"Comparison study of optical friction and temperature meters" published

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The aim of the study was to assess the accuracy and functionality of optical friction meters as a support tool for winter maintenance quality control. Additionally, the study analyzed the accuracy of other road weather variables (road weather type, road and air temperature, dew point, air humidity) registered by the meters. The objects of the study were the optical friction meter RCM411 by Teconer Ltd and the optical friction meter DSP310 by Vaisala (figure 1).

Optiset näyttöpäätteet
Figure 1. The objects of the study: Vaisala’s optical friction meter DSP310 (above) and Teconer’s optical friction meter RCM411 (lower). The mobile phones were acting as displays and user interfaces of both devices.

The optical meters were compared to three different braking friction meters: Eltrip-45n, μTEC and Gripman. The measurements showed that the accuracy of the optical meters has improved since tests conducted 2 years earlier. RCM411 had a markedly narrower dispersion of the friction results than DSP310. On the other hand, the friction profile of DSP310 was steeper, which is a desirable feature and helps the separation of different friction levels. Moreover, the steep and wide friction scale of DSP310 automatically increases the dispersion of the DSP310 results. DSP310 was more accurate on highways, where thick snow or ice layers occur less frequently, than on minor roads. In contrast, RCM411 appeared to work equally well on minor roads and on highways (figure 2).

Braking friction
Figure 2: RCM411 and braking friction meter uTEC compared. Correlation is quite good if you remember, that the accuracy of braking friction meters is not absolute and that meters measure different areas on the road. The road weather classification has been made by measurement person.

The study also compared road weather classes reported by the optical meters to the class registered by the measurer. As a rule, the classifications showed a rather logical correlation. RCM411 reported icy conditions more often than DSP310. RCM411 reported ice quite often in situations where the measurer registered snow or slush conditions. However, it should be noted that no objective reference method exists for road condition classification (chart 1).

Chart 1. The road weather condition measured by DSP310 compared to road weather registered by measurement person. The occasions, when DSP didnn’t register the road weather class at all, are excluded.

Road weather conditions

Additionally, the study compared the other road weather variables (road and air temperature, dew point, air humidity) reported by the optical meters to measurements conducted by road weather stations. The most significant differences could be found in road surface temperatures (about ± 1°C). However, it should be noted that a road weather station reports the temperature of only one spot, which may be in the sun or in the shadow regardless of the road section around. Optical meters, on the other hand, always analyze longer road sections. The air temperature, dew point and humidity measurements conducted with DSP310 appeared quite accurate. RCM411 does not register dew point and humidity at all.

According to the tester, the optical meters helped to observe especially black ice situations, and therefore the meters can be recommended as a support tool for winter maintenance quality control. However, the accuracy demands for winter maintenance quality control are extremely high, since measured friction levels can be used as a basis for sanctions. Therefore braking friction meters should always be used to verify optical measurements in situations where optical devices indicate friction near or below friction demands.

A long abstract of the study in english can be seen on Transport Agency web pages.

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