Frequently Asked Questions Regarding Road Surface Temperature Sensors

What is the difference between passive & active pavement sensors?

The active sensor uses an integrated PELTIER element for the heating & cooling process. In case of wet conditions (existing water film) and if the road surface temperature is less than 5°C, the measuring cycle will be started. One cycle lasts 20 minutes. The cooling procedure stops once the freeze point temperature has been reached. The cooling time depends on water film level & concentration of de-icing chemicals. After cooling, the heating procedure is re-activated.

Why is it necessary to use a passive pavement sensor in conjunction with an active sensor?

Since the active sensor has a cooling and heating cycle, the road conditions are altered. This leads to condensation & pavement temperature changes. This is the reason the active sensor will ONLY measure freeze point temperature. All other road measurements such as road surface temperature, depth temperature, water-film & road conditions are reported by passive sensors.

Why do so many networks use passive sensors only?

As long as only one de-icing chemical, such as NaCl or MgCl is applied, a state-of-the-art passive sensor will measure the conductivity, "salt in water" and calculates the freeze point temperature. A prerequisite for an accurate result is the precise measurement of the water film.

Why is the water film thickness important information for winter road maintenance operations?

The freeze point temperature depends on concentration of de-icing chemical & the existing water film thickness. The higher the water film, the more chemical is necessary. Vice versa, in case of a sloping road & a very thin water layer, there is a risk of “over-salting” & to exceed the so called EUTECTICAL POINT.

What are the main benefits of non-invasive road condition sensors?

The non-invasive sensor is not part of the pavement and is easily accessible for repair & maintenance/calibration. Both, surface temperature & road conditions can be measured. A disadvantage is in that the freeze temperature itself cannot be measured.The water film measurement is not nearly as precise as the measurement of an embedded pavement sensor.

Is it feasible to design a Road Weather Network based on different road sensor technologies (active/passive/non-invasive)?

Networks of the future will be modular, open, & in concept extendable. Wherever possible, embedded pavement sensors will be deployed to provide all data in a particular application. In situations of very thin water layers (busy highways), an active sensor will determine the precise freeze temperature. Locations (bridges) where in-pavement sensors cannot be installed, a non-invasive senor is the best choice to deliver road condition information.

Are regular sensor calibrations recommended?

Due to the replaceable electronics of pavement sensors, a calibration cycle of three years is recommended, in spite of the available long-term stable sensor elements with minimal drift. Replacement Lufft sensors can be installed within minutes, allowing for minimal down-time of operation as well as lane closures. Sensors are to be sent to a qualified laboratory for inspection/re-calibration.

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