PPM Meter

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EC/TDS/PPM Meter On Limited Budget

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Construction

Since most of the parts for this were scavenged from old junk there is not much point in discussing construction techniques in depth (That and I'm getting tired of writing sooo much). Construction is not critical so long as all the wires go the the right spot it should work.

These pictures show the prototype and the 'finished' module squeezed onto a small piece of perferated board. Eventually this one will go into a box with PSU, microcontroller, and a bunch of other I/O. So put on a .156" connector for the power/ouput and a RCA jack for the probe so it could be removed easily.


Prototyped version	More permanent version

Operation

After Calibrating with the 2Kppm (4 EC) solution, tested with the 1K (2 EC) solution. Also tried it with a few other values down to 250ppm, seemed to worked well. Btw, you may notice my meter is falling apart, if you feel this information is of use and have a spare meter or something please send thanks.

Reading 2000PPM

Reading 1000PPM

There are other ways to do this. For instance if the probe were to be connected across or in place of R1 or R2 the frequency of the oscilator will vary a bit depending on conductivity of the solution and the values of C1, C2, etc. Connecting the oscilator output to a microcontroller will allow you to determine the conductivity by the frequency without the need for the other 3 op-amp stages.

This is not done at all, ill post more as its completed.

Long Term Observations

2006-09-22-Observations: I've been using this thing for about 5 months now, keeping the probe submerged all of the time. This worked great for a few weeks, but slowly it started to get cruddy every 3-4 days and readings were off by up to 100ppm. Wiping off the probe gets it back to within about +/-40ppm but before I take a critical reading (like when refilling the tank it's monitoring) will allways clean and recalibrate. Looking at it, the probe's once smooth and shiney gold surface now seems dull, cruddy, and pited.

The probe is holding up alot better than i first thought it would being constantly submerged. If used intermittently it would probably stay stable without recalibration longer.

Found places that sell just the ppm probe itself and i have a feeling using one of these will increase the meter's accuracy and stability greatly. Also an email from someone who has worked at places where EC meters were manufactured suggested that i use nickle instead of gold. But i've not had a chance to check either of these.

Someone hinted to me that this could be an even more useful circuit if only it would operate off a single polarity power source. I need another one of these so in the process of building it may make a few mods.

Also, to make things easier and alot more ereet I have installed the circuit in this box and connected it to the adc input of a microcontroller. By doing this i can zero/calibrate it using software simply pushing a few buttons. It has a rs-232 port, and USB too so can connect it to a computer for loging or possibly automating some process. I'll put up more documentation on this part of things soon. Note: The display is reading ppm on the left and ph on the right, with the probes in calibration solutions.

Microcontroller Code

Here is a snippet of the meter's code. These command functions are called via the shell or assigned to button presses and use fixed point maths:

/* zeroppm command */
int c_zeroPpm(int  argc,char **args) {
	m_cfg.ppmoset=readADC(ADC_CH0); //Read ADC channel 0, store as offset
	meterSave(m_cfg); //Save settings to eeprom
	return(m_cfg.ppmoset);
}
 
/* scaleppm command, if argument calibrates to ARG1 ppm, otherwise defaults to 1000 */
int c_scalePpm(int  argc,char **args) {
	if(argc==1) {
		printf("usage: calibppm PPM\r\n");
		return(ERROR);
	}
	if(argc==0) { // calcuate scale using CALIBPPM/(ADC-offset)
		m_cfg.ppmscale=((long)(((long)100000)/(readADC(ADC_CH0)-m_cfg.ppmoset)));
	}
	if(argc==2) { //same but with arg1 as calib, note the *100 for fixed point
		m_cfg.ppmscale=((long)(((long)numtoint(args[1])*100)/(readADC(ADC_CH0)-m_cfg.ppmoset)));
	} 
	meterSave(m_cfg); //Save settings to eeprom
	return(m_cfg.ppmscale);
}
 
/* readppm */
int c_readPpm(int  argc,char **args) {
	long ppm;
	ppm=((long)(readADC(ADC_CH0)-m_cfg.ppmoset)*m_cfg.ppmscale)/100;
	//display the results
	printf("PPM: %04i\r\n",(int)ppm); //print upper 4 digits of ppm value (/100)
	return(ppm);
}

Other Stuff

Thanks to all u pplz who've emailed me! Part of motivation for wikifing was all the great suggestions and good questions wish i coulda easily thrown in, not to mention the Talk:PPM_Meter page! \^_^/!!

This article was originally written late 2005/early 2006..

Disclaimer

Good Luck! Be carefull, dont do anything stupid and shock yourself to death.

Disclaimer: I am not responsible for what you do.

Experiment, learn, improvise, take what is otherwise junk from your enviroment and making of it something more.

PPM Meter

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Construction

Operation

Long Term Observations

Microcontroller Code

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Disclaimer

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