I'm really really new to all this stuff. I've successfully completed an LM34 temperature sensor project with no problems thanks to the great tutorials available. However, I'm having a difficult time interfacing an analog pressure sensor to an Arduino UNO board.
I have an SSI P51-1000-A-B-I36-5-R pressure sensor (Digi-Key # 734-1069-ND) which measures PSIA. Here are links to data information about the sensor. http://search.digikey.com/scripts/DkSearch/dksus.dll?lang=en&site=us&KeyWords=734-1069 http://www.ssitechnologies.com/PDF/Application%20Notes/PS_AN2_MediaSensor_P51_Product_Overview.pdf
Note that the sensor requires 8-30vdc input so I have to use external power instead of taking power directly from the UNO board. I'm currently using a spare laptop AC adapter that according to my voltmeter is delivering a steady 18.74vdc. The sensor uses three wires Vin, Vout, and GND.
My voltmeter shows that the sensor output is a very steady 1.05v however the value returned by analogRead() is jumping around between 203 and 210 which doesn't make sense to me since it seems the UNO should be returning a consistent value from analogRead given the very consistent output from the sensor. This also makes sense since the atmospheric pressure being measured from the sensor (sitting on my workbench) isn't changin enough to cause such values from the UNO.
I'm also having trouble determining a formula for converting the analogRead() value to PSI. This particular sensor is calibrated to output 1vdc at 0psi. Since it is measuring PSIA it will output something above 1vdc just sitting on my workbench due to atmospheric pressure and that voltage is currently 1.05vc, and it will never output less than 1vdc because that indicates a perfect vacuum. The maximum output is 5vdc at 1000psi. So the voltage output range is 1-5vdc representing a span of 0-1000psi.
What is causing the analogRead() value to drift so much and how do I derive a formula that converts tha adc output to a PSI value?
Any help appreciated.