How will this work in a swimming pool environment?
59 views
Skip to first unread message
goinmobile
Apr 17, 2012, 6:04:24 PM4/17/12
to pHduino
Aloha!
Thank you very much for creating and documenting your pH project.
I was considering using your circuit to control the pH of my pool and
I was wondering how this circuit would work in an electrically "noisy"
environment with possible dirty ground issues?
Here's some info to describe my concerns:
Begin article:
Often in pools and industrial control applications a noise voltage
develops between the solution and electrical earth. We assume that
this happens because of poorly constructed pools, faulty pumps, or
even static build up on the PVC pipes; however it's not something we
or the controller manufacturer typically has control over and so we
have to work around it.
The circuit diagrams should help explain the problem. The typical pH
meter and electrode is shown in the 1st diagram. They use a single-
ended amplifier for the pH signal. The pH electrode's reference is
tied to the ground of the circuit, and so the voltage between them is
assumed to be 0mV. When the ground of the pH circuit is otherwise
untied, there is no problem, because it simply floats to the potential
of the solution and there is no noise voltage applied across
R_REFERENCE. For example, this is what happens when using a battery-
powered handheld meter.
However, typical pool/control installations have a fixed power source
which ties the ground potential to the same ground as the noise
source, or another fixed potential with respect to the noise source.
This in turn forces a noise voltage across R_REFERENCE which has a
direct effect on the pH reading. Often the noise voltage is high
enough to exceed the common mode input limits of the amplifier as
well, causing it to saturate and even be damaged in some situations.
In addition the extra current flowing through the electrode's
reference polarises the reference giving another error source.
One solution to the problem is shown in the 2nd diagram. A
differential instrumentation amplifier is used instead of the single-
ended amplifier. The solution noise voltage is reduced to near 0 by
the low resistance solution earth connector directly connected to the
circuit ground; and the amplified voltage is now correctly the voltage
between the pH and reference instead of the voltage between the pH and
ground. Instrumentation amplifiers also typically have much better
common mode rejection than traditional operational amplifier
configurations.
Recommended is the TI INA116 instrumentation amplifier and other
manufacturers also have amplifiers which are suitable. There are
likely other techniques that solve the problem as well - for example
one could design their own instrumentation amplifier; or use optical
isolation. The key specifications for pH amplifiers are that the input
impedance is much higher (typically 1000x) than the electrode
resistance, so as a rule the input impedance should be greater than
1x1012 ohms (at 25C); and for the same reason the input bias current
must be very low, as a rule less than 5pA (at 25C). Be aware that the
resistance of a pH electrode doubles with every 6~10C drop in
temperature and so lower temperatures require better performance from
the amplifier.
End Article
Do you think your "single-ended" circuit will work in a pool
environment without any problems?
Thank you very much!
Craig
Thank you very much for creating and documenting your pH project.
I was considering using your circuit to control the pH of my pool and
I was wondering how this circuit would work in an electrically "noisy"
environment with possible dirty ground issues?
Here's some info to describe my concerns:
Begin article:
Often in pools and industrial control applications a noise voltage
develops between the solution and electrical earth. We assume that
this happens because of poorly constructed pools, faulty pumps, or
even static build up on the PVC pipes; however it's not something we
or the controller manufacturer typically has control over and so we
have to work around it.
The circuit diagrams should help explain the problem. The typical pH
meter and electrode is shown in the 1st diagram. They use a single-
ended amplifier for the pH signal. The pH electrode's reference is
tied to the ground of the circuit, and so the voltage between them is
assumed to be 0mV. When the ground of the pH circuit is otherwise
untied, there is no problem, because it simply floats to the potential
of the solution and there is no noise voltage applied across
R_REFERENCE. For example, this is what happens when using a battery-
powered handheld meter.
However, typical pool/control installations have a fixed power source
which ties the ground potential to the same ground as the noise
source, or another fixed potential with respect to the noise source.
This in turn forces a noise voltage across R_REFERENCE which has a
direct effect on the pH reading. Often the noise voltage is high
enough to exceed the common mode input limits of the amplifier as
well, causing it to saturate and even be damaged in some situations.
In addition the extra current flowing through the electrode's
reference polarises the reference giving another error source.
One solution to the problem is shown in the 2nd diagram. A
differential instrumentation amplifier is used instead of the single-
ended amplifier. The solution noise voltage is reduced to near 0 by
the low resistance solution earth connector directly connected to the
circuit ground; and the amplified voltage is now correctly the voltage
between the pH and reference instead of the voltage between the pH and
ground. Instrumentation amplifiers also typically have much better
common mode rejection than traditional operational amplifier
configurations.
Recommended is the TI INA116 instrumentation amplifier and other
manufacturers also have amplifiers which are suitable. There are
likely other techniques that solve the problem as well - for example
one could design their own instrumentation amplifier; or use optical
isolation. The key specifications for pH amplifiers are that the input
impedance is much higher (typically 1000x) than the electrode
resistance, so as a rule the input impedance should be greater than
1x1012 ohms (at 25C); and for the same reason the input bias current
must be very low, as a rule less than 5pA (at 25C). Be aware that the
resistance of a pH electrode doubles with every 6~10C drop in
temperature and so lower temperatures require better performance from
the amplifier.
End Article
Do you think your "single-ended" circuit will work in a pool
environment without any problems?
Thank you very much!
Craig
Carlos Neves
Apr 17, 2012, 6:28:35 PM4/17/12
to phd...@googlegroups.com
Hi Craig,
Thank you for share your problem with us. I am studying your case. I really appreciate some contribution from some member from the group.
I attached the two diagrams from your first email (it is the same text).
Carlos A. Neves
Thank you for share your problem with us. I am studying your case. I really appreciate some contribution from some member from the group.
I attached the two diagrams from your first email (it is the same text).
Carlos A. Neves
2012/4/17 goinmobile <comb...@yahoo.com>
--
You received this message because you are subscribed to the Google
Groups "pHduino" group.
To post to this group, send email to phd...@googlegroups.com
To unsubscribe from this group, send email to
phduino+u...@googlegroups.com
For more options, visit this group at
http://groups.google.com/group/phduino?hl=en
Carlos Neves
Apr 21, 2012, 10:24:09 PM4/21/12
to phd...@googlegroups.com
Hi Craig,
Sorry about the delay.
First, I need to say that I am not responsible if someone promote any kind of damage for people, materials, etc. You really need to know what is doing.
The pHduino is not the best option for outdoor application. But, if you want to test the idea, go ahead and report us some news.
A standard pH glass electrode is not recommended for outdoor applications. A IS-FET is a better replace. Pay attention in an environment that can develop algae and sludge.
A battery and an optical cable could be used to put the pHmeter ground on a float potential. Wireless is more power consume expensive.
Different configurations and chemicals will generate different results.
* Mechanic parts
The device will be exposed to the environmental conditions. Thermal stresses and humidity variations will damage the trimpots (variable resistors) of the pHduino.
* pH precision
Text is right about the measurement system need has a input impedance 1000x higher the the output impedance of the glass electrode. I select the J-FET Operational Amplifier TL082 because it is cheap and highly available. We will have 100x or 10x in a bet situation. It is acceptable for measurements with a precision from 1 to 0.1 pH units. I told at the project page that it is not an analytical instrument. It is a lab-bench instrument or in a environment well controlled.
You will have bad readings with pH higher than 9 (low hydrogenionic concentration). In this condition, the sodium ion competes with hydrogen ion on the glass electrode. High sodium ions concentration will interfere too when you use a cheap glass electrode made. For high sodium concentration, you need a specific glass electrode made with a glass with low sodium. It is expensive. But, let's return about the precision using the pHduino. How precise need be the pH measurement? What is the range of an acceptable pH? How does fast your automatic system respond for the changes? These are important questions that will determine if the pHduino is acceptable or not.
I don't know the swimming pool chemistry. I did not planned apply pHduino with a swimming pool. I would like to know about the chemicals, problems, etc and how they will affect the pH measurement using a glass electrode.
* Electric isolation
You could power the pHduino up with a 12V big battery, close from it, and send the data by an optical cable. It will put the pHduino ground in a float potential.
You could use wireless communication, but which one is better? How much power will consume?
I can not guarantee that a electric filter will protect the electronic circuit.
* The sensor
The sensor is the most fragile part. It is not like a temperature sensor. Algae and sludge could adsorb on the surface of the sensitive bulb and change the pH performance. It can block the porous junction for the reference electrode. High temperatures will promote evaporation of the solvent of the reference electrolyte. You can not close the hole at the top of the pH sensor to avoid the evaporation. This will Increase the pressure inside of the electrode and could change the measurement.
A good replace for the glass electrode is the IS-FET pH electrodes. It is a solid state and suitable for neural solutions.
http://en.wikipedia.org/wiki/ISFET
I suggest read about pH glass sensor. Take a look at the books:
Skoog et al., "Fundamentals of Analytical Chemistry"
http://www.amazon.com/Fundamentals-Analytical-Chemistry-Seventh-Edition/dp/B0032Q2BKG/ref=sr_1_2?s=books&ie=UTF8&qid=1335060961&sr=1-2
Chapter about Potentiometry and pH glass electrode or any other book about Analytical Chemistry with electrochemistry.
Carlos A. Neves
Sorry about the delay.
First, I need to say that I am not responsible if someone promote any kind of damage for people, materials, etc. You really need to know what is doing.
The pHduino is not the best option for outdoor application. But, if you want to test the idea, go ahead and report us some news.
A standard pH glass electrode is not recommended for outdoor applications. A IS-FET is a better replace. Pay attention in an environment that can develop algae and sludge.
A battery and an optical cable could be used to put the pHmeter ground on a float potential. Wireless is more power consume expensive.
Different configurations and chemicals will generate different results.
* Mechanic parts
The device will be exposed to the environmental conditions. Thermal stresses and humidity variations will damage the trimpots (variable resistors) of the pHduino.
* pH precision
Text is right about the measurement system need has a input impedance 1000x higher the the output impedance of the glass electrode. I select the J-FET Operational Amplifier TL082 because it is cheap and highly available. We will have 100x or 10x in a bet situation. It is acceptable for measurements with a precision from 1 to 0.1 pH units. I told at the project page that it is not an analytical instrument. It is a lab-bench instrument or in a environment well controlled.
You will have bad readings with pH higher than 9 (low hydrogenionic concentration). In this condition, the sodium ion competes with hydrogen ion on the glass electrode. High sodium ions concentration will interfere too when you use a cheap glass electrode made. For high sodium concentration, you need a specific glass electrode made with a glass with low sodium. It is expensive. But, let's return about the precision using the pHduino. How precise need be the pH measurement? What is the range of an acceptable pH? How does fast your automatic system respond for the changes? These are important questions that will determine if the pHduino is acceptable or not.
I don't know the swimming pool chemistry. I did not planned apply pHduino with a swimming pool. I would like to know about the chemicals, problems, etc and how they will affect the pH measurement using a glass electrode.
* Electric isolation
You could power the pHduino up with a 12V big battery, close from it, and send the data by an optical cable. It will put the pHduino ground in a float potential.
You could use wireless communication, but which one is better? How much power will consume?
I can not guarantee that a electric filter will protect the electronic circuit.
* The sensor
The sensor is the most fragile part. It is not like a temperature sensor. Algae and sludge could adsorb on the surface of the sensitive bulb and change the pH performance. It can block the porous junction for the reference electrode. High temperatures will promote evaporation of the solvent of the reference electrolyte. You can not close the hole at the top of the pH sensor to avoid the evaporation. This will Increase the pressure inside of the electrode and could change the measurement.
A good replace for the glass electrode is the IS-FET pH electrodes. It is a solid state and suitable for neural solutions.
http://en.wikipedia.org/wiki/ISFET
I suggest read about pH glass sensor. Take a look at the books:
Skoog et al., "Fundamentals of Analytical Chemistry"
http://www.amazon.com/Fundamentals-Analytical-Chemistry-Seventh-Edition/dp/B0032Q2BKG/ref=sr_1_2?s=books&ie=UTF8&qid=1335060961&sr=1-2
Chapter about Potentiometry and pH glass electrode or any other book about Analytical Chemistry with electrochemistry.
Carlos A. Neves
2012/4/17 Carlos Neves <can...@gmail.com>
Reply all
Reply to author
Forward
0 new messages