Which pH meter

Wayne Sallee wrote on 5/3/2007 12:27 PM:

Most people calibrate their Ph meters for us in salt water with 7, and
4.

oops, That was supposed to read "meters for use in
salt water".

Wayne Sallee
Wayne's Pets

On Thu, 03 May 2007 19:17:17 +0200, Wayne Sallee
wrote:

http://www.bpp.com.pl/?left=dysleksja&right=dysleksja
http://www.terapia-kregoslupa.waw.pl

Well what a coincidink :-) It's in Polish :-)

Now who here speaks Polish besides Pszemol?

Now Pszemol if you wanted to post some links to web sites you could have
used your own handle, instead of making it look like you were someone
else.

While Pszemol's nick suggests he is Polish (at least for someone that
speaks Polish) he is not the only Polish posting on usenet. And I am
definitely not him

Borek AKA Mr.pH
--
http://www.chembuddy.com/?left=BATE&right=pH-calculator
http://www.ph-meter.info/pH-electrode
http://www.bpp.com.pl/?left=dysleksja&right=dysleksja
http://www.terapia-kregoslupa.waw.pl

"Wayne Sallee" wrote in message ink.net...
Mr.pH wrote on 5/3/2007 5:13 AM:
http://www.bpp.com.pl/?left=dysleksja&right=dysleksja
http://www.terapia-kregoslupa.waw.pl


Well what a coincidink :-) It's in Polish :-)

Now who here speaks Polish besides Pszemol?

Now Pszemol if you wanted to post some links to web
sites you could have used your own handle, instead
of making it look like you were someone else.

Wayne - stop being paranoic... "Mr.pH" is not me :-)

There is about 38 milions residents in Poland alone
and about 10 milions somewhere around the World.
Including 2-3 milions (or more?) immigrants in USA.
The chances are - more than one of them is here in our group.

And your "coincidink" is not a good proof of anything...
I hope your are not often chosen to do jury duty, Wayne! ;-)

"Wayne Sallee" wrote in message nk.net...
I did not write the software for the Ph meters, but
I do know what the results are when I use my
calibration methods, so I do know what the end
results are. And what is most important here, is
what the end results are.

Repeat after me:
calibration at pH 4 is not needed for sal****er measurements.
Only pH 9 or 10 makes sense in relation to zeroing in pH 7.0

Most people calibrate their Ph meters for us in salt
water with 7, and 4.

Only if they do not read the instructions included with pHmeter.

I was stating that it is better to calibrate with 10.

You were correct in this statement.
But you were wrong suggesting that additional calibration at pH 4
improves measurements between pH 7 and 10. It does not.
If anything - it might decrease pH meter accuracy in the 7-10 range.

I also stated that I like to look at 4 as well, and see where it is.
And if I use a ph meter (I have more than one ph meter) mainly
for fresh water I will go with the 4 ph calibration
as the standard, but still like to look at the 10.

Let me asure you that pH meter uses these ranges SEPARATELLY.

You are doing more harm than help adjusting
your pH meter for acurate pH 4 when you measure
marine water.

That's silly. I am not doing more harm than good by
using all 3 calibration fluids.

Yes, you are.
Calibration at pH 4 influences corrections you made for pH 10.
These are two separate ranges your pH meter operates and
it usually does not have a memory to keep separate settings
for both ranges even if the calibration is automatic/digital.
It is most certainly bad, if the meter has a turn knob (trimmer
setting the amplifier gain directly with the variable resistance).

This is the problem with your thinking:
pH probe is not ideal and is not linear! The calibration
curve will need a DIFFERENT correction above pH 7
and below pH 7. THAT IS THE REASON YOU DO
CALIBRATION WRAPING YOUR EXPECTED SAMPLE pH.

I don't understand what you are trying to say here.

Ask the question... I am electronic engineer involved in
design of electronic circuits - I will try to explain as good as I can... :-)

The problem is that when you adjust reading at pH 4
you influence reading at pH 10. Each range will have
conflicting needs and calibration point should always
wrap around the expected sample pH.

You need to re-read what I said.

When you calibrate at pH 4 you DISTURB calibration made for pH 10.

Now if you were to purchase a Ph meter that only had one calibration
knob on it, which calibration knob would you like to be without. I
would not want to be without either one :-)

Calibration at pH 7 is in fact compensating "zero point"
for the pH probe and the whole measuring equipment.
It is more important than pH 4 or 10 calibration points,
which is setting only the DC amplifier gain - this varies
less from probe to probe and can be preset at factory
in cheaper, less precise units.


Yes, 7 is the most important point. That's why it
should be set first. So wha'ts your point?

My point is that only very expensive, lab-grade pH meters costing
well above $1000 have ability to keep multipoint calibration curves
for measurements in wide ranges of sample pH. Our regular meters
can keep only one curve in memory (if digital) or have fixed aplifier
gain set with the "knob" (which is a variable resistor in a feedback
loop of the operational amplifier). Due to the nature of the beast,
you should always calibrate with the buffors reflecting your expected
sample pH the way, sample pH lays inbetween the calibration points.
Calibration for pH 4 made after calibration at pH 10 does not make
sense if you measure pH 8.3 and it affects previously made
calibration for pH 10. In effect makes your meter LESS accurate
for measurements in the pH 7-10 range.

Pszemol wrote on 5/5/2007 9:20 AM:
"Wayne Sallee" wrote in message
nk.net...
I did not write the software for the Ph meters, but I do know what the
results are when I use my calibration methods, so I do know what the
end results are. And what is most important here, is what the end
results are.

Repeat after me:
calibration at pH 4 is not needed for sal****er measurements. Only pH 9
or 10 makes sense in relation to zeroing in pH 7.0

Most people calibrate their Ph meters for us in salt water with 7, and 4.

Only if they do not read the instructions included with pHmeter.

I was stating that it is better to calibrate with 10.

You were correct in this statement. But you were wrong suggesting that
additional calibration at pH 4
improves measurements between pH 7 and 10. It does not.
If anything - it might decrease pH meter accuracy in the 7-10 range.


It helps show degradation of the accuracy of the
probe as it gets older.



I also stated that I like to look at 4 as well, and see where it is.
And if I use a ph meter (I have more than one ph meter) mainly for
fresh water I will go with the 4 ph calibration as the standard, but
still like to look at the 10.

Let me asure you that pH meter uses these ranges SEPARATELLY.


No they don't. Not the ones that I've used.
The Ph 4,10 knob adjusts both 4 and 10 ends of the
scale a the same time.

You are doing more harm than help adjusting
your pH meter for acurate pH 4 when you measure
marine water.

That's silly. I am not doing more harm than good by using all 3
calibration fluids.

Yes, you are.
Calibration at pH 4 influences corrections you made for pH 10.
These are two separate ranges your pH meter operates and
it usually does not have a memory to keep separate settings
for both ranges even if the calibration is automatic/digital. It is most
certainly bad, if the meter has a turn knob (trimmer
setting the amplifier gain directly with the variable resistance).


Like I said in a few lines up, the Ph 4,10 knob
adjusts both 4 and 10 ends of the scale a the same time.


This is the problem with your thinking:
pH probe is not ideal and is not linear! The calibration
curve will need a DIFFERENT correction above pH 7
and below pH 7. THAT IS THE REASON YOU DO
CALIBRATION WRAPING YOUR EXPECTED SAMPLE pH.

I don't understand what you are trying to say here.

Ask the question... I am electronic engineer involved in
design of electronic circuits - I will try to explain as good as I
can... :-)

And I have been involved with writting machine code
to operate computer circuits, but what does this
have to do with the circuits in the ph probes? What
experience have you had with Ph probe circuits?


The problem is that when you adjust reading at pH 4
you influence reading at pH 10. Each range will have
conflicting needs and calibration point should always
wrap around the expected sample pH.

You need to re-read what I said.

When you calibrate at pH 4 you DISTURB calibration made for pH 10.


Exactly my point. You are adjusting both the 4 and
the 10 end of the scale when you adjust one of them.
Of course you are also affecting the whole scale.
You can look a this as being like compressing or
stretching a meter stick.

Now if you were to purchase a Ph meter that only had one calibration
knob on it, which calibration knob would you like to be without. I
would not want to be without either one :-)

Calibration at pH 7 is in fact compensating "zero point"
for the pH probe and the whole measuring equipment.
It is more important than pH 4 or 10 calibration points,
which is setting only the DC amplifier gain - this varies
less from probe to probe and can be preset at factory
in cheaper, less precise units.


Yes, 7 is the most important point. That's why it should be set first.
So wha'ts your point?

My point is that only very expensive, lab-grade pH meters costing
well above $1000 have ability to keep multipoint calibration curves
for measurements in wide ranges of sample pH. Our regular meters
can keep only one curve in memory (if digital) or have fixed aplifier
gain set with the "knob" (which is a variable resistor in a feedback
loop of the operational amplifier). Due to the nature of the beast,
you should always calibrate with the buffors reflecting your expected
sample pH the way, sample pH lays inbetween the calibration points.
Calibration for pH 4 made after calibration at pH 10 does not make
sense if you measure pH 8.3 and it affects previously made
calibration for pH 10. In effect makes your meter LESS accurate
for measurements in the pH 7-10 range.

It does make sence if you are going to be using your
ph meter to measure fluids from 5.5 to 8.5

When I calibrate my ph meters, I like to know how
they are going to read across the entire scale.

Wayne Sallee
Wayne's Pets

"Wayne Sallee" wrote in message ink.net...
I was stating that it is better to calibrate with 10.

You were correct in this statement. But you were wrong suggesting that additional calibration at pH 4
improves measurements between pH 7 and 10. It does not.
If anything - it might decrease pH meter accuracy in the 7-10 range.


It helps show degradation of the accuracy of the probe as it gets older.

not really... It only shows how different characteristic of the probe
is in these two ranges: 7-10 and 4-7.

I also stated that I like to look at 4 as well, and see where it is.
And if I use a ph meter (I have more than one ph meter) mainly for fresh water I will go with the 4 ph calibration as the
standard, but still like to look at the 10.

Let me asure you that pH meter uses these ranges SEPARATELLY.


No they don't. Not the ones that I've used.
The Ph 4,10 knob adjusts both 4 and 10 ends of the scale a the same time.

That is exactly what I said: this meter uses one setting for two separate ranges. If you pick one setting of this knob good for pH 4
it will not be good for pH 10 and vice versa. If you get something between, averaging two calibrations, you will get both ranges not
accurate as good as they could be.
From the meter perspective both ranges are separated and it does not have intelligence to connect them anyhow. They are separated by
the probe characteristic which is different for each range and is aproximated with linear at different angle for each measurement
range.

Like I said in a few lines up, the Ph 4,10 knob adjusts both 4 and 10 ends of the scale a the same time.

pH 4/10 knob is ONE, and is responsible for ONE setting of operational amplifier gain.
This sets the angle for the linear aproximation of non-linear probe characteristic in one of these two ranges.

I don't understand what you are trying to say here.

Ask the question... I am electronic engineer involved in
design of electronic circuits - I will try to explain as good as I can... :-)

And I have been involved with writting machine code to operate computer circuits, but what does this have to do with the circuits
in the ph probes?

It has a lot to do with circuits in pH meters.
The measuring principle is the same in many other elecronic meters.
You need to set the zero point and then adjust angle of the linear
aproximation - this second setting is just gain of the amplifier...

output voltage = gain(input voltage) + offset.
or
output voltage = gain(input voltage + offset)
depending on the circuit.

pH probe is just weak source of electric DC voltage with very high internal resistance ranging 100-200 megaohm.
Your pH meter is just very precise DC volt meter with adjusted gain the way that miliVolts are translated to pH readings.

What experience have you had with Ph probe circuits?

Well, I have build one simple pH meter myself using one LF444 integrated circuit containing 4 opamps.
If you have basic electronic skills and know how to read and understand electronic schematics you can try to understand how the pH
meter works from the LF444 datasheet. Manufacturer of this simple chip is showing its usage in measuring devices using pH meter guts
as an example:
http://cache.national.com/ds/LF/LF444.pdf

Check out the page 9 - you will find two adjustable resistors, "knobs" like you call them, one is marked "calibration" and this one
is adjusting zero point of the amplifier (pH 7 adjust) and the other, marked as "temperature" adjusts gain of the amplifier, for the
user this will be your "pH 4/10" adjust.
Output from the circuit is just variable voltage which can be read with any digital or analog voltmeter. Your pH meter will have
both parts in one box: voltmeter with digital readout and the preamplifier with adjustments to prepare DC signal generated by the pH
probe to be read by the voltmeter.
BTW - there is no electronics in the probe itself. Probe is a pure electrochemical devices with chemical processes causing voltage
difference measured by the high imput resistance operational amplifiers. Sometimes probe is embedded with a temperature sensor in a
form of a thermistor or PT100 thermopair sensor.

As you can see, adjustment is one for pH 4 as for pH 10, so if the probe characteristic in these two ranges differs significantly,
you better adjust the meter for only one range at the time, take the measurements and than if second sample is in a different range,
calibrate again in the range reflecting the measured sample.

The problem is that when you adjust reading at pH 4
you influence reading at pH 10. Each range will have
conflicting needs and calibration point should always
wrap around the expected sample pH.

You need to re-read what I said.

When you calibrate at pH 4 you DISTURB calibration made for pH 10.


Exactly my point. You are adjusting both the 4 and the 10 end of the scale when you adjust one of them. Of course you are also
affecting the whole scale. You can look a this as being like compressing or stretching a meter stick.

Dont say exactly my point because you were suggesting calibration of the meter with two solutions,
which is wrong. You will have better calibration results for given range if you calibrate with only calibration solution.
Calibration with pH 4 disturbs previous calibration made with pH 10 and meter will NOT BE AS ACURATE for measurements in sal****er
as it would be if calibrated with only pH 10.

It does make sence if you are going to be using your ph meter to measure fluids from 5.5 to 8.5

When I calibrate my ph meters, I like to know how they are going to read across the entire scale.

The best option is to calibrate meter with pH 4 for measurements of fluid of pH 4-7 and to calibrate with pH 10 formeasurements of
fluids of pH 7-10.
As you will see in above discussion calibration with pH 4 makes readings in 7-10 range LESS ACURATE. How much less? It depends on
specific probe you use - each probe will be different.

Pszemol wrote on 5/6/2007 9:04 AM:
"Wayne Sallee" wrote in message
ink.net...
I was stating that it is better to calibrate with 10.

You were correct in this statement. But you were wrong suggesting
that additional calibration at pH 4
improves measurements between pH 7 and 10. It does not.
If anything - it might decrease pH meter accuracy in the 7-10 range.


It helps show degradation of the accuracy of the probe as it gets older.

not really... It only shows how different characteristic of the probe
is in these two ranges: 7-10 and 4-7.

I also stated that I like to look at 4 as well, and see where it is.
And if I use a ph meter (I have more than one ph meter) mainly for
fresh water I will go with the 4 ph calibration as the standard, but
still like to look at the 10.

Let me asure you that pH meter uses these ranges SEPARATELLY.


No they don't. Not the ones that I've used.
The Ph 4,10 knob adjusts both 4 and 10 ends of the scale a the same time.

That is exactly what I said: this meter uses one setting for two
separate ranges. If you pick one setting of this knob good for pH 4 it
will not be good for pH 10 and vice versa. If you get something between,
averaging two calibrations, you will get both ranges not accurate as
good as they could be.
From the meter perspective both ranges are separated and it does not
have intelligence to connect them anyhow. They are separated by the
probe characteristic which is different for each range and is
aproximated with linear at different angle for each measurement range.


Well of course, if they were not that way, then it
would not be as accurate. And some Ph meters have
been made with a knob for 7, a knob for 4, and
another knob for 10. So what's your point?

Like I said in a few lines up, the Ph 4,10 knob adjusts both 4 and 10
ends of the scale a the same time.

pH 4/10 knob is ONE, and is responsible for ONE setting of operational
amplifier gain.
This sets the angle for the linear aproximation of non-linear probe
characteristic in one of these two ranges.


No. You can check the calibration of a ph meter, and
have 10 and 4 dead on, and then recalibrate the 10
to 10.something, then go to the 4, with a 4
solution, and then bring it back to 4, and the 10
will be back to 10.

I don't understand what you are trying to say here.

Ask the question... I am electronic engineer involved in
design of electronic circuits - I will try to explain as good as I
can... :-)

And I have been involved with writting machine code to operate
computer circuits, but what does this have to do with the circuits in
the ph probes?

It has a lot to do with circuits in pH meters.
The measuring principle is the same in many other elecronic meters.
You need to set the zero point and then adjust angle of the linear
aproximation - this second setting is just gain of the amplifier...

output voltage = gain(input voltage) + offset.
or
output voltage = gain(input voltage + offset)
depending on the circuit.

pH probe is just weak source of electric DC voltage with very high
internal resistance ranging 100-200 megaohm.
Your pH meter is just very precise DC volt meter with adjusted gain the
way that miliVolts are translated to pH readings.

What experience have you had with Ph probe circuits?

Well, I have build one simple pH meter myself using one LF444 integrated
circuit containing 4 opamps.
If you have basic electronic skills and know how to read and understand
electronic schematics you can try to understand how the pH meter works
from the LF444 datasheet. Manufacturer of this simple chip is showing
its usage in measuring devices using pH meter guts as an example:
http://cache.national.com/ds/LF/LF444.pdf

Check out the page 9 - you will find two adjustable resistors, "knobs"
like you call them,

Yea and I could have also called them trimmers, but
most people would not have known what I was talking
about if I did that.

one is marked "calibration" and this one is
adjusting zero point of the amplifier (pH 7 adjust) and the other,
marked as "temperature" adjusts gain of the amplifier, for the user this
will be your "pH 4/10" adjust.

It looks to me like this design was not designed for
a 4,10 adjustment.

The instructions says:

"To calibrate, insert probe in pH =7 solution. Set
the “TEMPERATURE ADJUST” pot, R2, to correspond to
the solution temperatu full clockwise for 0°C, and
proportionately for intermediate temperatures, using
a turns-counting dial. Then set “CALIBRATE” pot so
output reads 7V."



By the way, the pinpoint probes do automatically
adjust for temp.


Output from the circuit is just variable voltage which can be read with
any digital or analog voltmeter. Your pH meter will have both parts in
one box: voltmeter with digital readout and the preamplifier with
adjustments to prepare DC signal generated by the pH probe to be read by
the voltmeter.
BTW - there is no electronics in the probe itself. Probe is a pure
electrochemical devices with chemical processes causing voltage
difference measured by the high imput resistance operational amplifiers.
Sometimes probe is embedded with a temperature sensor in a form of a
thermistor or PT100 thermopair sensor.

As you can see, adjustment is one for pH 4 as for pH 10, so if the probe
characteristic in these two ranges differs significantly, you better
adjust the meter for only one range at the time, take the measurements
and than if second sample is in a different range, calibrate again in
the range reflecting the measured sample.

The problem is that when you adjust reading at pH 4
you influence reading at pH 10. Each range will have
conflicting needs and calibration point should always
wrap around the expected sample pH.

You need to re-read what I said.

When you calibrate at pH 4 you DISTURB calibration made for pH 10.


Exactly my point. You are adjusting both the 4 and the 10 end of the
scale when you adjust one of them. Of course you are also affecting
the whole scale. You can look a this as being like compressing or
stretching a meter stick.

Dont say exactly my point because you were suggesting calibration of the
meter with two solutions,
which is wrong.

No I was saying that I like to calibrate with 3
solutions.


You will have better calibration results for given range
if you calibrate with only calibration solution.
Calibration with pH 4 disturbs previous calibration made with pH 10 and
meter will NOT BE AS ACURATE for measurements in sal****er as it would
be if calibrated with only pH 10.

It does make sence if you are going to be using your ph meter to
measure fluids from 5.5 to 8.5

When I calibrate my ph meters, I like to know how they are going to
read across the entire scale.

The best option is to calibrate meter with pH 4 for measurements of
fluid of pH 4-7 and to calibrate with pH 10 formeasurements of fluids of
pH 7-10.
As you will see in above discussion calibration with pH 4 makes readings
in 7-10 range LESS ACURATE. How much less? It depends on specific probe
you use - each probe will be different.

Not everybody uses their Ph probe for only 7 to 10,
or 4 to 7.

Wayne Sallee
Wayne's Pets

On Sun, 06 May 2007 16:04:24 +0200, Pszemol wrote:

From the meter perspective both ranges are separated and it does not
have intelligence to connect them anyhow. They are separated by the
probe characteristic which is different for each range and is
aproximated with linear at different angle for each measurement range.

Note that theoretical response IS linear, so calibration means only
adjusting for the lack of perfectness

Well, I have build one simple pH meter myself using one LF444 integrated
circuit containing 4 opamps.
If you have basic electronic skills and know how to read and understand
electronic schematics you can try to understand how the pH meter works
from the LF444 datasheet. Manufacturer of this simple chip is showing
its usage in measuring devices using pH meter guts as an example:
http://cache.national.com/ds/LF/LF444.pdf

See other example at
http://www.ph-meter.info/pH-meter-construction

Two calibration resistors called - not surprisingly - slope and zero

Mr.pH
--
http://www.chembuddy.com/?left=BATE&right=pH-calculator
http://www.ph-meter.info/pH-electrode

http://www.bpp.com.pl/?left=dysleksja&right=dysleksja
http://www.terapia-kregoslupa.waw.pl

On Sun, 06 May 2007 21:42:58 +0200, Wayne Sallee
wrote:

No. You can check the calibration of a ph meter, and have 10 and 4 dead
on, and then recalibrate the 10 to 10.something, then go to the 4, with
a 4 solution, and then bring it back to 4, and the 10 will be back to 10.

You said it by yourself: you calibrate at 10.00, then you move electrode
to 4.00 buffer, it shows 4.something, so you calibrate it to show 4.00,
now you go back to 10.00 buffer - and your indications are no longer 10.00
but 10.-something. That's exactly Pszemol's point and you have just
confirmed he is right

I will stick with two-point calibration, depending on the range I am
interested in. Above 7 - calibrate for 7/10. Below 7 - calibrate for 4/7..
You want to register titration curve and you need 2-11 range - I am in
troubles

Mr.pH
--
http://www.chembuddy.com/?left=BATE&right=pH-calculator
http://www.ph-meter.info/pH-electrode

http://www.bpp.com.pl/?left=dysleksja&right=dysleksja
http://www.terapia-kregoslupa.waw.pl

Borek wrote on 5/7/2007 2:48 AM:
On Sun, 06 May 2007 21:42:58 +0200, Wayne Sallee
wrote:

No. You can check the calibration of a ph meter, and have 10 and 4
dead on, and then recalibrate the 10 to 10.something, then go to the
4, with a 4 solution, and then bring it back to 4, and the 10 will be
back to 10.

You said it by yourself: you calibrate at 10.00, then you move electrode
to 4.00 buffer, it shows 4.something, so you calibrate it to show 4.00,
now you go back to 10.00 buffer - and your indications are no longer
10.00 but 10.-something. That's exactly Pszemol's point and you have
just confirmed he is right

Don't twist my words. That's not what I said.

Wayne Sallee
Wayne's Pets