Freezing Temperature Does Not Indicate If Road Is Safe - DRS511 - RWCC - RWS200

Embedded Road and Runway Sensor DRS511 Application Note

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Challenges of Active Road Sensors in Real Road Environments

Active road sensors measure freezing temperature by cooling and heating the sensor surface. The measurements are based on measurement cycles, which are several minutes long and easily disturbed by splashing water or tires running over the measurement point. When the measurement cycle is interrupted, random values can be generated. Filtering out these random values is very challenging.

Independent studies show that active road sensors perform well in laboratory conditions but poorly in real road environments.

Passive Road Sensors Are Less Vulnerable to Disturbances

Vaisala uses only passive road sensor technology. Passive sensors such as Vaisala DRS511 calculate the freezing temperature from the conductivity measurement and the thickness of the solution on the sensor. The measurements are less vulnerable to disturbances because they are based on continuous measurement. The observations are also more reliable, because modern filtering techniques can be used to remove irregularities in the data.

Why Use Amount of Chemical Measurement?

However, freezing temperature itself does not indicate if there is enough chemical on the road to tolerate weather changes.

As the picture shows, there is often only a thin layer of precipitation and chemical on the road. When the solution is diluted, the thickness of the layers increases. The relative proportion of the chemical in thicker layers may be enough to keep the freezing temperature down, whereas thinner layers quickly freeze.

But how do you know if the layer is thick enough to tolerate the change? By monitoring the amount of de‑icing chemical on the road.

The amount of chemical measurement is less sensitive to errors than freezing temperature measurement, because the value does not change with evaporation or precipitation.

If you are only monitoring freezing temperature, the road may appear to be safe while the amount of chemical clearly shows that this is not the case.

Vaisala DRS511 Measures Amount of Chemical Directly

The amount of chemical clearly indicates how much residual chemical there is on the road and how the road will tolerate the expected change in weather.

Because of this, Vaisala has designed the amount of chemical to be measured directly from the solution. Other manufacturers' sensors require concentration and layer thickness measurements for the amount of chemical measurement.

Reliable, accurate measurements give you the tools to know when to treat the road. Using the same units for monitoring and treating the road makes the calculation easier: when you know how much residual de‑icing chemical there is, you know how much more you need to apply.

Figure 1. Amount of Chemical Provides the Most Relevant Information in Changing Conditions