From my testing, the calibration pots on the Extech 445815 model simply DO NOT WORK. I’ve owned 5 of these units over the past ten years. All of these 445815 units (except one) could not be adjusted or calibrated as advertised. The only unit of the 5 that had a functioning calibration pot is my NIST-certified, Extech 445815 unit that retails for over $200.00. The new unit from Amazon that I'm reviewing here was nearly 10% off (higher) than my NIST-certified, Extech 445815 unit. I returned that unit for a replacement of another 445815-model unit and that unit failed as well. I contacted Extech in March of this year regarding the non-adjustability of this new unit. I explained that I already own two Extech 445815-model hygrometers, both of which were purchased over ten years ago, and the same non-adjustability problem was present with the older 445815 unit that was NOT NIST-certified. I made it clear that I was testing this new unit against my NIST-certified Extech 445815 unit, which works perfectly well and is adjustable (see below). We exchanged a half-dozen or so emails. The following is a summary of those exchanges: I initially contacted Extech asking whether they knew of any problems with the calibration pots on the Extech 445815 model. The first response from Extech customer service was: "we do not have any problem reported from the model 445815, new units should be working completely fine." I responded with more details on the new unit that I purchased from Amazon, explaining the non-adjustability problems. I also pointed out that I suspected this unit was probably not defective, but that all non-NIST-certified Extech 445815 models were likely non-adjustable from the factory by design, i.e., either the calibration pot is disabled or not present. From the numerous reviews on Amazon and elsewhere that report the exact same non-adjustability problem with the 445815 model, along with my 10+ years’ experience with the 445815 model, I concluded it was more likely that the 445815 model is manufactured without the calibration feature enabled or is absent altogether. It seems to me far more unlikely that so many Extech 445815 model units would be defective given my experience and similar reports from others. The next series of responses from Extech went as I expected; they completely ignored the main issue at every step. First, they claimed that my NIST-certified Extech 445815 unit was out of calibration, and the new unit was likely more accurate. I disagreed and explained why that was not true (see #1 below). Second, they claimed that I didn't conduct the test properly. Again, I disagreed and explained why that was not true (see #2 below). Third, they claimed that only their proprietary RH300-CAL kits could properly test their hygrometers. This last claim was the most absurd of all. I disagreed and explained why that was not true (see #3 below). #1) I tested each of the three units twice (older NIST-certified Extech 445815-unit, older non-certified Extech 445815 unit, and the new non-certified Extech 445815 unit). The older NIST-certified Extech 445815 unit was within 1% accuracy (74% RH with the Sodium Chloride solution). I adjusted the unit in accordance with the Extech instructions, and the unit then measured accurately at 75% RH -- the test threshold for a Sodium Chloride solution. The older non-certified Extech 445815 unit was 6% off (lower) – I already knew that but the test confirmed it; I attempted to adjust the unit in accordance with the Extech instructions, but the unit would NOT adjust; again, I already knew that as well. Finally, the new Extech 445815 unit was 8-9% off (higher); I attempted to adjust the unit in accordance with the Extech instructions, but the unit would NOT adjust. I conducted three sets of tests in total. #2) For the first set of tests, I used a standard Sodium Chloride solution, which is used for a higher humidity fixed point at a 75% relative humidity threshold. For the second and third set of tests, I used both a standard Sodium Chloride solution test (75% RH) and a standard Magnesium Chloride solution test; the Magnesium Chloride test is used for lower humidity fixed point at a 33% relative humidity threshold. All the tests were conducted by inserting the unit probes into a short glass bottle, with each the Sodium Chloride and Magnesium Chloride solutions respectively – NOT TOGETHER. For each test, the bottle was capped off with an airtight seal and placed along with the unit in an airtight Ziploc bag. The tests were carried out for both 12 and 24-hour periods. For each test (on the non-NIST-certified units) the calibration pots failed to adjust, and each of the units remained between 6% and 9% off from an accurate RH reading. I tested a total of 5 Extech 445815-model units. #3) There are a multitude of humidity fix points that can be tested using a variety of saturated salt solutions (over 20 in all). The two most commonly used for testing hygrometers are the Sodium Chloride solution (75% RH) and the Magnesium Chloride solution (33% RH). Both Sodium Chloride and Magnesium Chloride are readily available and inexpensive. For only a few dollars (literally) anyone can make their own solutions -- do a Google search. If you can manage to mix salt and water, then you're more than capable of creating these solutions. The standard reference for hygrometer calibration is an article published in 1977 by Lewis Greenspan, titled "Humidity Fixed Points of Binary Saturated Aqueous Solutions." If you wanted to test your hygrometer at a 43% RH fixed point, you would use Potassium Carbonate and so on (See table 2 on page 92 in the Greenspan article). I include the full reference below. But here's the kicker regarding Extech's claim that you must use their proprietary test kit to accurately calibrate the 445815 model. The Chinese manufacturers who produce Extech's RH300-CAL test kit NOT ONLY use the same information from Greenspan's 1977 article, but they have copied and pasted directly from Table 2 on page 92 in the article itself. The fonts and the table row/column values printed on the kit labels for the calibration jars are exactly the same as those found on Table 2 in the article. They couldn't even bother to print out the labels in a modern font! You can see this for yourselves but do it quickly. Finally, in every single communication that I had with Extech, they side-stepped, ignored, and refused to address the primary issue – why is the Extech 445815 model hygrometer non-adjustable and, thus, incapable of being calibrated? The advertising for this model clearly states that the 445815 model can be calibrated, but from my testing and given the many reviews of this model, it seems clear to many of us that the 445815 model cannot be calibrated. Lewis Greenspan (1977) "Humidity Fixed Points of Binary Saturated Aqueous Solutions." Journal of Research of the National Bureau of Standards - A. Physics and Chemistry, Vol. 81A, No.1 (January - February): 89-96.
