PH meter design question

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aliyesami

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have a PH probe that will give me known readings for a solution of PH value 4.0 , 7.0 and 10.
So if I dip the probe in 4.0 solution it will say give me a value of X. if I dip it in solution 7.0 it will give me a value Y
And if I dip it in solution 10.0 it will give me a value Z.
What I want is to be able to predict the value the probe will give me in any PH solution.
Now there is another twist to this problem is that each of these PH rated solutions comes with a temperature table e.g

For 4.0 solution for 7.0 solution for 10.0 solution

4 deg C - 4.01 4 deg C – 6.99 4 def C – 10.02
10 deg C – 4.02 10 deg C – 7.01 10 deg C – 10.03

etc etc


Is there a mathematical solution to this? I am trying to build a PH meter using Avr ATmega1284

thanks
 

There are two factors in operation: 1)Temperature affects the dissociation/activity coefficient of the standard buffer; and 2) Temperature affects the probe itself. The latter can be calculated, but in general, a calibration line is used that that takes into account both effects. Here is one reference: https://www.coleparmer.com/TechLibraryArticle/559

How closely do you need to measure the pH and over what temperature range?

John
 
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    tonny_t

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hi !
so where can I find this calibration line and is it applicable to all electrodes?
I don't know exact values I need but I am building it for aquarium use to don't have to be very precise but good enough.
and temp range can be from 75-85 deg centigrade.

I know lots of people are making it on their own.
Also are their any good PH electrodes that will not cost n arm n leg?
 



Hi,

In practical terms for aquarium use you will calibrate using 7 and 10 solutions in a water bath warmed to 25c / 77f
( assuming you are using it for saltwater tanks)

Trying to calibrate at 4 as well as 7 and 10 will be pointless.

A decent probe at a reasonable price for you would be one of these Pinpoint ones, though you should find decent generic ones locally.
The PinPoint sachets of calibration fluids always seem very good value.
https://www.marinedepot.com/American_Marine_Pinpoint_pH_Monitor_Single_Item_Monitors_Controllers_for_Saltwater_Aquariums-American_Marine_Pinpoint_Monitors-AM1111-FITEMOID-vi.html

My simple ph control board is below which uses hardware calibration though many commercial designs use software calibration with a much simpler input op amp though not sure how that is achieved.
 

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  • PH_Amplifier(1).pdf
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Over a narrow range in an aquarium, you can probably get by with just the straight-line correction given in the Cole Palmer link I posted above. Or, forget about any correction. Your calibration solutions will be close enough at room temperature to what your aquarium is (assuming you meant °F) that it won't matter.

I suspect 75° to 85°C is a bit high. Did you mean °F?

John
 
lol yes sorry . .i meant Farenheit
ok i will take a look at the link as i am looking for software caliberation

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jpanhalt said:
Over a narrow range in an aquarium, you can probably get by with just the straight-line correction given in the Cole Palmer link I posted above. Or, forget about any correction.
John
Click to expand...

I am not sure i am understanding the article and what this "straight-line" correction is , can you please give and example ?

thanks
 

so 0.003 is negligible as you pointed out earlier and I will simply ignore it.
so now I am at the second step of integration . I will have known values at 4.0 7.0 and 10.0 , how do I map them to the analog range of atmega from 0- 1023 ?
e.g with the potentiometer I am mapping zero 0 - 10k ohm with
0-1023 values of the ADC.
 

i would really like to know how do people do this ? i.e mapping the analog reading from the PH probe to the numbers generated by the ADC .
anyone?
 

Hi,

With the 10k pot you are testing a voltage range Vss to Vdd 0 to 5v for your 1024 adc steps.

Your PH probe over its full range or ph 4 to 10 will not be 0 to 5v unless you input circuitry has been built to produce a 0 to 5v swing .

If you see page 7 of the pdf I posted it will give you the idea of how to take a reading and convert it.

That will of course depend on what voltage your ph probe input op amp / hardware circuit design produces.
 

excellent point that clears up a lot of things for me. ok so I will work on your circuit to get a 0-5v swing and report back.

thanks a lot
 

aliyesami said:
excellent point that clears up a lot of things for me. ok so I will work on your circuit to get a 0-5v swing and report back.

thanks a lot
Click to expand...

Hi,

Things vary according to the ph probe itself and the op amp circuitry, hence the adjusters.

Trying to make the hardware produce 0 to 5v for ph 4 to 10 is perhaps not the best way.

That would mean you have a swing of 6 ph units, so 1 ph would equal 5v/6 = .833v and 0.1 ph would equal 0.0833v
The 1024 steps of the ADC unit produce .00488v per step so you can see there is no way you can neatly match the two figures.

Have a look at this circuit which covers the design in greater detail.

https://damien.douxchamps.net/elec/ph_meter/
 

i will take a look at the circuit, but what do you recommend ?

- - - Updated - - -

i looked at the circuit ,its a copyrighted circuit .
I avoid using copy righted circuits for legal issues.
 

aliyesami said:
i will take a look at the circuit, but what do you recommend ?

- - - Updated - - -

i looked at the circuit ,its a copyrighted circuit .
I avoid using copy righted circuits for legal issues.
Click to expand...

Hi,

Perhaps have a look as the countless Arduino ph projects, they should work fine on your mega 1284

Here are just 2 I quickly found.

The first is as basic as you can get but he details his program logic
Be aware that due to the amplification of the probe stray wires etc can make reading very irratic; thats why a good pcb is essential for anything other than basic testing.
https://www.sparkyswidgets.com/portfolio-item/ph-probe-interface/

The second is a complete project with pcb, code and documentation, looks good.
https://code.google.com/p/phduino/


On that previous link, copyright ?? - think all he is saying is for diy use feel free, but don't use my design to sell and make money from.
 

aliyesami said:
what do you think of the following design by Texas Instrument ? looks incredibly simple compared to the designs I have seen so far, and considering such a huge name in electronics cant be wrong ?

https://www.ti.com/lit/an/snoa529a/snoa529a.pdf
Click to expand...



Hi,

Manufacturers often present such datasheets to show off their latest chips.

That circuit is, quote "an optimum " ph circuit, thats not necessarily the best practical circuit.

However, can you source those chips and at what price, they are expensive over here.

There are no software examples and that circuit means the software must be used for calibration as well as reading and converting the adc values

Again no pcb layout is provided which as said earlier is quiet important for a stable reading.


That Phduino seems very similar in design to my board, it also uses readily available and lo cost parts and looks a complete project.


Not sure what else you are really looking for ..?
 

hi WP my first preference is software caliberated ph meter , I love your design it is very simple and effective .
I hope someone can tell us about this software caliberation of the ph probe .
 

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