PH Meter

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Amplifier

The circuit here will allow adjusting for probe and temperature variants and produce output in Volts/pH or ADC compatible ranges.

  • Volts/pH configuration: output will be between 0 and 14Volts with the voltage directly representing the pH value. (Such as: 3.4volts out would equal pH of 3.4) This is most useful if you want to plug it directly into a volt meter.
  • ADC configuration: output will be between 0 and 5V for interfacing with an ADC or microcontroller's input range.

It uses op-amp U1a to amplify and buffer the high impedance signal from the probe and adjust the slope which is done by adjusting the gain loop. The output of this stage if adjusted properly would be beetween -7v to+7v with -8 being pH of 14 and +7 being pH of 0. What is needed is for this voltage to be inverted and offset, and both those tasks are accomplished using op-amp-u1b. The output at this stage should be 0 to 14FV indicating the pH of the sample being tested.

Prolly should note that the circuit has no AC noise limiting, this could be a problem if the probe is located near electromagnetic sources such as pump motors. If you get sporadic readings might try a .1uF or so capacitor across R3. Being in the gain loop of U1b it will work as a integrator and easily pass DC such as the probe reading, while nullifying AC signals. :D

Microcontroller Stuffs

If using in ADC mode, you can use the microcontroller to scale/offset. just set the offset a little above 0v and set 7pH to be about 2.5v. Then use the controller to sample the offset and scale test solutions, then calculate the ph of test sample. Like this for fixed point maths:

/* zero (7pH) ph meter */
void c_zeroPh(void) {
	m_cfg.phoset=delayADC(ADC_CH1);
	meterSave(); // save settings to eeprom
}
 
/* calibrate ph to 4.0 solution */
void c_scalePh(void) {	
	m_cfg.phscale=((long)(delayADC(ADC_CH1)-m_cfg.phoset)*10000)/-300;
	meterSave(); // save settings to eeprom
}
 
void c_readPh(void) {
	long pH;
	pH=((long)(delayADC(ADC_CH1,)-m_cfg.phoset)*10000/m_cfg.phscale)+700; // read the results
	printf("pH: %2i.%02i\r\n",(int)pH/100,(int)pH-(int)((pH/100)*100));
}

Temperature Compensation

Really need to add temp compensation to this project. seems to change by about .10pH for ever 5F change in temp. Need to research this more.

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