One of the things I never see in reports like this is what sensor is being used (is it a Bosch unit? Sensiron? Plain thermistor? Pixies and Fairy Dust? Some bodged together 1wire deal?) and what MCU (Nordic? Espresif? Microchip? Something else?) as those have a huge bearing on what the sensor is actually capable of, and would say a lot about precision/accuracy and battery life.

Sadly it's missing Ikea one. They are cheap and most importantly, use AAA standard batteries instead of the coin cell type.

Ikea selle rechargable ones for really cheap additionally.

I put the Ikea Thread sensor in my fridge a while back and it's been a lot more useful than I expected. My fridge had been periodically getting too cold or too hot, I suspect the issue is that the captive touch buttons that control the temp get occasionally bumped, changing the set temperature. With the thread sensor I can have Home Assistant send me an alert when the fridge leaves the safe zone for too long so I can manually correct it.

It's also been interesting to see when placing hot food in the fridge how long the temperature stays elevated for. I put the sensor in a ziplock bag and sucked the air out of it so it has weatherproofing.

Do _not_ get the Aqara ones. The things will not stay connected for shit.

I have a couple of the Tuya ones and I'm pleased with them. The humidity accuracy is questionable, but it's fine.

I also have an airgradient (not zigbee, uses wifi) for high accuracy the one place I care about that. Works great in Home Assistant with esphome.

> To get an accurate base for benchmarks, I decided to calibrate it against a lab-certified thermometer at a friend’s butcher shop, as weirdly as that sounds. His particular thermometer gets checked and calibrated by the state on a schedule (every 3 months) because it sits inside a commercial meat refrigerator and it is required by law. After calibration, my MHO-C401 needed an offset of minus 0.5°C and plus 4% humidity to match the reference reading exactly.

> It is worth clarifying from the start that my setup does not represent typical room comfort conditions, since the calibration was done at a much lower temperature, around 4°C. Sensors do not always respond in a perfectly straight line across a wide temperature range, so this fixed point is not a guarantee of the exact same accuracy at room temperature.

This can be fixed pretty easily if you are up to a little simple DIY.

Instead of using the MHO-C401, calibrated at one point against a lab-certified thermometer, build on a solderless breadboard your own temperature/humidity sensor using an ESP32 or ESP8266 dev board and a Sensiron SHT45 temperature/humidity sensor and program it using Arduino.

The SHT45 communicates with the ESP over I2C. The wiring is simple: ESP 3.3V, GND, SDA, SCL to SHT45 VCC, GND, SDA, SCL, in that order. Just 4 wires. The ESP dev board will be powered by USB. (If you need it to be battery powered, just use a small USB power bank).

The SHT45 has an accuracy of ±1% RH and ±0.1℃ out of the box.

If you want even better temperature accuracy add a tmp119 temperature sensor. This too uses I2C so the hookup is the same as the SHT45 hookup. It has a temperature accuracy of ±0.08℃ max and it typically is ±0.03℃ and is NIST-traceable out of the box. This is across a range of 0℃ to 45℃. (Outside that range it is ±0.2℃ from -55℃ to 150℃).

Adafruit has all the sensors [1][2]. Those have JST connectors using a standard pinout for daisy chaining I2C devices, which would be good for this application so when measuring temperature in the fridge or freeze you don't have to put the whole breadboard assembly in there. They have cables up to 400mm long [3].

You know what...you don't even actually need the breadboard! Here's a JST to female socket cable [4] that could connect one of the sensors to the pins on the ESP dev board.

ESP, that cable to the sensor, and if using two sensors, the sensors daisy chained with JST to JST cable.

Adafruit has ESPs also, but they seem to be out of stock on most of them. There are plenty of suitable ones on Amazon that are pretty inexpensive.

I'd say go for an ESP32 C5 or C6. They both support 2.4 GHz wifi and Zigbee, so your have the choice of reporting reading to a web server or making your DIY reference temp/humidity sensor use Zigbee. The Arduino library from ESP includes built in classes to make making Zigbee sensors easy.

Oh, they also support Matter over wifi, so you could make this a Matter device.

The C5 adds 5 GHz wifi.

Just remember that an ESP32 generates enough heat due to the radio that these sensors will pick it up if the are too close. If you build on a breadboard don't be the sensors right next to the ESP.

This is a fairly reasonable first project if you've not done any MCU stuff before but do have programming experience, especially nowadays with LLMs. They seem to have a pretty good grasp of Arduino stuff. (Don't just vibe code it! Just use them when you get stuck on something or want some documentation clarified or things like that. That will be a lot more interesting).

[1] https://www.adafruit.com/product/5665

[2] https://www.adafruit.com/product/6482

[3] https://www.adafruit.com/product/5385

[4] https://www.adafruit.com/product/4397

No mention of models that can survive outside. I'm not sure how useful an indoor sensor is??

It's not Zigbee, but I've been very happy with Yolink and several of the products in their ecosystem.

Hah. I did something similar a while ago, but I also did full saturated salt solution humidity calibration. It's very simple to do: get several different salts, create a saturated solution in a sealed container, and observe the humidity there.

The best sensor in my tests was CentraLite 3310-G. It was the only one actually precise to 1% of RH and 0.1C They are also pretty robust and long-lasting.

> you absolutely must press the pairing button on the sensor first to wake it up before sending the new reporting configuration. Sending the payload while the device is asleep just gets ignored

This stuff is still so user unfriendly.

Why cannot the software just save the desired setting to set it when the device comes online next? I feel like many things in zigbee2mqtt are like that: I click something, it fires off some message and it often isn't clear if that worked, if it'll work later, and so on.