloride and at 75% with table salt (and optionally at 100% with pure water), and if it is close enough at the first two percentages it is most likely good for luthiery as that is the range that we need.
    My Caliber IV was spot on at all three values.
    I've got a old-school lab-grade hair hygrometer that can be calibrated in two variables.
    Adrian

  8. #7
    Registered User PT66's Avatar
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    Default Re: Humidity gauge

    Quote Originally Posted by HoGo View Post
    For instrument work all you need is check your device at 33% with magnesium chloride and at 75% with table salt (and optionally at 100% with pure water), and if it is close enough at the first two percentages it is most likely good for luthiery as that is the range that we need.
    My Caliber IV was spot on at all three values.
    I've got a old-school lab-grade hair hygrometer that can be calibrated in two variables.
    I guess I don’t have enough scientific background to know about checking with magnesium chloride.
    Dave Schneider

  9. #8
    Adrian Minarovic
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    Default Re: Humidity gauge

    Quote Originally Posted by PT66 View Post
    I guess I don’t have enough scientific background to know about checking with magnesium chloride.
    Place the hygrometer into airtight container (or zip bag) with a small cup or bottle cap with few teaspoons of salt and add to it water till it is completely saturated with water but not too diluted. Close thhe bag (leave air inside so it can reach all areas) and wait few hours till the humidity inside stays level.
    If you use ordinary tablesalt NaCl you should shoul get almost exactly 75%. With Magnesium chloride(hexahydrate) you should get 33% reading. if you use just wet sponge the humidity should approach 100% (especially if you place it on window so water will evaporate).
    Make sure your device won't get in drect contact with water or the salt.
    Many salts have this affinity to water and you can find what RH they produce.
    Google "salt test hygrometer".
    Adrian

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    Default Re: Humidity gauge

    Quote Originally Posted by PT66 View Post
    I guess I don’t have enough scientific background to know about checking with magnesium chloride.

    The basic idea is that a saturated salt solution (one with enough salt that some solid salt is visibly present) has a unique equilibrium RH associated with it. For NaCl (table salt) it's 75% . For MgCl its 32%. For pure water it is 100%. Other salts give other RH. These are also slightly dependent on temperature. If you put your RH sensor in a closed container with the saturated salt solution and let it equilibrate, you can calibrate your sensor.

  11. #10
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    Default Re: Humidity gauge

    Clay is good for moderating humidity .Old Japanese houses had internal walls made of clay.If humidity suddenly rises the clay will absorb moisture.If humidity drops the walls will release moisture .Clay tends to allow surrounding air to settle at 50 % humidity .Mabe you could use that to make a meter checker box .

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