pH sensor

[Vikky]

hi all,

I am developing pH meter should be working from 0- 14 pH. For this i got a pH sensor from Î人ÄÜÊÀ·ÖÎö´«¸ÐÆ÷ÓÐÏÞ¹«Ë¾.

But i don't know how to procede with it. The datasheet says that at 7 pH solutions it would read 10 mv and it has a slope of ~58 mv /pH. I tried to verify it with the multimeter. The reading changed but not as been indicated.

Whether the process i am following is correct or some external circuitary required to read the voltage change from the pH sensor??

 

[ericgibbs]

hi all,

I am developing pH meter should be working from 0- 14 pH. For this i got a pH sensor from Î人ÄÜÊÀ·ÖÎö´«¸ÐÆ÷ÓÐÏÞ¹«Ë¾.

But i don't know how to procede with it. The datasheet says that at 7 pH solutions it would read 10 mv and it has a slope of ~58 mv /pH. I tried to verify it with the multimeter. The reading changed but not as been indicated.

Whether the process i am following is correct or some external circuitary required to read the voltage change from the pH sensor??

hi,
The link shows a number of pH sensors, which one is yours.?
A datasheet would help.

 

[Vikky]

In the lab sensor tab, you will find ASP200C -2 (blue).

I am using that one.

 

[ericgibbs]

In the lab sensor tab, you will find ASP200C -2 (blue).

I am using that one.

hi,
I have found ASP200C, but where can I find the datasheet.?

I must say, that linked web site is one of the most irritating sites I have seen.

 

[Vikky]

I must say, that linked web site is one of the most irritating sites I have seen.

I too agree with you. But what can we do other than using the google translator.

By the way i got some printed doucment about it. It all contains about how to clean it, what is the temperature range etc.. rubbish.

The main thing which i think is usefull is

Zeropoint @ 7pH
Asymmetry 12mv
slope 58.17 mv/pH
slope at 25 C 98.32%

 

[ericgibbs]

I too agree with you. But what can we do other than using the google translator.

By the way i got some printed doucment about it. It all contains about how to clean it, what is the temperature range etc.. rubbish.

The main thing which i think is usefull is

Zeropoint @ 7pH
Asymmetry 12mv
slope 58.17 mv/pH
slope at 25 C 98.32%

hi,
As you know, 7pH is neutral and it it slopes at 58.17mV/ph.

Say you require to cover from 0pH thru 14pH, I would use an OPA amplifier.

If it reads 10mV at 7pH, at 14pH it should read +407mV and at 0pH [+10mV - (7*58.17mV)] = -417mV.

An overall range of approx 824mV, if you want to raise to ~+/-2.5V then a OPA with a gain of ~6.

What do you plan to do with the output signal.?

 

[Vikky]

I just want to sense the output voltage, convert it into digital by ADC and send send it to system for capturing the data via I2C.

So, from your reply i think i need an instrumentation amplifer with its input connected the the leads of the pH probe, set the gain and test it.

Is this will be fine??? how to calibrate it??

 

[ericgibbs]

I just want to sense the output voltage, convert it into digital by ADC and send send it to system for capturing the data via I2C.

So, from your reply i think i need an instrumentation amplifer with its input connected the the leads of the pH probe, set the gain and test it.

Is this will be fine??? how to calibrate it??

Use this method.

**broken link removed**

 

[Vikky]

I do have such solutions capsules of 4,7,10 pH. But what should be the actual calibration steps.

Do i need to change the gain value as time passes or some other method??

 

[ericgibbs]

I do have such solutions capsules of 4,7,10 pH. But what should be the actual calibration steps.

Do i need to change the gain value as time passes or some other method??

The calibration methods I have seen on the web say the the probe should be cleaned in neutral water, twice before each use.

The amplifier would have a ZERO pot and a SPAN pot, so I expect the procedure would be to clean the probe, immerse in 7pH liquid and adjust the ZERO pot, immerse in a known pH and adjust the SPAN pot [slope] to give the correct pH, repeat this once again and then use the probe to check the unknown liquids pH.
This calibration method is common to most sensor circuits.

ie: ZERO on a null reference, SPAN on a known value and repeat.

 

[jpanhalt]

The amplifier would have a ZERO pot and a SPAN pot, so I expect the procedure would be to clean the probe, immerse in 7pH liquid and adjust the ZERO pot, immerse in a known pH and adjust the SPAN pot [slope] to give the correct pH...

I suspect you mean to set to pH 7.0 using the zero pot. The the other end of the range is calibrated to what is closest to what you will be using with the span or slope pot, e.g., if you are measuring between 7.0 and 12, then use the ph10.x standard. Some procedures check both ends of the range, but that is not necessary in most situations.

As for the water that you use, most water will have a pH that is slightly acidic. That is due to dissolved CO2 and possibly trace organic compounds (distillation and deionization will not completely get rid of organic compounds). The pH of pure water is hard to determine, because really pure water has such a low ionic concentration that the meters do not work well with it. So, it is common practice to add a drop of concentrated KCl to distilled water to determine its pH.

As for care of the electrode, follow the manufacturer's recommendations. If you do not have them, then store with the tip submerged in KCl solution for long periods or in distilled or deionized water for shorter periods. Rinse with distilled/deionized water before use so as not to contaminate the solution you are measuring. Be sure to open the vent on the probe so the electrodes are at atmospheric pressure.

John

 

[Vikky]

Thanks for you help,

I tried to built the instrumentation amplifier with the gain of 2. Hence for each pH change i would get the change of 120 mv at the ouput. The circuit didn't run as expected though giving different voltage reading at different pH.

After thinking about the cause, i came to conclusion that i didn't kept the provision for the amplification of -ve voltages. The probe output is -413 mv to +413 mv.

I have some how convert this range into the positive side. i.e 100 - 1100mv with 7pH showing 600 mv. Any suggestion how to get this done?? Or can anybody send me the schematic of the same??

 

[Roff]

I don't know if this is the source of your problem, but the pH sensors I have seen have extremely high output impedance, and therefore need to be buffered by a CMOS input amplifier.

 

[Vikky]

but the pH sensors I have seen have extremely high output impedance, and therefore need to be buffered by a CMOS input amplifier.

Yes,

I allready buffered the input of the pH probe sources and then tried to amplify them. But i have only one polarity signal and i think the 0 to -413 mv signal get loss when using this source. Hence trying to convert into the positive side

 

[Roff]

Your transfer function is Vout=1.2107*Vin+0.6. There are various ways of realizing this with op amps, resistors, and a voltage reference.

 

[Vikky]

I have prepared a schemaitc. Please let me know whether it is correct?/

Also how to simulate the circuit for the negative side of the voltage

 

[ericgibbs]

After thinking about the cause, i came to conclusion that i didn't kept the provision for the amplification of -ve voltages. The probe output is -413 mv to +413 mv.

I have some how convert this range into the positive side. i.e 100 - 1100mv with 7pH showing 600 mv. Any suggestion how to get this done?? Or can anybody send me the schematic of the same??

I have prepared a schemaitc. Please let me know whether it is correct?/

Also how to simulate the circuit for the negative side of the voltage

Hi,
The circuit is over complex and will not give the required result.
Are you saying that you want to shift the -413mV thru +413mV input range to, 0V thru +826mV or 0V thru +5V.???

 

[Vikky]

Are you saying that you want to shift the -413mV thru +413mV input range to, 0V thru +826mV or 0V thru +5V.???

yes i that to do that. Need help for the schematic

 

[ericgibbs]

yes i that to do that. Need help for the schematic

OK,
What OPA's do you have access too.?

 

[Vikky]

I have MCP607. I even tried to built it my adding the voltages at the initial stages. Here is the new schematic attached. I gives 600mv at 0mv input. But at the extreme it fails like +413 should give 100mv and -413 mv should give 1100.

Their is the differene of 50mv on both side.