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Anniversary Story – Part 2

75 Years SIGRIST: Part II:

The innovative oscillating mirror technology

The first innovation made by SIGRIST was the development of the oscillating mirror technology – the basis for a long-term stable instrument for precise turbidity measurement for continuously monitoring beer filtration.

Since its foundation in 1946, SIGRIST-PHOTOMETER AG have been firmly associated with turbidity measurement in beer filtration. The first instruments in their portfolio were the UP series, which were then also used for turbidity measurement in drinking water monitoring from the 1960s onwards.

The oscillating mirror technology: the solution for precise and long-term stable measurements

principle of the oscillating mirror technology
Graphical representation of the principle of the oscillating mirror technology
At that time, the manufacture of a precise and long-term stable measuring instrument was a great challenge. Neither the light sources nor the optical detectors or amplifiers were long-term stable. The ingenious idea now lay in splitting the light beam into two separate beams with the help of an oscillating mirror. One beam was directed through the measuring cell and then to the photocell. The second beam served as a reference beam and was dimmed before the photocell with the help of an optical shutter until the light intensity corresponded to that of the measuring beam. Based on the shutter position, the measured value could be determined.
Test station for oscillating mirrors
Test station for oscillating mirrors. Each one passed a three-month test before final installation in the device.

Ageing of the light source or the detector had the same effect on both beams and thus had no influence on the measurement result. The amplifiers only had the task of determining which of the two beams was brighter and of readjusting the optical shutter accordingly, which was within the scope of the tube technology of that time.

KTL 30 & KT3011 SO
Left: KTL 30 for final quality assurance in the laboratory Right: KT3011 SO for turbidity measurement in the beer brewing process

When market and technology requirements changed, SIGRIST parted with the existing tube technology. With the advent of transistors, the new K-series emerged in 1980. " K" stands for compact and is also the most important difference to the UP series. The entire optics and electronics were reduced to such an extent that the instrument was contained in a characteristic grey round housing. The two main instruments were the KT3011 SO for turbidity measurement in the beer brewing process and the KTL30 for final quality assurance in the laboratory. Both instruments provided two measuring angles, 90° and 25°.

The SIGRIST turbidimeter CT3011 SO
The SIGRIST turbidimeter CT3011 SO

In 1997, the K-instruments were replaced by the C-instrument line. The C instruments were also developed within the "one instrument philosophy". This meant that the basic unit with optics and electronics was largely the same for all instruments and the individual adaptation to the measuring task was carried out with an adapted measuring cell approach. Technical progress, especially in the field of microcontrollers and amplifiers, made it possible to dispense with the very proven but expensive oscillating mirror. The dual-beam method continued to be used for the C-series. The light beam was split into a reference beam and a measuring beam by a partially transparent mirror. By means of a chopper disc (hence C series), the light was guided alternately through the measuring and the reference channel. For the first time, absolute light levels were measured. This technology increased the sensitivity of the instruments by a factor of 10. The dynamic range was also massively increased, so that an optical measuring range selector could be dispensed with. The C-units were characterised by the shortened base units and the stainless-steel housings.

Maybe you have not read the first part yet? No problem! Here you can go directly to the article about the history of our company's foundation with the first process photometer for beer filtration.

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René Gehri
Water, Beverages & Food, Industrial Processes
Pascal Schärer
Traffic & Environment