This is a follow up to Measuring water pressure in a tank. I've decided to try and go a different direction so I'm asking a new question.
I have an application in which I need to electronically measure the pressure inside of a pool filter. Because the sensor will be subjected to chlorinated water I can't use a simple pressure sensor such as the MPX5700 from Freescale.
I've got a cheap 150 PSI Pressure Transducer from Ebay with the following technical details:
Input: 0-150 psi. Output: 0.5V – 4.5V linear voltage output. 0 psi outputs 0.5V, 75 psi outputs 2.5V, 150 psi outputs 4.5V.
Works for oil, fuel, water or air pressure. Can be used in oil tank, gas tank, etc.
Accuracy: within 2% of reading (full scale).
Thread: 1/8”-27 NPT.
Wiring connector: water sealed quick disconnect. Mating connector is included.
Wiring: Red for +5V. Black for ground. Blue for signal output.
I tested it out on the bench using an oscilloscope and multimeter and measured the following:
- When no pressure is applied it is producing about 418mV. This seems correct based upon the above.
- When I force some air into it using my mouth the voltage goes up as expected.
- The resistance between 5V and GND is 42.7K ohm
- The resistance between 5V and signal OUTPUT is 120K ohm
- The resistance between GND and signal OUTPUT is 69K ohm
I connected the sensor signal wire to my LPC1768 micro-controller on P17 ( analog in ), sensor Red wire to +5V and the sensor ground wire directly to ground.
When I read the 12 bit AD convertor output I saw wildly varying output such as the following in the Pressure column:
Cycle Level Pressure ================================  W: F:  W: F:  W: F:  W: F:  W: F:  W: F:  W: F:  W: F:  W: F:  W: F:  W: F:  W: F:  W: F:  W: F:  W: F:  W: F:  W: F:  W: F:  W: F:  W: F:  W: F:  W: F:  W: F:  W: F:  W: F:  W: F:  W: F:  W: F:  W: F:  W: F:  W: F:
I'm using the same software routine to read both the Level and Pressure column. The Level column is very stable unlike the Pressure column.
My questions are as follows:
- If the sensor is supposed to output .5V with no pressure how is it that I'm reading 0?
- Why such variations between each ADC reading?
- Do I have to add some components to the circuit interface because of the resistance noted above in points 3,4 and 5 such as a unity gain buffer? If so, what op-amp would you recommend? Note that I'm not an electrical engineer, just a hobbyist new to op-amps. Thanks Joe Hass for the unity gain buffer idea.
- What about power supply filter capacitors? Should they be needed and if so have much?
- Should I add pull-down or pull-up resistor?
- Is it possible to get this to work or would I need to use a much more expensive item such a Honeywell industrial pressure sensor?
Note: This is both an electronics and software question. Right now I am working on the electrical interface.
Update: Things appear for the moment to have been solved. I had the sensor signal connected to the LPC1768 P18 which is shared as Analog In and Analog Out. The sensor signal is now connected to P17 ( Analog In only ). That change and adding a 0.1uF cap to filter the sensor output has made the output at 0 PSI appear correct. I just have to test it with an air compressor and certain PSI settings to ensure the ADC readings remain correct.