The Impact of Thermal Variation
This article describes the impact temperature has on the ASP measurement performance, and how these can be mitigated.
The Automatic Settlement Profiler (ASP) is an hydraulic system, using pressure and the specific gravity of a fluid to determine change in elevation.
A fluid’s specific gravity will change with temperature, and while the ASP has integrated compensation, these effects must also be taken into consideration when planning your ASP project and interpreting the data, as shown below.
Key Points
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Temperature changes are not a problem, but temperature variation across a system will reduce measurement performance
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Sections of the ASP that are exposed to larger temperature changes can be isolated when processing the data, reducing the impact on the rest of the system
The Impact
While the impact is small, it is always good to know what to expect and how to get the very best out of your monitoring system.
In the below example, we can see that shallow cover over an ASP monitoring a railway embankment slope is not enough to isolate the ASP from daily temperature changes, with some areas warming more than others.
When looking at the displacement results, it can be seen that the temperature variation itself is not directly correlated to the measurement performance, but rather the variation in temperature is causing the largest deviations.
While the ASP is good at accommodating temperature changes over time, temperature consistency across the length of the system provides the best results.
Thermal Ballasting
The best way to create a consistent temperature across the ASP system is to surround it with a large thermal ballast.
Burial
The most obvious thermal ballast to use is soil. By burying the system deep enough, localised temperature changes are unlikely. The optimal depth will depend on site conditions, but typically 0.5m for temperate climates, deeper where the climate is more extreme.
Sandbags
Where trenching or burial is not an option, sandbags will reduce the impact of exposure to direct sunlight, but not entirely. If the installation area has inconsistent shading, it is possible you will see temperature variations across the system during the day. Depending on the monitoring criteria, this may mean accepting a lower precision, or limiting measurements to night, when the sun is not creating hot spots.
Water
Submerged installations are best case scenario as they offer consistency across the system. Be aware that systems are from submerged to non-submerged will have a temperature gradient across them, so the use of two references should be considered.
Water can be used to fill conduit in which the ASP is installed to provide temperature homogeneity along the system. In smaller diameter conduit, flowing water is best.
Concrete
While concrete is an excellent thermal ballast, it is rare to install an ASP more than a few centimetres below the surface.
Most monitored concrete slabs will have structures or earthworks placed on top, offering protection from direct sunlight, but where that is not the case, the mitigation measures below should be considered.
Selecting a Reference
The ASP operates based on differential pressure between a measurement sensor and a reference sensor. The reference sensor is used to offset any environmental changes and a control assumed to be in a fixed position.
Temperature changes occurring between the measurement sensor and the reference sensor will impact the measurement performance, so it is important to ensure a reference sensor is selected that is both stable, and shares thermal conditions with the measurement sensor.
In the example below, sensors one and two were exposed to the sun during the day, and cool air at night, while the others were installed in a conduit underground, offering temperature stability.
When using the sensor with large temperature variations as the reference, we can see it significantly reduces measurement performance:
Whereas selecting a sensor with the same temperature conditions as the measurement sensors removes these errors, giving much more stable data:
Where an ASP system spans two or more temperature “zones”, for example where part of the ASP is submerged, a reference sensor should be used for each zone for optimal performance.
Next Steps
If you are concerned about temperature effects on your ASP application, please contact Osprey’s support team at support@ospreymeasurement.com or +44 (0) 20 3355 4447, and they will be able to assist you with advise to get the best results for your project.