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TEMPERATURE COMPENSATION OF pH READINGS?

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TEMPERATURE COMPENSATION OF pH READINGS?

Postby Glovisol » Thu Oct 13, 2016 3:01 pm

I am using the USB DrDAQ with a standard DD011 pH electrode and also have a DD100 temperature sensor.

I understand that the pH electrode is calibrated by inserting readings taken from standard buffer solutions into a scaling table for the pH channel. All is clear up to this point and works well.

But

The DrDAQ User's Guide, paragraph 2.9.3, page 14, under pH MEASUREMENT AND TEMPERATURE CALIBRATION, states: "..........To minimize these errors, a temperature sensor can be connected to EXT 1 and the DrDAQ will compensate for temperature changes".

The above seems to imply there is an automatic correction inside Picolog/DrDAQ mechanics to compensate for temperature, as read from the DD100 on EXT 1. I have examined the channels involved, but I cannot find any provisons for this... Clarification and/or additional istructions on this point would be very welcome!

glovisol
Last edited by Glovisol on Wed Oct 19, 2016 6:34 am, edited 1 time in total.
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Re: TEMPERATURE COMPENSATION OF pH READINGS?

Postby Gerry » Fri Oct 14, 2016 10:04 am

Hi Glovisol,

The value for pH is a measure of the activity of the Hydrogen ions in the solution being measured. Increasing the temperature increases the activity of the ions, so the slope of the pH response for a pH sensor varies linearly with temperature. This direct dependence enables us to compensate (calibrate) the pH response (as mentioned in the Dr DAQ User Guide) by incorporating temperature into the measurement. To do this you need to set up the EXT 1 input to take temperature measurements with a specific temperature sensor, and then connect the appropriate sensor.

So, the information that you need is:
1) how to connect a temperature sensor (including autodetect)
2) how to set up the input scaling (e.g. when reducing an excitation voltage to the input range of the Dr DAQ channel for a resistance based temperature sensor)
3) how to set up PicoLog (e.g. with a scaling file to linearize a thermistor)

All of that is explained starting from page 24 of the Dr DAQ User Guide. (Note that a thermocouple would not be appropriate because of the low sensitivity, and need for cold junction compensation, and an RTD sensor wouldn't be a good choice either because of the extremely low sensitivity)

Regards,

Gerry
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Re: TEMPERATURE COMPENSATION OF pH READINGS?

Postby Glovisol » Fri Oct 14, 2016 10:47 am

Dear Gerry,

Thanks for your reply, which, unfortunately, does not help. Of course it is generally well known how to apply a temperature sensor to the EXT 1 input of the DrDAQ, in fact I mentioned I am using a DD100 temperature sensor. Moreover I have done extensive work and publication on this, pse see my previous postings on DDS files.

MY ORIGINAL QUESTION WAS AND IS: where is the connection between the EXT 1 reading and the PH channel reading? Pico literature (and also your reply) seem to IMPLY THAT THE TEMPERATURE CORRECTION of THE PH READING IS DONE AUTOMATICALLY BY THE DrDAQ, starting from the temperature reading on EXT 1. I do not see how this can happen, also considering the math complexity of the necessary correction.

Please just reply: is the temperature correction done automatically? YES or NO. And if YES, HOW?

Kind regards,

glovisol
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Re: TEMPERATURE COMPENSATION OF pH READINGS?

Postby Glovisol » Fri Oct 14, 2016 2:31 pm

Dear Gerry,

I have connected the DD100 Temperature Probe to the EXT 1 input of the DrDAQ and can confirm that temperature variations as measured by the probe indeed automatically change the pH reading as shown on the pH channel. Perhap you can explain how this is done....

I shall now check the Probe/DrDAQ system with buffer solutions at various temperatures and submit the results in the next post.

Kind regards,

glovisol
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Re: TEMPERATURE COMPENSATION OF pH READINGS?

Postby Glovisol » Sat Oct 15, 2016 9:14 am

PH TEST RESULTS UNDER SAMPLE TEMPERATURE VARIATION

The tests were conducted by reading the pH of buffer solutions graded 4.01 and 7.01. The solution for the test was placed inside a small plastic container placed inside a glass container. Water at a given temperature was then poured into the glass container and pH of the buffer solution measured by the pH probe. The temperature probe was also immersed in the buffer solution to track solution temperature.

TEST DATA AS FOLLOWS
Acidic buffer solution pH = 4.01 by MILWAUKEE - M 10004.
Neutral buffer solution pH = 7.01 by MILWAUKEE - M 10007.
pH Test Probe: Pico DD011 on DrDAQ pH channel.
Temperature Test Probe: Pico DD100 on DrDAQ EXT 1 channel.

pH PROBE CALIBRATION LOOKUP TABLE
RAW............SCALED
3.65.............4.01
6.55.............7.01
The calibration data was obtained by bringing the buffer solution up to 25°C as measured by the temperature probe. It takes 120" approx. for the temperature and the pH readings to stabilise.

TEST RESULTS
ACIDIC..........................................NEUTRAL
Nominal 4.01............MILWAUKEE.........Nominal 7.01....... MILWAUKEE
33.3°C...pH = 4.05.....4.02..................33.3°C pH = 7.08....6.99
25.0°C...pH = 4.01.....4.01..................25.0°C pH = 7.01....7.01
23.3°C...pH = 4.01.....4.01..................23.3°C pH = 7.01....7.02
13.3°C...pH = 3.97.....4.00..................13.3°C pH = 6.97....7.05

The columns "MILWAUKEE" list the nominal pH value vs. temperature shown on the packet and tell us that the automatic compensation tends to overcompensate a bit. In any case the DrDAQ/probe system shows excellent accuracy over the tested 20°C range.

One problem is the long measurement time due to the big mass of the DD100 Temperature Sensor, which has a big thermal inertia. Additionally, if the mass of the solution under measurement is small, the temperature of the Temperature Sensor before immersion would alter the temperature of the solution. A smaller Temperature Sensor such as the LM35 would drastically reduce these problems.
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Re: TEMPERATURE COMPENSATION OF pH READINGS?

Postby Glovisol » Sun Oct 16, 2016 6:59 am

Dear Jerry,

The temperature compensation of the pH reading works only with the DD100 Temperature Sensor and with sensors based on an NTC (negative temperature coefficient) resistor having the same characteristic R-vs-T of the DD100. This happens because the internal, hidden and user not accessible compensation software does not read the sensor's temperature, but the sensor's voltage.

The present arrangement rules out not only any other temperature sensing device, such as the LM35, PTC's, etc., but also NTC's different from the one used in the DD100. In my opinion the compensation software should be modified to read temperature, thereby allowing the use of ANY sensor.

The measured characteristic of the NTC used in the DD100 is as follows:

TEMPERATURE °C.............................VOLTAGE mV
5................................................1822
10...............................................1687
20...............................................1395
30...............................................1100
40................................................840
45................................................725

I am asking you to state the NTC type used in the DD100, so that anyone can build a low mass temperature sensor for fast pH reading compensation using the present software arrangement.

Best regards,

glovisol
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Re: TEMPERATURE COMPENSATION OF pH READINGS?

Postby Glovisol » Mon Oct 17, 2016 7:59 am

TECHNICAL DETAILS OF THE DD100 TEMPERATURE SENSOR AND DrDAQ MODE OF OPERATION

When using the DD100 Temperature Sensor, the DrDAQ exploits the internal 100 KOhm pull-up resistor (Pin 1 of EXT 1) to feed the NTC resistor inside the probe, then reads the voltage across the NTC (beween Pin 1 and Pin 2 (ground) of EXT 1). The open circuit voltage at Pin 1 is 2.475, e.g. 2.5 V.

The measured value of the sense resistor built into the DD100 is 330 KOhm. Note that the DD100 does not use the +5V present on Pin 4 of EXT 1.

Measuring the voltage at Pin 1 of the EXT 1 input at various temperatures and applying Ohm's Law, by simple mathematics we obtain the following data:

TEMP.,°C..................Pin 1, V.....................NTC Resistance, Ohm

5............................1.822........................268,732
10...........................1.687........................207,503
15...........................1.543........................161,233
20...........................1.395........................126,244
25...........................1.246.........................99,362
30...........................1.100.........................78,571
35...........................0.965.........................62,866
40...........................0.840.........................50,602
45...........................0.725.........................25,217

Therefore the NTC used by the DD100 Temperature Sensor has a value of 100 KOhm @ 25°C, which is a standard value, generally available. With the data given above it is possible to build a very low mass Sensor for pH probe temperature compensation.
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Re: TEMPERATURE COMPENSATION OF pH READINGS?

Postby Gerry » Mon Oct 17, 2016 12:27 pm

Hi Glovisol,

Although I implied it, I didn't explicitly confirm whether the automatic compensation is done by PicoLog by default, so I apologise for leaving that out. Also, I was tied up on Friday so I wasn't able to respond quickly to your posted reply. So, to answer your first question in more detail, historically the compensation was done automatically, when the following setting existed in the plw.ini file:
[Preferences]
TempComp=4
However, over a period of time, the usage has changed and there are a number of settings that are done automatically, without needing to be specified in the ini file. The problem is that while most settings can still be controlled (they can be turned off in the ini file, e.g. 'AllowPause=No') TempComp doesn't appear to be one of them.

Regarding your comment about the temperature compensation using temperature values instead of voltage, changing the reference used for calculations is one solution. Another way to avoid having to use the internal sensor would be to have the switch for the ini file enabling you to turn off compensation and use a calculated channel instead to multiply the pH reading with the scaled temperature of the sensor.

Thanks for your work in characterizing the behaviour of the DD100 sensor with the pH sensor, and determining both it's scaling, and the issue with it's thermal mass. This is valuable information for other users.

I need to perform a check and, if the the functionality of the temperature compensation switch has been removed (or not initially implemented), I'll put in an enhancement request for PicoLog to enable compensation via an external temp sensor. In the meantime as you mentioned, your data can be used to modify a scaling file or calculated parameter.

Regards,

Gerry
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Re: TEMPERATURE COMPENSATION OF pH READINGS?

Postby Glovisol » Mon Oct 17, 2016 1:01 pm

Hi Gerry,

Thank you for the helpful and exhaustive comments and reply. Just a quick one: how do I get to the plw.ini file?

Thanks,

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Re: TEMPERATURE COMPENSATION OF pH READINGS?

Postby Gerry » Mon Oct 17, 2016 1:14 pm

Hi Glovisol,

You should find it here: C:\Users\<your user name>\AppData\Local\Pico Technology\42XXXXXX-XXXX-XXXX-XXXXXXXXXXXX.

Regards,

Gerry
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Re: TEMPERATURE COMPENSATION OF pH READINGS?

Postby Glovisol » Mon Oct 17, 2016 4:37 pm

I need to perform a check and, if the the functionality of the temperature compensation switch has been removed (or not initially implemented), I'll put in an enhancement request for PicoLog to enable compensation via an external temp sensor.


Hi Gerry,

I fear there is a misunderstanding....At present the automatic compensation I have described is done by the Pico model DD100 EXTERNAL Temperature Sensor connected to the EXT 1 port of the DrDAQ and NOT by the internal temperature sensor.... The problem is that the Picolog software, in order to correct the pH reading, directly reads the voltage on Pin 1 (input) of EXT 1 and NOT the temperature computed from that voltage.

Trust this is clear now.

I found the plw.ini file, changed the extension to .txt to read it, but contains little. Is a commands list available? Sorry for these stupid looking questions.

Kind regards,

glovisol
Last edited by Glovisol on Tue Oct 18, 2016 2:03 pm, edited 1 time in total.
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Re: TEMPERATURE COMPENSATION OF pH READINGS?

Postby Gerry » Tue Oct 18, 2016 9:57 am

Hi Glovisol,

I posted a reply, but as this is now a technical support issue, a better medium to use would be our help desk. So, I will create a ticket and continue my response on the ticket.

Regards,

Gerry
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Re: TEMPERATURE COMPENSATION OF pH READINGS?

Postby Glovisol » Fri Oct 28, 2016 1:42 pm

ADDITIONAL ADVANCE INFORMATION ON pH OPERATION

Works is being done to obtain a complete description of the operation of the DrDAQ in the pH measurement mode, thus completing the info already given in the previous postings. I shall publish this information at the conclusion of this work.

In the meantime the user must appreciate the fact that, as soon as he plugs the BNC connector of the pH probe into the DrDAQ socket, a "special relationship" starts working between the pH reading and the EXT 1 input. In other words the status of the input/detect pins of EXT 1 influences the pH reading, even though channel EXT 1 is switched off!!

This happens as soon as the DETECT input of EXT 1 sees a high value autodetect resistor (330 KOhm for the DD100 Temperature probe). If you have a NTC type temperature probe connected to EXT 1 and you do not wish to have temperature compensation, switching off EXT 1 on Picolog is not enough: you must disconnect the probe, otherwise the pH reading will be in error.
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Re: TEMPERATURE COMPENSATION OF pH READINGS?

Postby Glovisol » Thu Nov 24, 2016 7:45 am

As previously stated, work on this subject is now available in the "APPLICATIONS" section.

Topic 27111

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