Last week I bought a Arduino Duemilanove board and a SHT 15 breakout from Sparkfun.
I use the practicalarduino / SHT1x code from Github written by Jonathan Oxor (June 16, 2009).
At my job (I work on a technical laboratory) I placed the SHT15 sensor together with another commercial
temperature / humidity sensor above a hermetic closed saturated sodium chloride solution.
The theoretical value of the humidity of the saturated sodium chloride solution at 20 °C is 75,5%.
I measure with the commercial temperature / humidity sensor above the saturated sodium chloride solution an temperature 21,2°C and 75,8% humidity.
With the SHT15 breakout I measure above the saturated sodium chloride solution an temperature 21,3°C and 67,1% humidity.
Before the measurements I conditioned the SHT15 breakout 20 hour with >75% humidity.
Who can help me?
Is there maybe a error in the calculation of the humidity in the arduino code SHT1x.cpp?
Temperature
Looking at the formulas they almost agree with the datasheet , - Smart sensor solutions -
Raw temperature is in 1/100 Celsius with zero == -40C : -40 C equals -40 F That is why the functions/formulas look identical
the params D1 in the lib code for Celsius is off with one tenth [table 8], exactly your difference. You need to change the two lines marked <<<<<<<<<
float SHT1x::readTemperatureC()
{
int _val; // Raw value returned from sensor
float _temperature; // Temperature derived from raw value
// Conversion coefficients from SHT15 datasheet
const float D1 = -40.1; // for 14 Bit @ 5V <<<<<<<<<<<<<<<<<<<<<<<
const float D2 = 0.01; // for 14 Bit DEGC
// Fetch raw value
_val = readTemperatureRaw();
// Convert raw value to degrees Celsius
_temperature = (_val * D2) + D1;
return (_temperature);
}
float SHT1x::readTemperatureF()
{
int _val; // Raw value returned from sensor
float _temperature; // Temperature derived from raw value
// Conversion coefficients from SHT15 datasheet
const float D1 = -40.2; // for 14 Bit @ 5V <<<<<<<<<<<<<<<<<<<<<<<
const float D2 = 0.018; // for 14 Bit DEGF
// Fetch raw value
_val = readTemperatureRaw();
// Convert raw value to degrees Fahrenheit
_temperature = (_val * D2) + D1;
return (_temperature);
}
The formula in The library for humidity is different than in the datasheet , Check chapter 4 and adapt the constants in the function c1,c2,c3 T1, T2 according to Table 6 and 7
did you make the changes to the humidity formulas too?
(code not tested)
float SHT1x::readHumidity()
{
int _val; // Raw humidity value returned from sensor
float _linearHumidity; // Humidity with linear correction applied
float _correctedHumidity; // Temperature-corrected humidity
float _temperature; // Raw temperature value
// Conversion coefficients from SHT15 datasheet
const float C1 = -2.0468; // for 12 Bit
const float C2 = 0.0367; // for 12 Bit
const float C3 = -1.5955E-6; // for 12 Bit
const float T1 = 0.01; // for 14 Bit @ 5V
const float T2 = 0.00008; // for 14 Bit @ 5V
// Command to send to the SHT1x to request humidity
int _gHumidCmd = 0b00000101;
// Fetch the value from the sensor
sendCommandSHT(_gHumidCmd, _dataPin, _clockPin);
waitForResultSHT(_dataPin);
_val = getData16SHT(_dataPin, _clockPin);
skipCrcSHT(_dataPin, _clockPin);
// Apply linear conversion to raw value
_linearHumidity = C1 + C2 * _val + C3 * _val * _val;
// Get current temperature for humidity correction
_temperature = readTemperatureC();
// Correct humidity value for current temperature
_correctedHumidity = (_temperature - 25.0 ) * (T1 + T2 * _val) + _linearHumidity;
return (_correctedHumidity);
}
When I measure the temperature and the humidity above a hermetic closed saturated sodium chloride solution I got the next results:
Commercial temperature/humidity sensor (Vaisala) temperature 21.1 °C and humidity 75.8%
SHT15 breakout temperature 21.3 °C and humidity 67.1%
When I measure the temperature and the humidity in a thermostatic calibration room of our laboratory I get the next results:
Commercial temperature/humidity sensor (Vaisala) temperature 20.0 °C and humidity 53.3%
SHT15 breakout temperature 20.1 °C and humidity 53.2%
It seems if the SHT15 above 65-66% humidity saturate.
The correction formula in the SHT1x.cpp and the datasheet of Sensirion is the same, but I don’t know if this formula is right.
Could it be that the sensor is calibrated for water vapor and that sodium chloride vapor reacts differently?
From my physics I recall that the amount of molecules in a gas is constant (P.V)/(n.T) = R. but also that water molecules are smaller than sodium chloride.
It seems if the SHT15 above 65-66% humidity saturate.
Is it possible for you to check the sensor with above 65% humidity with water iso ?
From the datasheet:
*1.3 Storage Conditions and Handling Instructions It is of great importance to understand that a humidity sensor is not a normal electronic component and needs to be handled with care. Chemical vapors at high concentration in combination with long exposure times may offset the sensor reading. *
At my job we have 3 commercial Vaisala temperature/humidity sensors. Every year we calibrated these sensors with a saturated sodium chloride solution (75.5% humidity) and a saturated lithium chloride solution (11.3%). Allways we measure for the sensors humidity 74.5 – 76.5% with a saturated sodium chloride solution. This method works perfect.
Commercial calibration laboratory works on the same maner.
I am afraid that Sensirion not calibrate each sensor (they say yes).
The support of this company is very bad (arrogant!)
Could it be that the sensor is calibrated for water vapor and that sodium chloride vapor reacts differently?
From my physics I recall that the amount of molecules in a gas is constant (P.V)/(n.T) = R. but also that water molecules are smaller than sodium chloride.
sodium chloride vapor?? At 1000 degrees maybe! The saturated salt solution is used to provide a repeatable partitioning of the moisture between the air and the salt, so you can calibrate humidity sensors. The sodium and chloride ions are bound to dozens of water molecules each and won't evaporate at any measurable rate, only the individual water molecules can do that. In the saturated solution the water molecules are bound to the salt ions and evaporate less easily than pure water, hence the relative humidity being less than 100%.
Clearly the SHT15 sensor isn't conforming to its datasheet? If its performing repeatably then you've calibrated it!
