[werk_ag]

Since some months that I'm testing the HTU21 from Measurement Specialties, for outside use.
I have to say that I'm quite impressed with its performance.
I installed a SHT21 and an HTU21 on the same radiation shield, and most of the times that I check the readings, both give almost the same values for the temperature, with very small differences on the humidity readings. Considering the super low cost of the HTU21 (around 3€ in China) it may be a very interesting alternative.

Both sensors are read through the I2C protocol, so long cables should be avoid, but despite of this, I manage to get stable readings from the HTU21 with a 3 meters two par twisted cable.

Some sources, refer that reading from the HTU21 is faster than from the SHT21, which is not very important for the WeatherDuino Pro2 system.

[Mikko]

My experience about HTU21 are not so good for outside usage. I have several humidity sensors outside and normally they last less than one year. One example is below, there is SHT11 and HTU21 sensor inside stevenson shield, fan aspirated H24. First picture is from July 2015, both sensors have the same readings. The second picture is about one year later, HTU21 sensor is giving much higher values. During the winter time HTU21 was showing 100%. For inside use HTU21 is very good, but for outside only Sensirion sensors have been working more than one year. Now testing SHT31 and that looks promising.
Mikko.

[hvalentim]

Just by looking at the graphs, one month apart (similar season), without further knowledge, for me (not having access to further elements, which I am sure you have) it would be more easy to hypothesize it is the Sensirion underreading in 2016.

[Mikko]

It's difficult to say what is the absolute truth. My personal feeling is that Sensirion is closer to the real value. The following graphs have data for one week. I am planning to buy Vaisala sensor (less than 400€) in order to have reliable values in case SHT31 does not work well. I started with HIH-4000 sensors more than ten years ago but they also lasted only one winter season.

[werk_ag]

My experience about HTU21 are not so good for outside usage. I have several humidity sensors outside and normally they last less than one year.

Hi Mikko

Thanks for sharing your experience. My HTU21 is installed just since January 2016, maybe it is too new!

Have you ever did any comparison between a Sensirion SH1X and SHT2X? I have the felling that the SH1X underreads moisture, but may be the opposite, and be the SHT2X that is overreading.

[Mikko]

Hi Werk,
I haven't used SHT2x at all. Not sure if they (china) are using that name for HTU21 sensors also. I have testbed with SHT31 and BME280 but it's still inside. BME sensor is giving constantly lower reading than SHT for humidity, but temperature is less than 0.2° degrees from SHT values. BME is combined pressure, temperature and humidity sensor.

[IzzyRiess]

Hi..i do not have used it much but as per my experience about HTU21 are not so good for outside usage. I have several humidity sensors outside and normally they last less than one year. One example is below, there is SHT11 and HTU21 sensor inside stevenson shield, fan aspirated H24.

pcb prototypes

[spudstrawb]

I might as well add my 2 cents worth. My system is still sitting on the desk beside me whilst I get things operational. It has 2 HTU21D sensors, one for inside and one for outside. At the moment, with both of them sitting on the desk less than 1m apart, one is reading 19.6 degrees C and 60.4% humidity, whilst the other is reading 20.0 degrees C and 55.5% humidity. This difference in humidity that I'm seeing is not uncommon since I've had it switched on. I should swap the sensors over and see if the difference stays the same or reverses.

[werk_ag]

Hi Werk,
I haven't used SHT2x at all. Not sure if they (china) are using that name for HTU21 sensors also...

The HTU21 chip have the same pinout than the SHT2x (can use the same board), but is a different chip, which requires it's own reading method.
It is produced by Measurement Specialties, with Headquarters almost everywhere on the world.

"Based on a unique capacitive cell, digital humidity sensors from Measurement Specialties are specifically designed for high volume automotive, medical, printer and humidifier applications."

The specs sound good, but the very low price may also means something else, which by your experiences, may be a shorter longevity.

Here is a link to the manufacturer specs: http://www.meas-spec.com/product/humidity/HTU21D.aspx#

[spudstrawb]

I swapped my 2 HTU21D sensors over (after stopping CumulusMX and depowering the RX and TX units). Keep in mind that they are almost side by side (less than 1m apart) on my desk at the moment. Upon repowering, the lines have now crossed over on the Relative Humidity chart in CumulusMX. So the issue is with the sensors, not with anything else. The problem is that I don't know which one is correct and which one isn't. I purchased them both at the same time from the same ebay seller, so you would think they would be from the same batch. Currently one is reading 53.2% and the other is reading 56.2%. I'll see how it goes for a while and then decide whether I should buy something else. I won't be losing any sleep over it though.