OK, one more time from the beginning starting with the sensor. I will explain how I would go about this using what you have. You have a
PX26-001 DV 0-1 PSID which is a 0 to 1 PSID differential pressure sensor. For your particular sensor the sensitivity or output is (16.7mV, 1.67mV/V for 1 PSI range) meaning the sensor full scale output (Full Scale = 1 PSI) will be 16.7 mV when 10 Volt excitation is applied. Earlier I mentioned and covered (as other forum members did also) the importance of stable excitation. I strongly suggested using a
10 Volt Reference for the excitation. Using a stable excitation is the only way to assure your sensor will provide a stable and accurate output and remain stable in use.
Your goal is to run your sensor signal to an Arduino Mega analog input channel. Therefore you want to amplify the signal from the sensor (0 to 17.7 mV) to something useable like 0 to 5 volts. Therefore you need a Gain of about 282.5. You have an
Analog Devices AD620 IA (Instrumentation Amplifier). I provided a drawing as to how I would go about doing this. There are several ways to go about it. I would use a dual power supply I would use a +15 and -15 Volt supply. One reason for choosing 15 Volts is because using the +15 Volt supply you can power the 10 Volt reference I mentioned earlier. While the AD 620 can be used with a single supply there are drawbacks to doing so. Should you wish to use single supply I suggest you configure exactly as shown in the data sheet: Figure 38. A Pressure Monitor Circuit that Operates on a 5 V Single Supply. Personally I would do as mentioned and use a dual supply, we also covered this in detail in another thread.
When all is said and done at this point you have 0 to 1 PSID = 0 to 17.7 mV = 0 to 1 PSID = 0 to 5 V. I suggested you use a 10 turn potentiometer for Rgain with the AD620. So how to calibrate at this point? Unless you have a precision pressure calibration system you need to find an alternative means to achieve a 0 to 1 PSI pressure. I explained this in detail in another thread. 0 to 1.0 PSI = 0 to 27.670 inches of water column. I explained how to use a simple aquarium air pump to get accurate pressure readings. I suggest you
read this and modify the concept to get an accurate 0 to 1 PSI reference for calibration. For example if 27.7 inches of water column = 1.0 PSI then each 6.92 inches of water = 0.250 PSI. There are many ways to derive accurate pressure using a plastic tube, a ruler and some water.
As to the 0 to 5 Volts going to the Arduino that becomes 0 to 5 volts 0 0 to 1023 bits, I gave you example code samples to convert all of that to PSID. I also pointed out that the Arduino is only a 10 bit (0 to 1023) A to D system. I showed how using my Arduino Mega with 2.5 Volts applied read 524 bits and not the expected 512 or so. I showed in a code sample how to calibrate that.
Finally I will point out that when using delta pressure to measure flow the results are not a linear function, I assume you know that? This is a square law response. That means for a given pipe diameter, a known fluid temperature and density that if 1.0 PSID = for example 10.0 GPM of flow that 0.5 PSID will not be equal to 5.0 GPM of flow. It is not a linear curve.
Ron