AcuRite Humidity Monitors Fail Salt Test

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I have two AcuRite temperature and humidity monitors.  One is an old (~2yrs) Model 00309SBDI Indoor Temperature and Humidity Monitor and one is a new Model 01083M Pro Accuracy Indoor Temperature and Humidity Monitor.  While both monitors were reporting nearly identical temperature and humidity readings I felt like the humidity readings were low.  I decided to test them using the "salt calibration" method or "salt test" where the sensors are placed in a closed environment with a small amount of damp salt.  When using table salt the environment should equilibrate at 75% RH (ref1 or ref2) allowing one to see how accurate or inaccurate their sensor is.

When I did this with the AcuRite humidity sensors, the measurements were pretty far off of 75%. In the picture (see below) you can see that the maximum measured humidity is 63 and 64 percent.  I repeated this experiment several times using table salt (kosher salt) and sea salt and with more and less water (very dry mixture, very wet mixture, and in between). Each time the sensors were left for between 12 and 24 hours and each time they reached a maximum in the 62-64% range.

My question to AcuRite is, can you comment/explain the apparent 12 or 13% error in the RH readings? These seem like pretty large errors for sensors that have +/- 2 or 3 percent accuracy specifications (see specifications on ref3 and ref4). I'm aware that the 01083M can be calibrated, but it should not need to be calibrated that much right out of the box.

To any other users out there, have you or can you confirm my results with similar AcuRite sensors?  I would think that your results would be similar considering my two monitors are two different models purchased about 2yrs apart.



(note that the Model 01083M is not an option in the "Related Categories" below.)
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Ryan

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Posted 5 months ago

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George D. Nincehelser

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Your salt looks too dry and too closed off from the sealed environment.  The salt should be thoroughly saturated, yet not enough for water to pool.
(Edited)
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AcuRite Rachell, Employee

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Hello Ryan,

We do not recommend the salt test because if the salt test is not done exactly right, you will get inaccurate readings. The Pro accuracy model 01083M has an accuracy rating is +/- 0.5°F, +/- 2% RH with manual calibration option. The 00309SBDI accuracy rating is +/-2° and 5% per unit.  These two units are reading within accuracy of each other at this time. When you take them out of the bag and leave them side by side for 8 full hours, what does the humidity read?  Thank you.
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GrantK

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Thanks George.  I think I'll buy the kit from the second link.  It's about $10CAD.  I didn't even know they had these kits.  Thanks!
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Ryan

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Rachell,

Thank you for the reply.  When outside of the bag and placed next to each other, the two sensors read within 1 or 2 percent of one another.  However, the two sensors indicating nearly the same value suggests that they are precise, but not necessarily accurate. I'm convinced that the AcuRite sensors are precise, but I wanted to test their accuracy by exposing them to a known environment and seeing if they accurately indicate the correct RH. This is why I tried the salt test in the first place.

Can you elaborate on doing the salt test "exactly right"? As I hinted at in my original post, I ran the experiment 6 time (two different salts, three levels of moisture/wetness, and all with distilled water) and got similar results each time.

Also, Omega Engineering has a document referring to saturated salt solutions as a "very convenient method to calibrate humidity sensors" with tables of equilibrium RH levels for several different types of salt.

Omega

Finally, I know YouTube is not the best place to get good scientific evidence, but there are several videos of people casually performing the salt test (usually for cigar humidor hygrometers) and getting results with 5 or 10 percent of the target 75 percent, although the results always seem to be below 75 percent, no above. I was hoping mine would be a little closer to 75 percent then they are.
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George D. Nincehelser

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Like many scientific endeavors, technique is important.  The salt test is not something a beginner is likely to get right off, and it takes practice and experience to know just what the salt and water combination should look like.
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Ryan

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Grant,

Thanks for the extra data point.  Your readings are even lower than mine, but the general behavior is similar.

George,

Thanks for the suggestion about the calibration kit.  I may give one of those a try.
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GrantK

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Ryan,

My Boveda kit just shipped so hopefully I'll get it early next week.  Once I test with it, I'll post my results.  I think there isn't really anything for me to do but just put the hygrometers in the bag, seal it and wait.


Grant
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AcuRite Jennifer

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Hello All,

 We do not recommend the salt test because the measurements have to be precise when doing the test. If the measurement of salt or water is off then it will affect the reading making it inaccurate. For accuracy we do recommend that you compare them side by side for at least 30 minutes without touching them. Please let us know if there is anything we can assist you with.
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GrantK

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I received my Boveda One Step Calibration Humidipak.  I followed the instructions and left two of my meters in the bag for just over 24 hours.  I use my analog meter by Accutemp, and my Acurite 00613B.  The bag was too small to also fit my Thermopro TP-50, so I had to take measurements of that the following day.

All measurements below are based on my measurement in a house that varies temperature from 18 Celcius  up to 21 Celcius.  All three devices were left inside the One Step Calibration bag for between 24-26 hours.

The One Step Kit is claimed to hold a 75% RH inside the bag.  With that as a basis, here are my measurements:

Accutemp analog meter: 90%
ThermoPro TP-50: 67%
Acurite 00613B: 73%

Clearly my cheap analog meter is way off.  It is 20% above the actual humidity.
The ThermoPro is about 89% of the actual humidity.
The Acurite is about 97% of the actual humidity.

So at lower humidity levels, the ThermoPro and Acurite will display fairly similar numbers.  For example, if the actual RH is around 35%, the ThermoPro will display 31%, where as the Acurite will display 34%.  If the humidity drops to 25% on those extremely cold days, the ThermoPro will display 22%, where as the Acurite will display 24%.

Since I don't need an exact amount of humidity, either the ThermoPro or the Acurite will fit my needs, though the Acurite appears more accurate.

Grant
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AcuRite Rachell, Employee

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Hello Grant,
Thank you for taking the time to write your experience through testing. The accuracy rating is +/-2° and 5% per unit. Side by side with a unit of the same accuracy standard you can show +/-2° and 5% per unit.  You are reading within the accuracy standard according to your findings above.  Please let us know if you have any questions. Thank you.
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Ryan

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For another data point, I purchased one of the 32% Boveda calibration kits. The bag is too small to fit both monitors so I tested them for 48 hours each, one right after the other. The results are

AcuRite 00309SBDI 32-34%
AcuRite 01083M       33-35%

The measurement seemed to vary a little bit with temperature, and they are both slightly biased above the target 32%, but both monitors read very close to the targeted 32% RH.

All said an done, it does appear that the low cost AcuRite monitors are within their stated accuracy. I'm not sure how so many people (referring to several websites and youtube videos) seem to get good results with the homemade salt test. I could not; my results were routinely off by 5-13%. I guess I have to take those results with a grain of salt. Sorry I couldn't resist.
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Jon008

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I've found the acurite thermo/hygro tower sensors to be fairly accurate but sometimes slow to respond to rapid changes in humidity outdoors.
(Edited)
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MuskokaJohn

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The only true way to determine relative humidity, is with a wet and dry bulb thermometer.