Measuring pH probe mV reading directly with a multimeter.

Thread Starter

cgw94

Joined Jun 11, 2020
41
Hi all, I am trying to determine whether my pH probe is giving me accurate data or not. I was hoping to measure the voltage with a multimeter. I have the probe in a pH buffer of 4 and according to pH to voltage scale. 1 pH is about 59 mV/pH, Starting at 7 pH = 0. IF thats the case i should be seeing around 150mV but i only get about 10mV

not sure if this is a good place to ask but everyone here is very helpful.

Thanks!
 

ci139

Joined Jul 11, 2016
1,696
there are quite large errors even inside the brand/model - the good point is the nature of the errors inside the "model" are more determined/similar/predictable/adjustable/correctable than in between different brands/manufacturers ...

... e.g. you must know/determine your error throughout the measurement range at various working conditions -or- you have to accept it's unknown , with the range that may be larger the one specified in the manual

 
Last edited:

Danko

Joined Nov 22, 2017
988
Hi all, I am trying to determine whether my pH probe is giving me accurate data or not. I was hoping to measure the voltage with a multimeter. I have the probe in a pH buffer of 4 and according to pH to voltage scale. 1 pH is about 59 mV/pH, Starting at 7 pH = 0. IF thats the case i should be seeing around 150mV but i only get about 10mV
"The source impedance of a pH electrode is very high because the thin glass bulb has a large resistance that is typically in the range of 10 MΩ to 1000 MΩ.
This means that the electrode can only be monitored by a high-impedance measuring device"

See solution here
1601055107089.png
 
Last edited:

Thread Starter

cgw94

Joined Jun 11, 2020
41
"The source impedance of a pH electrode is very high because the thin glass bulb has a large resistance that is typically in the range of 10 MΩ to 1000 MΩ.
This means that the electrode can only be monitored by a high-impedance measuring device"

See solution here
View attachment 218018
Ah that is a well explained circuit. That will be my next steps. Currently I just want to take my signal and boost the hell out of it, just need to know how much. I built a amplifier circuit to boost the expected ~150mV to about 2V but i can't seem to get any input from the probe
 

Thread Starter

cgw94

Joined Jun 11, 2020
41
That may be the problem. The TL082 has a much higher input impedance than a LM324.
ahhhhh i see. well that would explain one issue i might be having. But shouldn't i still be able to see how many mV the probe is producing without circuitry?
 

Thread Starter

cgw94

Joined Jun 11, 2020
41
You can't do that with a standard multimeter, the impedance of the probe is much too high.
I am a little confused now, because I was also told in this thread that I cant use a LM324 op amp due to its low input impedance? Not sure what to do with that.
 

sghioto

Joined Dec 31, 2017
1,428
But shouldn't i still be able to see how many mV the probe is producing without circuitry?
IF the input impedance of your meter is the same as the LM324 then your readings would be about equal.
EDIT: What readings were you getting with the LM324?
SG
 
Last edited:

OBW0549

Joined Mar 2, 2015
3,544
The problem with trying to use an LM324 is not just its low input resistance compared with, say, a TL082, but also the fact that its input bias current is so much higher-- up to 100 nA versus just 400 pA (0.4 nA) for the TL082. That current has to flow through the pH bulb's internal resistance and, with an LM324, will drastically alter the voltage you read to the point where your measurement is meaningless.
 

Thread Starter

cgw94

Joined Jun 11, 2020
41
Alrighty I am misunderstanding something for sure. Here is what i've read.

Can't use LM324 due to having lower input impedance than TL082.
Cant' use multimeter directly because input impedance is too high.
IF the input impedance of your meter is the same as the LM324 then your readings would be about equal.
EDIT: What readings were getting with the LM324?
SG
About that same as the meter reading the raw voltage. Here is my circuit more or less for the lm324. Which I know is wrong on its own but I want to confirm my input voltage is correct before I continue on.

1601056736570.png
 

OBW0549

Joined Mar 2, 2015
3,544
Cant' use multimeter directly because input impedance is too high.
Wrong. The input resistance of your meter is too LOW, not too high; compared to the gigohm-level source impedance of your pH probe, your meter appears as a dead short.
 

Thread Starter

cgw94

Joined Jun 11, 2020
41
The problem with trying to use an LM324 is not just its low input resistance compared with, say, a TL082, but also the fact that its input bias current is so much higher-- up to 100 nA versus just 400 pA (0.4 nA) for the TL082. That current has to flow through the pH bulb's internal resistance and, with an LM324, will drastically alter the voltage you read to the point where your measurement is meaningless.
oohhhhh gotcha gotcha ok. I think my next steps are to aquire a correct op amp and see where i go from there
 
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