ExtechInstruments Digit Thermometer

We have two and have used them for years to measure both dryness and moisture. The readings differ by just a little bit. I think I returned a third one for being much further off calibration, so it may take a couple rounds when you purchase these. The device allows detach the sensor from the base and thus some ability to measure into places slightly distant from the base. Mostly, though, they reside on furniture and measure ambient air humidity. It does a good job of supplementing my dehumidifier readings as well, as those dehumidifier readings can be way off.

The Extech 445715 Digital Thermometer is a solid purchase if you need accuracy. I use it for everything from cooking to checking room temperatures, and it’s always spot-on. The display is clear, and the device itself feels well-made and durable. It’s quick to respond, and the easy-to-use buttons make it simple to operate. If you need a reliable thermometer, this one’s definitely worth it!

Works great in my music studio. Guitars require between 40-60% humidity. This is perfect measuring device for me.

Out of the box the unit would not power up (or perhaps just the display simply would not power up). After replacing the battery, which I thought must be DOA, I discovered that the display powered up after pressure was applied near the battery compartment. Upon further troubleshooting, the sweet (dead) spot turned out to be an area on the left size of the bezel just below where the probe is mounted. Applying very slight pressure at this point shut the unit down. (Again, it could be just the display losing power.) I could get the unit to stay on, but the slightest touch to the left bezel again shut it down.I don't object to overpriced Chinese-made goods--it's today's reality--I do object, though, to sloppy quality assurance. (Neither the shipping box nor the unit packaging showed any signs of distress, by the way.)Thanks to Amazon's no-hassle return policy, this unit is being sent back. Since I'm desperate for a hygo-thermometer that is easy to read, I ordered a replacement unit. I'll give it another try.Update 2015-03-08: The replacement unit from Amazon powered up without issue. In comparing the temperature reading to three other hygro-thermometers, it was within 1 degree. The humidity reading is within 4 percent of my other units, which is within tolerance. The unit responds very quickly to temperature changes. After three weeks in my wine cabinet, I am satisfied with the performance.Further, I'm impressed that a rep from Extech Instruments offered to assist. (See comment to original post.) The value of good customer support cannot be overstated. I've changed my rating from 1 to 4. Keeping me from rating a 5: Value. At a Chinese labor rate of about $2.50/hour, I expect a better price.

Works as stated and numbers are a nice size.

From my testing, the calibration pots on the Extech 445815 model DO NOT WORK. I’ve owned 5 of these units over the past 10 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 9 percent off (higher and lower). I’ve returned several od 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.