SHTx Temperature and Humidity Sensor (Part 1)

Part 123

I introduced the temperature and humidity sensors from SENSRION a couple a months ago [link to this post], and I recently decided to use one of them for building a sensor which would aggregate multiple physical measurements. For the sake of saving time, I ordered the PARALAX module which is handy and has a good quality vs. price ratio.

sht11

Important notice: Multiple revisions of the data sheet for the SHT1x have been released by SENSIRION. Too bad, the early ones are BAD, yes, really BAD, with mistakes which will make you loose your lengendary temper! In order to avoid such drama, get to the official page of SENSIRION and docwnload the latest data sheet revision.

I did not like the sample code neither so that I decided to build a brand new library that I hope you will like as much as the code available from here.

There is no reason why I should paraphrase SENSIRION documentation, and will just pick up the most significant information in order to describe this product. The very nice thing about SHT1x is that these sensors incorporate both thermal and moisture sensor. In this way, it is possible to achieve highly reliable humidity measurements, and dew point estimates. More tan this, the sensor features a builtin heater which allows the baking of the active cell. Heating the cell will desorb contaminants which could interfere with the measurements of moisture. At normal room temperature, it takes less than 1 min to reach approximately 35°C. After an other minute at such temperature (desorption takes a little tim which depends upon the vapour pressure of the contaminant), the sensor will require up to to 10 min for cooling down and allow accurate measurements. In the same spirit, SENSIRION advises not to exceed 10% duty cycle in order to prevent self heating of the measurement cell. The SHT11 communicates with external world through a “look alike” I2C serial protocol, using an input/output data line and a clock line which strobes data. The chip can be fed with voltages as low as 2.7V, up to 5.5V, and its typical supply voltage is 3.3V.

SHT11 performance specifications

Humidity

  • accuracy: ±3.5%RH
  • resolution: Low res 0.5%RH, high res 0.03%RH
  • repeatability: ±0.1%RH
  • response time: 4s

Temperature

  • accuracy: ±0.5°C @25°C
  • resolution: Low res 0.4°C, high res 0.01°C
  • repeatability: ±0.1°C
  • range: -40 to 123.8°C
  • response time: 5 to 30s

Note: The SHT11 features a 14 bits ADC which is used for both thermal and humidity measurements. In low resolution mode, the temperature is converted in 12 bits while the humidity is converted in 8 bits. In high resolution mode, 14 and 12 bits are respectively used. The major difference lies in the time required to acquire and convert data which can be has long as (11+ 55) 66ms in low res mode and (55+210) 265ms in high res mode. Taking into account the response times which are pretty long, I would encourage to leave the sensor in its default, high resolution, mode.

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