From my testing, the calibration pots on the Extech 445815 model DO NOT WORK. I’ve owned 5 of these units over the past 12 years. All these 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 non-NIST units were all between 6 to 10 percent off (higher and lower). I’ve returned several of these units for replacement and those units failed as well.I contacted Extech numerous times and explained the non-adjustability problem. I suspect the 445815 models are not defective, but that all non-NIST-certified Extech 445815 models are likely non-adjustable from the factory by design, i.e., either the calibration pot is disabled or not present. Numerous reviews on Amazon and elsewhere report the exact same non-adjustability problem with the 445815 model.Extech's follow-up responses went as I expected. They ignored the main issue at every step:First, they claimed that my NIST-certified Extech 445815 unit was out of calibration, and the new units were likely more accurate. I explained why that was not true (see #1 below).Second, they claimed that I didn't conduct the test properly. I explained why that was not true (see #2 below).Third, they claimed that only their proprietary RH300-CAL kits could properly test their hygrometers, and again I explained why that was not true (see #3 below).#1) I tested each of the units twice (older NIST-certified Extech 445815-unit, older non-certified Extech 445815 unit, and the new non-certified Extech 445815 units). 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 units were 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) as well as a standard Magnesium Chloride solution test, which is used for a lower humidity fixed point at a 33% relative humidity threshold. All the tests were conducted by inserting the unit probes into separate short glass bottles, i.e., the Sodium Chloride and Magnesium Chloride solutions were tested in their own distinct bottles. 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 many humidity fix points that can be tested using a variety of saturated salt solutions (over 20 in all). The two most common 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. 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." -- referenced below.Here's the kicker. Extech claims 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 and test solutions from Greenspan's 1977 article but they have copied and pasted directly from Table 2 on page 92 in the article itself. You can see this for yourselves. The fonts and formatting on the kit labels for the calibration jars are exactly the same as found in the article. They couldn't even bother to print out the labels in a modern font!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.

We have three of these at different locations in our warehouse to make sure that our temperature and humidity are at proper levels.