Ioannis
- 4th February 2013, 22:23
Trying to figure out how to design a pH Meter with Automatic Temperature Compensation (ATC), I found that it is more complicated than what I thought.
The design is based on the LMP91200 chip that can be controlled by SPI bus. It just has a good op-amp for the probe with bias level shift (because probe gives + or - voltages for pH values from 0 to 14) and a programmable gain amplifier for the temp sensor. Which is a PT1000 type.
Now, after some maths, I ended with this equation: pH value=(Vprobe-1.388*Temp)/(0.1984*Temp)
where:
Temp is in Kelvin
Vprobe in mVolts
and pH value needs to be with two decimal places from 0.00 to 14.00.
Having an ADC set with a Vref at 2.5 volts, Temperature will range from ~409 to 567 for 0-100 C.
The Vprobe at 100 C, will range in the ADC from 852 to 428 (negative slope) and at 25 C from 810 to 470.
Now,I cannot figure how the ADC values can be combined and be displayed on an LCD.
Any help appreciated.
Ioannis
P.S. Some more info regarding the pH. At pH value of 7.00 the probes have an output of 0mV in theory (maximum 30mV). At 25 C the slope is 59.16mV/pH, and 100 C, 74.04mV/pH.
The slope is negative, with high pH value, giving less voltage with isopotential point at 0mV for pH=7.
The design is based on the LMP91200 chip that can be controlled by SPI bus. It just has a good op-amp for the probe with bias level shift (because probe gives + or - voltages for pH values from 0 to 14) and a programmable gain amplifier for the temp sensor. Which is a PT1000 type.
Now, after some maths, I ended with this equation: pH value=(Vprobe-1.388*Temp)/(0.1984*Temp)
where:
Temp is in Kelvin
Vprobe in mVolts
and pH value needs to be with two decimal places from 0.00 to 14.00.
Having an ADC set with a Vref at 2.5 volts, Temperature will range from ~409 to 567 for 0-100 C.
The Vprobe at 100 C, will range in the ADC from 852 to 428 (negative slope) and at 25 C from 810 to 470.
Now,I cannot figure how the ADC values can be combined and be displayed on an LCD.
Any help appreciated.
Ioannis
P.S. Some more info regarding the pH. At pH value of 7.00 the probes have an output of 0mV in theory (maximum 30mV). At 25 C the slope is 59.16mV/pH, and 100 C, 74.04mV/pH.
The slope is negative, with high pH value, giving less voltage with isopotential point at 0mV for pH=7.