Product category:
Gas, Chemical, Radiation and Magnetic Sensors
News Release from: Mettler Toledo | Subject: InClean pH measuring and cleaning system
Edited by the Engineeringtalk Editorial
Team on 19 October 2000
Self-cleaning sensor in pH control
The Mettler Toledo InClean pH measuring and cleaning system utilises conventional combination pH sensors, whilst alleviating the problems of clogging and coating
The control of pH in pulp for papermaking is vital to achieve the correct quality of the final product One of the major problems of measuring pH reliably and accurately in the process is sensor fouling
Most frequently this is due to contamination or blockage of the reference system diaphragm, but can also be caused by fouling of the pH sensitive membrane.
Historically, glass combination pH electrodes have not been used in pulp and paper manufacture because of their short operational life.
This is caused both by clogging of the diaphragm and coating of the pH glass; typical of the problems that can sometimes beset the user of such sensors.
Thus papermakers have tended towards the use of the more expensive and bulky antimony electrode system.
Although these resist the effects of coating, they are still prone to clogging and also to contamination by sulphide species present in the pulp.
This leads to early failure of the sensor and the inconvenience of costly refurbishment or replacement.
Another problem is that the antimony electrode requires a special amplifier, since it has a different zero point than glass electrodes.
Furthermore, due to the nature of the antimony electrode system, results are subject to appreciably more scatter than is the case for glass electrodes.
Reasons for the scatter include effects of changing ionic strength, temperature, redox potential and sulphide content of the solution, both from calibration and during operation.
This scatter could be minimised by calibrating the electrode under conditions similar to those in which it is used, but such procedures are awkward and inconvenient.
The Mettler Toledo InClean pH measuring and cleaning system utilises conventional combination pH sensors, which are cheaper and more convenient, whilst alleviating the problems of clogging and coating.
The InClean system retracts the sensor at user programmable intervals, washes it (applying a cleaning agent if necessary) and then returns the sensor to the process.
Test runs in a pulp stream at temperatures of 50C to 60C have shown no contamination or clogging problems over periods of more than 8 weeks.
The sensor was retracted periodically (30min.
to 60min.
intervals), simply washed with water, and then returned to the measurement.
Doing this avoided the clogging problems that affect both kinds of sensors.
For more difficult applications, and where continuous measurement is not required, the InClean's alternative programme sequence can be used.
In these circumstances, the sensor is held outside the process and only inserted during the critical period when measurement is required.
What about the sulphide problem that is known to beset sensors with silver/silver chloride reference systems? InClean's cleaning ability would minimise this problem anyway, but the use of Mettler Toledo's unique silver-ion-barrier reference system eliminates the reaction of silver ions with sulphides in the process liquid.
Thus the problem simply does not arise.
While pay-back periods are very acceptable, the biggest benefit is the ease of maintenance of the system.
InClean's watchdog software provides early warning of electrode deterioration, thus permitting intervention at the user's convenience.
Furthermore, the provision of an alarm on electrode failure, draws the operator's attention to the need for an immediate action.
Electrodes are easily changed without stopping the process, neither is there the need for expensive refurbishment nor replacement.
In addition to these reductions in overhead, calibration is easier.
Scatter is significantly reduced by the use of a conventional glass combination electrode that is not susceptible to the solute problems referred to earlier in this article.
Wherever reliability of pH measurement is paramount, and process conditions make access to the electrode awkward, InClean can offer increased confidence, improved performance, and better yields over conventional pH measuring systems. Request a free brochure from Mettler Toledo ...
• Mettler Toledo: contact details and other news
• Email this article to a colleague
• Register for the free Engineeringtalk email newsletter
• Engineeringtalk Home Page
Search the Pro-Talk network of sites
