AcuRite Wireless Digital Indoor Thermometer and Hygrometer Monitor for Inside Temperature and Humidity Measurements (00613)

The first question ask by users of AcuRite humidity monitor is how accurate is the reading of relative humididty (RH) especially when the spec for RH is +-4% due to unit to unit variation. (Obviously, the company would like to ship all units they made to make more money. Tighter spec causes high rejects and the company will make less money or the cost for the AcuRite will have to go up.) Also the accuracy could degrade over time due to contamination of the humidity detector. There is no way for users to find out if they have bought an accurate monitor or one that is 4% or more off unless they do a two point calibration at room temperature using two saturated (with some undissolved salt left at the bottom) salt solutions each sealed in a ziplock bag together with the monitor. This is very easy to do for anyone. Just put the AcuRite in the sealed bag overnight then check the RH reading the next morning and you are done. Fortunately, the needed salts are also easy to come by as one is table salt (sodium chloride; the monitor should read 75% RH at 70 deg F) and the other could be driveway ice melt (calcium chloride; the monitor should read 31.5% RH at 70 deg F). Since bacteria could not grow in such salty solution, the sealed saturated solutions could be saved and reused over and over again allowing re-calibration whenever the user desires. I used two ziplock bag, each of which has two plastic cups. In one of these bags, each cup has 1/8 inch of water on top of 1/8 inch of undissolved table salt. Likewise, in the other bag each cup has saturated driveway ice melt solution.The procedures I used to do a comprehensive calibration are given below.Using these two saturated solutions, my measurements on the AcuRite I bought showed that for temperature from 39 deg F to 79 deg F and for relative humidity from 29% to 75% , the max error is 3% off in humidity occurred at very low temperature (39 deg F) (i.e 37%RH is actually 40%) and near room temperature, the error is only 1%RH. (Lucky me!)Actually, the AcuRite is both a digital humidity monitor (hygrometer) and a digital thermometer. The AcuRite I tested is surprising accurate over a practical range of temperatures and a practical range of relative humidity values (see details given below). The temperature reading is also quite accurate. It is very desirable to have a high accuracy humidity monitor and thermometer for use at home. In the summer, a high humidity house is uncomfortable. Worse yet, a humid basement smells foul allowing mold to grow and humid closets cause clothes to smell bad - a nightmare for many housewives. Knowing accurately the humidity allows one to take measure to lower it. With seemingly high accuracy, the AcuRite is surprisingly cheap.The comfortable relative humidly(RH) range to human body is recommended to be a region around 45%. American Society of Heating, Refrigerating and Air-conditioning Engineers Inc. (ASHRAE) recommends:---RH ---------temperature ---- season30- 60%----- 68-75degF -------winter30- 60%----- 73-79degF ------summer"Elevated relative humidity can promote the growth of mold, bacteria, and dust mites, which can aggravate allergies and asthma" and "Relative humidity levels above 70% may lead to the development of condensation on surfaces and within the interior of equipment and building structures. Left alone, these areas may develop mould and fungi." Typically, room temperature is set at 70degF. Also I preferred a RH of 50%. At 45%RH, I feel dry and my skin starts to wrinkle.Sodium Chloride saturated solution is commonly used to calibrate a hygrometer (humidity monitor) at room temperature (70 deg F) which should give 75%RH. Thus the hygrometer needs to be calibrated again with another source with lower RH, such as Calcium Chloride (31.5% RH at 70 deg F) or Magnesium Chloride (33%RH at 70 deg F). If the hygrometer agrees with one of these values then one can assume it is accurate around 50% RH. If the hygrometer is used only at room temperature, it only needs to be calibrated by two saturated solutions (for example sodium chloride and calcium chloride). However, I have calibrated from 39 to 79 deg F.Table salt (sodium chloride) and driveway ice melt (calcium chloride) provide an inexpensive and convenient way to calibrate (or check accuracy) of a hygrometer over a range of temperatures and humidity values. I have calibrated my AcuRite unit by using sodium chloride and calcium chloride saturated solutions, each of which was placed in a ziplock bag together with the AcuRite.An accurate hygrometer is expensive and bulky and I was surprised at the seemingly high humidity accuracy of the Acurite which I purchased from Amazon. The attractive features of the AcuRite are: seemingly very accurate humidity and temperature readings across a useful range of humidities and temperatures(see calibrations given below), records the max and min humidity and temperature occurred within 24 hr interval and low cost. Also the display font for humidity is large enough for convenient viewing. These features are not expected from a $10 device.Calibration using saturated table salt (sodium chloride) solutionCalibration is based on published measurements of equilibrium relative humidity(RH) vs temperatures above saturated salt solution by scientist(s). I will use these published RH values as standard. For table salt saturated solution, the published RHs, given below, surprisingly are almost a constant from 32 to 104 degF(the measurement increment was 5 degC).Temp (deg F) RH(%)32-59 7668-104 75I found that there was no different from using table salt with and without iodine. So I used the former. I also found that salt slurry did not work well. I sealed the AcuRite and two plastic cups in a ziplock bag which was further sealed by Scott tape. Each cup has at least 1/8 inch of undissolved salt (at the highest temperature of measurement) with at least 1/8 inch of water above it. Each deg F change can cause 3% change in RH. Since the AcuRite with battery is a big thermal mass, I made sure that for each measurement, the temperature and RH were stable for at least 1/2 hr, a condition which turned out to be most difficult to meet. Measurement results are given below. I found that for temperature, the AcuRite seems to be about 1 deg F higher than another temperature meter which has fractional deg F. However, I will use the temperature displayed by the AcuRite. Low temperature measurements were taken in a refrigerator. It took about one week to do all the measurements.Temp (deg F) RH(%, measured) RH(%, from standard) Error(%)41 74 76 -272 75 75 075 77 75 +277 76 75 +188 77 75 +290 76 75 +1From the above table, at room temperature of 72 deg, the error is zero and from 41 to 90 deg F, the max error is 2%RH. I would say this is quite acceptable for home use. Next, I checked if the humidity is linear over a range of RH values and a range of temperatures by using another saturated salt solution.Calibration using saturated calcium chloride (CaCl2) solution at low humidityA better and purer source for calibration at low humidly is magnesium chloride. Over its saturated solution, the RH is 33% over practical temperature range. Pure Magnesium Chloride is available from amazon or ebay sellers. Another good source to use is Potassium Carbonate which another reviewer in this forum had used and which is also available from amazon or ebay. The equilibrium RH is 43% from 0 to 30 degC. However, I didn't want to spend the money and trouble to get them.I found that I have some Prestone Driveway Heat ice melter (available from Home Depot and Lowes) which turned out to be Calcium Chloride. But the purity of the source was unknown and this would cast doubt on the results. From the internet, I found that the published equilibrium relative humidity(RH) vs temperatures above saturated calsium chloride solution are:Temp (deg F) 41 50 59 68 77RH(%) 40.0. 38.0. 35.0. 32.0 29.5RH vs temp seems to be linear and the slope is bout -0.3%RH per deg FMy measurement results are:Temp (deg F) RH(%, measured) RH(%, from standard) Error(%)39 37 ~40 -379 28 ~29 -1Thus at each temperature, there are now two data points.If the calcium chloride measurements were believable due to the unknown purity of the calcium chloride source, then the linearity of the humidity of Acurite over a temperature range from 39 to 79 deg F and over a humidity range from 29% to 75% is quite good for home use as the max error is less than 3%RH occurred at 39 deg F, which is too low for normal use. Near Room temperature , however, the error is about 1%RH, which is remarkably small.I believe that the humidity detector is polymer on a capacitor. Capacitor is a linear device and should response linearly to humidity absorbed. Thus for each temperature if two measured data points agreed with published data by other scientists, then it is sufficient to determine the accuracy and linearity at that temperature. I have verified the data agreement from 39 to 79 deg F. (Note that the calcium chloride purity is questionable. But the measured data did agree with published values.) Thus, within this temperature range, the AcuRite unit I bought seems to be accurate and linear. Ideally, one would like to further calibrate the unit with magnesium nitrate saturated solution which has a RH of 50-55% across practical temperatures. This would give three data points on a straight line to verify the linearity. However, I do not know how to get it easily and cheaply.In Summary: for temperature from 39 deg F to 79 deg F and relative humidity from 29% to 75% , the max error is 3% off in humidity occurred at very low temperature (39 deg F) (i.e 37%RH is actually 40%) and near room temperature, the error is only 1%.Max humidityAfter I wrapped the AcuRite in a wet paper tower, the humidity was 99%, the expected value.Update rate and modify the back cover to increase airflow through the unitFor normal use, no modification is need as modification does not change the accuracy of humidity and temperature. Allowing more airflow than necessary may shorten the life of the unit by quicken the contamination of the humidity detector inside the unit. Modification only changes the response time when the humidity changes. For home use, the humidity and temperature are not expected to change so fast that the unit could not keep up.The unit updates the humidity and temperature about every 15 sec. After initial testings which showed that the unit was accurate, I invested time to modify the cover of the unit as the airflow through the unit is through a grill and could be increased to have a faster response time. Steps for the modification are given below:1) The cover is held by 4 small screws. Remove them by a jewelry screw driver.2) Separate the back cover from the main body gently and not too far apart as there is a ground wire of the battery housing connecting the two.3) Remove the ground wire from the battery housing. Now the two parts can be separated.4) There is a plastic stick used for changing deg C to deg F. It will fall out but is easy to put back during re-assembly.I drilled four 3/16 inch holes on each side of the back cover as well as one 3/16 inch at the center of the back cover.The above measurements were done on just one AcuRite which I bought. Because measured data points seem to agree with published values by other scientists, I am satisfied that the humidity and temperature of my unit are accurate and use it with confidence. However, beware that there are a large number of reviews in this forum who said their AcuRite is no good.

UPDATE - 8/23/2015OK. I've had this for a few days now. As before, the hygrometer seems to be working well enough for what I intend to use it for. Which is just making sure I'm not saturating my room with the humidifier. It continues to agree with the other analog hygrometer I have within a couple of percentage points. Practically exact unless you need absolute accuracy. The thermometer, I'm still not sure about but it rarely reads below 77, even though it often feels colder. My thermostat (which is located in the hallway) generally disagrees with this unit with about 5 degrees. So I'm gonna recommend if you need an absolute accurate thermometer, you should probably look elsewhere. I doubt it's that hard to find for about the same price. I just wouldn't trust this for temperature readings in a critical setting.-----------------------PREFACE - OK. Let's get this out of the way first. I just received this meter earlier today and have been watching it and comparing it to a cheaper analog hygrometer (Made by Best Air) I already had just to see what kinds of differences I would see. Let me say first off I am judging this product MAINLY based upon it's price. This thing sells for less than 10 bucks (US). It's obviously NOT intended to be a fine scientific instrument (even if the name might lead you to believe so). I also bought this mainly for the hygrometer function. The Thermometer function is nice to have (assuming it's accurate) but since I intend to use this in a bedroom where the temperature pretty much stays in the 70s all year, I don't have much use for that.MY EXPERIENCE -The first thing I noticed when I put the battery in was that it's nice that it runs on AAs. Kudos to them for making it run on AA batteries instead of some kind of button battery or something. Seems like such a small thing but honestly. AA batteries are so ubiquitous that it just doesn't make sense to use some kind of less common button battery unless size and weight is really an issue. With a device like this, who cares? The size and weight of a AA battery is not really going to matter on a meter that sits on your desk all the time.Anyway, I guess I just felt I needed to mention that since I just had to replace TWO of those button batteries in other devices I own in the last couple of days. It doesn't seem like such a big deal until you realize that people (generally) don't keep as many of them around.OK. The next thing I noticed. I put the battery in and sat the meter on my desk right beside the analog Best Air hygrometer and left them there for about an hour. The humidity readings were very close. By that, I mean within a couple of percentages. (I know this is not a reliable scientific test but bear with me).Next thing I tried was I opened my window and sat both of them right by the window. about a half hour later, the readings between the two were drastically different. The analog meter was reading somewhere in the 70% range (which was what the weather channel says the humidity was for this are) and the AcuRite was reading somewhere in the 50% range.So I took them both back to my desk and sat them both there for a while and they soon were (again) very close. And they have been reading (generally) the same since.The only explanation I have for this is the AcuRite was apparently only designed to measure indoor humidity. I'm not a weather scientist and I don't know why that might be (Which is why I bought a cheap meter instead of an insanely expensive one) but that's the only thing that makes sense to me.Temperature readings seem to be reasonably accurate no matter what (I'm not going to complain if a ten dollar meter is off within a few degrees). Humidity only seems to work well indoors (and presumably, well away from any open windows you might have). Since it says right on the screen "Indoor Humidity" I think it's reasonable to conclude this probably wasn't designed to measure outdoor humidity levels.That said, I understand there is a salt test you can do with hygrometers by putting them in a sealed plastic bag with some salt and water for a period of time. I believe I will try that within the next week or so and see what the results are. I would like to be reasonably confident in the humidity readings from this meter at least within a small room. If that works as it's supposed to, I will say this is a great deal for the purposes I intend to use it for (Simply making sure I'm not overdoing it with my room humidifier).I have to say though that for the price (About $5 locally) I'm very surprised the analog Best Air unit seems to be pretty accurate under pretty much all conditions. I didn't expect that. Which is the reason I bought a second hygrometer to start with.At any rate, this has been a good purchase.1. It wasn't expensive at all.2. It's probably a pretty good digital thermometer even if the hygrometer is not entirely reliable.3. It looks nice.4. It's not complicated. It has one button to switch between %F and %C. Although it's nice to be able to calibrate a hygrometer, that's an advanced procedure and is probably more trouble than most people would go to on a ten dollar meter. I am impressed that you can get something this decent for so little money.5. If nothing else, I am now reasonably sure the hygrometer I already had is pretty accurate. So if you put the price of the two together, I paid less than $20 for a reasonably accurate way to know my room humidity level. You could easily pay more than that for one meter and still have to calibrate it. And many of the more inexpensive hygrometers are apparently intended for cigar humidors. I don't know how much that matters but since this whole humidity thing seems to be hard to get right, I would not be surprised if there's a pretty big difference.

This is a handy item to have when you m want to check humity levels in your home, etc. It is a little smaller than I thought it would be, but it does the job well. Thanks, Amazon, for a good product!

Works great easy to use and set up. All you have to do is put a battery in. I just wish that the temperature read was bigger. It doesn’t light up so if you want to read it you need a light on if in a dark room.

Works great after a year!

We have a storage shed at work that has temp sensitive product in it. There's a small AC unit that's used to cool the shed in these hot Arizona summers. This thermometer is perfect to keep track of the high and low temps so we know what, if any, adjustments are needed. The humidity feature is a nice bonus. All in all very happy with this product.

Works great and is accurate for temp and humidity. Easy to read and tracks recent highs and lows.

Excellent value. Very accurate and a lack of backlighting is a non-issue.