Windmill Software Ltd
Windows Engineering Software

March 2006

The Newsletter for PC-Based Data Acquisition and Control
Issue 92          March 2006
--------------------ISSN 1472-0221----------------------

Welcome to Issue 92 of Monitor.  This month we give 
tips on using your PC to measure pH.  Should you wish to 
cancel your subscription to Monitor you can do so at

* Windmill News: Data Acquisition Glossary Updated
* How to Use a Computer to Measure pH
* DAQ News Roundup

Windmill News: Data Acquisition Glossary Updated

Thanks to readers pointing out terms we had missed, 
we have been able to further expand our glossary of 
data acquisition and control terms.  With links to 
further information and in-depth articles.

How to Use a Computer to Measure pH

pH is measured in many industries, including nearly all 
handling water.  In fabric dyeing the permanence of the 
colour and the speed of the process depend on pH.  pH 
determines product quality in paper mills.  A change in 
pH in a lake or river is an early indicator of 
pollution.  And the correct pH is essential to ensure 
proper beer production.

What is pH?

pH indicates the acidity of a solution.  The term "pH" 
was first used by W. M. Clark in 1920.  Previously the 
Danish biochemist Sorenson had used the term PH - H 
being a subscript of P.  The H stands for hydrogen. 
(An acid forms hydrogen ions in solution.)  It's unclear 
where the p came from.  It's often said that the p 
refers to the German word protenz, which apparently 
means power. This makes sense because the pH scale 
is a logarithmic one, so the p would refer to the 
power or exponent of 10.  However, in that case why 
was the German word for hydrogen not used? Jens Norby 
published a paper in 2000 arguing that the little p 
was simply a constant (Trends in Biochemical 
Sciences 25 (1) (2000) pp. 36-37.)

The pH scales ranges from 0 to 14. It is logarithmic, 
so each interval is 10 times more acidic or alkaline 
than the last. For example, pH 6 is ten times more 
acidic than pH 5, and one hundred times more acidic 
than pH 4.

pH Sensors

Various methods of measuring pH are available, but 
the most common one used in laboratory and industry 
is the glass electrode method. 

In this, the pH of a known reference solution is 
compared to the pH being measured.  Two electrodes 
are used: a glass measurement electrode and a 
reference electrode.

The measurement electrode comprises a glass bulb 
attached to a glass stem.  The bulb is a pH sensitive 
membrane filled with a conducting buffer solution. 
A silver wire is enclosed in the glass.  

The difference in pH between the solutions inside 
and outside the thin glass membrane creates an 
electrochemical force (voltage) proportional to 
the difference in pH. This is passed via the 
silver wire.

The reference electrode has a stable potential 
and also features a silver wire, enabling a 
complete circuit to be made and the voltage 
generated by the glass electrode to be measured.

The measurement and reference electrode may be 
individual and separate, or may be combined 
into one probe.  (There may also be a temperature 
compensating electrode.)  Individual electrodes 
are less practical than a combined probe, but 
may be more precise.

The probe may be connected to a pH meter that 
displays the current pH reading. 

How to Get pH Measurements into a Computer

- pH Meter -

First let's take the case of a pH meter - a 
measuring device which displays the pH of 
the sample.  This will often have an RS232 or 
USB interface.  This means that you can connect 
it to the serial COM or USB port on 
your computer.  

You will also need some software to collect 
the data from the pH meter.  Software like 
the Windmill COMIML program which reads 
data from the PC's RS232 COM port 

The Windmill software regularly collects 
the pH readings, every 5 seconds say, and 
saves them on the computer's hard disk.  
Windmill can also show live charts of the 
data, pass data in real-time to other 
programs like Excel and collect pH data 
alongside other data such as temperature 
or flow rate.

- pH Electrodes -

What if you are not using a pH meter but 
instead wish to connect the pH electrodes 
to the PC?  To do this you need a data 
acquisition (DAQ) device.  This device 
may be a card that you plug into the PC.  
More commonly, it is a unit sitting between 
the electrodes and the computer, connected 
to one of the PC's communication ports: 
USB, RS232, Ethernet or RS485 for example.  
You connect the electrodes to the DAQ device.  
The device then regularly takes pH readings 
and passes them to the computer.

pH electrodes have a very high output impedance 
and you cannot just connect them to a normal 
voltage input on your data acquisition unit.  
You will need instead to choose a DAQ unit that 
will amplify the signal to the appropriate level.  
For example, our small, portable, 
Microlink 751 unit - which plugs into the 
PC's USB port - has its own pH conditioning 

The larger Microlink 3000 system comprises a frame 
of modules.  Each module is dedicated to a specific 
task: voltage input, counting, current output and 
so on.  It has a special module dedicated to 
pH signal conditioning that you slot into the frame 
( You 
connect the electrodes to this module.   You 
then connect the pH module to the normal 
voltage measuring module and take pH readings 
at the same time as other measurements.

DAQ devices designed for pH measurement need input protected sockets to which you connect the electrodes. As pH readings change relatively slowly, if possible you should choose an integrating analogue-to-digital converter as this reduces noise interference. These typically offer maximum sampling speeds of around 40 readings per second. Further Reading =============== Trouble Shooting pH Measurements... More on Windmill Software... Example pH Monitoring Applications... pH Data Acquisition Hardware... ________________________________________________________ ________________________________________________________ DAQ News Roundup ________________________________________________________ Welcome to our roundup of the latest findings in data acquisition and control. If you would like to receive news updates as they happen then grab our RSS newsfeed at Read for information how. EU-funded sensor network goes a step beyond RFID An EU-funded project is going a step beyond existing radio frequency identification (RFID) systems by developing a sensor network that will allow items to communicate more information about their surroundings. The technology can be used with goods or equipment to make them "smart". Items such as cartons of foods will be able to warn operators when the storage limit in a warehouse is reached, if a leak occurs or if one is placed in the wrong location. Source: Internet Standards Gives Birth to a New Industry in Building Automation Systems The adoption of Internet Standards in the Building Automation Systems (BAS) market is giving birth to a new industry focusing on information management and analysis. According to the ARC Advisory Group, the worldwide market for BAS is expected to grow at nearly 5 percent over the next five years. The adoption of Internet communication standards in the BAS market is further extending the concept of smart buildings by allowing companies to create a single repository for all facilities data so that it can be easily retrieved and shared between all applications and all organizations within an enterprise. Source: ARC Advisory Group IEC Try to Solve Environmental Regulation Muddle As regulations about the environment increase, so life becomes more difficult for the electrical and electronics industries because the amount of paperwork increases in the form of green procurement surveys or supply chain questionnaires. The regulations are often a list of what one may not do and too much of this, with varying criteria, creates a real Tower of Babel. How to resolve the problem? IEC (International Electrotechnical Commission) thinks the answer is a Material Declaration Standard used worldwide and has started work on this. Source: IEC ________________________________________________________ ________________________________________________________ * Copyright Windmill Software Ltd * Reprinting permitted with this notice included * For more articles see We are happy for you to copy and distribute this newsletter, and use extracts from it on your own web site or publication, providing the above notice is included and a link back to our website is in place. An archive of previous issues is at and an index of articles at Windmill Software Ltd, PO Box 58, North District Office, Manchester, M8 8QR, UK Telephone: +44 (0)161 833 2782 Facsimile: +44 (0)161 833 2190 E-mail: [email protected]


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