humidity_sht.jal requires new coefficients for sht11 v4

[jfmateos]

This library uses the v3 coefficients for temperature compensation on
read relative humidity, but the most recent version of SHT11 is v4 an
requires different coefficients.

[vasile surducan]

AFIK SHT75 does not need any temperature compensation, as long the self heating and by purpose heating (with internal resistor) is avoided. But I'm not using the computational algorithm from the library for RHT display so maybe Eur can answer better to you.

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[Eur van Andel]

On 17 May 2012, at 18:27 , vasile surducan wrote:

AFIK SHT75 does not need any temperature compensation, as long the self heating and by purpose heating (with internal resistor) is avoided. But I'm not using the computational algorithm from the library for RHT display so maybe Eur can answer better to you.

I don't do temperature compensation of the humidity signal. I do apply a correction -as mentioned in the datasheet- of both the humidity and the temperature signal. The coefficients of these corrections are the same for SHT1x and SHT7x. 

Sensirion makes just a single humidity sensor. They test these sensors in an automated setup. They classify the sensors according to accuracy and repeatability and sell the accurate sensors for a higher price. The hardware is the same for all models. 

Intel does the same for microprocessors. They increase the clockspeed in an automated test until the processor fails and then sell it for a "rated" clockspeed that is, say 20% lower.

http://en.wikipedia.org/wiki/Clock_rate

This is common in semiconductor fabrication. 

The coefficients *have* changed since 2002. It seems that the old ones are in the code, but I'm reluctant to apply a quick fix. I will test with new hardware and put in new coefficients, but that will take a while. If anybody has new hardware lying around and means to calibrate, you are welcome to mess with my code. Please comment out the old calculation lines. 

Calibration works well over saturated salt solutions:

http://www.smartec.nl/pdf/apphs1002.pdf

i.e. NaCl, a common household chemical, gives 75.3% RH. 

On Thu, May 17, 2012 at 3:02 PM, jfmateos <juanfel...@gmail.com> wrote:

This library uses the v3 coefficients for temperature compensation on
read relative humidity, but the most recent version of SHT11 is v4 an
requires different coefficients.

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