Density measurement in concrete residual water
The development of the OLAS measuring system, an online measuring system for determining the solids content of concrete recycling water, began almost twelve years ago. Today, a reliable and economic measuring system is available that has reached series production maturity. The OLAS is named after its function: ‘Optical Light Absorption Sensor’. The OLAS makes use of a sophisticated infrared absorption measuring method, in which the measurement light is introduced into the medium via optical fibers. If desired, the OLAS can be supplemented by the OLAS-TPC (OLAS Touch Panel Controller), an optimal evaluation, display and control unit, to form a complete measurement system.
OLAS – functional principe & specific features
The Optical Light Absorption Sensor (OLAS) fabricated by the „Werne&Thiel GbR“ company sends light into the material of interest (medium) and makes use of the accurately measured absorption of light to determine the composition of medium. By this method not only the composition of water slurry, suspensions and composite materials of all kinds (e.g. cement slurry, chemical pulp, etc.) can be determined, but also the thickness of foils and coatings and much else. Whatever in production, manufacturing or processing results in a change of absorption of light of medium can be measured, monitored and controlled by the OLAS.
Distance between emitter and receiver:
As the light absorption can differ a lot from application to application, the light path of the OLAS can be adapted to the medium of interest: Highly absorbing media need a small distance between emitter and receiver, to allow enough intensity to reach the receiver, while for low absorbing media, on the other hand, a much wider distance between emitter and receiver must be chosen.
The OLAS can handle a change of intensity of light of up to 1 to 10,000,000, according to an internal signal of 0…700. „0“ is the result when measuring an entirely non-absorbing medium, while „700“ means maximum absorption.
The key task is now to choose the distance between emitter and receiver for each medium of interest in such a way, that the entire measuring range of „0…700“ is used. This can mean, that for a highly absorbing medium the distance is only a few millimeters, while for a low absorbing medium the distance can be one meter or even more.
Suppression of ambient light:
The OLAS provides a very high suppression of ambient light. Not only constant light (sunlight, etc.) is suppressed, but also modulated light, for instance the light of a fluorescent lamp.
If the optics is entirely immersed in the medium, ambient light plays no role anyway, because the ambient light is highly absorbed by the absorbing medium. But sometimes, if the distance between emitter and receiver exceeds the thickness of measured medium, for instance if the thickness of a foil is measured, or similar, then, nevertheless, in certain applications ambient light could eventually reach the receiver while the intensity of measuring light can be quite low. Even then the suppression of ambient light of OLAS will be high enough in most cases, unless you directly blind the receiver by a strong light source (e.g. strong fluorescent lamp).
It’s very easy to find out whether in your application the ambient light suppression is high enough: Bring a highly absorbing medium into the light path between emitter and receiver and switch the averaging time at the „Touch Panel Controller“ (TPC) „off“. In the „recorder“ mode a constant reading should be seen now, perhaps superimposed by some small noise peaks. Now, decrease the intensity of ambient light and make sure whether the reading is changing. If there is a change, you should shade the optics against the ambient light in a suited way. But please remember, that the influence of ambient light will also be tremendously minimized by the averaging.
The OLAS offers the best that is technically possible today. We have found an optimum compromise between fast settling and low noise. Who wants a very low settling time (c.a. 30msec) sets the averaging „off“. Who, on the other hand, needs a very low noise floor, for instance when measuring very high absorbing media, or who is not interested in fast changes of output signal but wishes an averaging, sets a convenient averaging time.
For many applications an averaging time of 0.3 sec will provide a good compromise.
The OLAS needs only one measuring range to cover the entire dynamic range of 1:10,000,000 (internally even 1:100,000,000). So, there’s no need to switch between several measuring ranges when measuring. This eliminates unacceptable dead times between switching, when there’s no valid signal. The use of only one measuring range makes the OLAS extremely fast and precise: The settling time for an instantaneous change of signal (10,000,000:1) is only ca. 30msec!
A fast settling time is crucial, if the absorption of fast changing media has to be measured, for instance when an inhomogeneous medium is pumped through a pipe and the OLAS is mounted there.
Contamination at the optics:
Many other optical measuring systems have great problems with dirt and scratches at the optics, because this contamination results in an additional and highly unwanted absorption of light. This is problematic, because for these products the contamination can easily absorb about as much light as the medium itself, which results in considerable measuring errors.
As the OLAS is designed to handle extremely high absorbing media, the OLAS is much less susceptible to contamination at the optics, because the medium is normally absorbing much more light than the contamination. So, normally, the influence of contamination at the optics on the measuring accuracy of OLAS is negligible.
To decrease this influence even further, for instance when measuring highly abrasive media, the customer can order „aged“ optics, if he wishes.
Sometimes, optical components have to be replaced or changed after the OLAS has already been calibrated by the customer. Has the OLAS to be calibrated again after-wards?
Not necessarily: Is the distance between emitter and receiver after-wards exactly the same as before (from window to window), then the OLAS offers a unique offset-adjust method, which only needs later carried out changes to be compensated. The initial calibration done by the customer remains valid and doesn’t need any change!
This offset-adjust method requires the determination of an additional characteristic parameter of your OLAS application, called „initial absorption“, when doing the initial calibration: Bring the optics of your
OLAS into a defined medium of well known absorption (e.g. clear water) and measure and store this „initial absorption“. When adjusting the offset after-wards, as consequence of a subsequent change or replacement of components of OLAS, bring the optics of OLAS again into this defined medium. Now, the OLAS calculates the difference (offset!) to the „initial absorption“ and stores this „offset“. All following measurements are then corrected by subtracting this offset from the measured value. The initial calibration done by the customer remains valid! This offset-adjust can be done as often as you like.
Of course, this offset-adjust method can also be used to correct the influence of contamination (dirt, scratches, etc.) at the optics (see above).
The crucial point of this offset-adjust method is, that the distance between emitter and receiver (concretely spoken the thickness of screened medium) must always be exactly the same. If the distance between emitter and receiver is changed for any reason, then the initial calibration is no longer valid!
The OLAS provides a self diagnosis, which all the time checks the operating status of OLAS. If an error occurs, an error message is sent to the process control.
There are five error messages:
This error message occurs, if the data transmission of RS485-interface is interrupted for a longer period.
„Data transmission error“
Here, data is transmitted, but the data transmission is incorrect.
This situation occurs, if the optics of OLAS is blinded by a considerable amount of ambient light. Or if the optics was manipulated in a way, that the receiver now gets much more intensity than normally and is over-driven.
This situation occurs, if the light source of measuring process shows a failure (which is highly unlikely, because the light source was designed for a working life of more than 100,000 hours).
This is not a real error message, but only an information for the operator, that the measured signal is very small and the measuring process begins to become nonlinear.
This error message was implemented, because this measuring situation could easily be overlooked when the averaging is activated.
This error message is especially useful during the initial calibration, done by the customer: When setting the calibration points of calibration curve, take care, that the measured value is always „reliable“, means, that the error message „unreliable measurement“ never becomes activated.