So i've been trying to get the pressure from my MPX12GP sensor (datasheet: here)
on my arduino, amplifying the signal with an instrumental amp (INA126P, datasheet: here). I use an 880 ohm resistor so that should be a gain of around 100.
I'm not too good at reading datasheets, and the amp makes it even harder to determine a formula or something to calculate pressure. That's why I was wondering if you guys can help out!
Thanks in advance!
Use 5V ?
10kPa : -> 115mV (INAxx output 11.5V !!!!) Hmmm
What about using 3V ?
Reduce gain ! Try 2.2 k resistor to get gain (80/2.2+5) 41.36
Use 5V
0 kPa -> 22mV * 5.0/3.0 * 41.46 -> 1.0033V (analogRead 205)
10 kPa -> 69mV * 5.0/3.0 * 41.46 -> 4.7564V (analogread 973)
Code:
float pressure = map(analogRead(A_pin),205,973, 0,1000000) / 100000.0 ;
The "offset" of your sensor is between 0 and 55mv with 20mv typical. An ideal sensor would output 0mv at zero differential pressure but this is not an ideal sensor. You can make the assumption that after removing that offset from your readings that the output will be linear from 0 kpa to 10 kpa but it would be best to calibrate it against another, known good gauge.
Thanks for your replies!
So I'm using the sensor with 5v, and am using 2440 ohm resistance on the INAxx, resulting in a gain of 37.79. knut_ny, you used the values at a +125 degrees, while we will be using it at 25 degrees. hence I needed a bit more resistance to get below 5v. The first value I get when reading the sensor value is 226 though, and calculating back with your formula (mv * (5/3) * gain) I end up getting 17mV, which is weird because it should be a bit more than 20mV I guess, due to the atmospheric pressure?
And Chagrin, yeah I guess an ideal sensor would output 0mV, but I can't really find a MPX..GP sensor which has that. The problem is though, that removing the offset with the INA amp between the measurement can make things inaccurate..
Guess I'm going to find me a different MPX which doesnt need an amp to be read by the arduino, these have a formula in the datasheet (including the error for temperature) so might be easier to get more accurate readings.
Is your USB 5V really 5V? (or more like 4.95?)
.. offset max is 35mV. Your vaues are perfectly OK.
Your sensor reads pressure difference, which means the atm-pressure doesn't care.
Get the circuit calibrated (in software) and it will perform well.
Use a Freescale sensor with inbuild preamp.
They connect directly to the Arduino.
AFAIK, the INA needs a negative supply to work with 20mV DC input signals.
Leo..