Temperature Meter: Application, Installation and Importance in Constructions
Application
Temperature measurements in geotechnical engineering fall into three general categories:
a) Where temperature itself is a primary parameter of interest;
b) Where temperature change causes a real deformation or stress change in the ground or in a structure;
c) Where a transducer itself is sensitive to temperature change.
A temperature increase causes most materials to expand and, if expansion is resisted, temperature increase will cause a compressive stress increase. Thus, steel struts across an open cut excavation can be subjected to substantial compressive stresses when heated by the sun: the greater the end restraint the greater is the stress. The same phenomena is illustrated by thermal expansion of railroad tracks.
Where a transducer itself is sensitive to temperature change, can be illustrated by systems such as pressure cells, load cells, strain gages etc. Temperature changes cause expansion or contraction of the liquid in the cell-thus an indicated pressure or load change-and temperature corrections must be applied if these changes are significant. If their thermal expansion and contraction characteristics are not matched with the monitored structure, show extraneous sensitivity to temperature.
Please refer to below graphs which clearly shows that Strain measurements are being affected by change in temperature.
How to take into account temperature sensitivity?
Three approaches are possible for taking account of temperature sensitivity:
i. Matching thermal co efficient of instrument and structure,
ii. Use of low coefficient of expansion materials such as invar or super-invar, and application of temperature corrections.
iii. The first two approaches are often only partially successful, owing to design and/or cost limitations.
iv. The third approach requires measurement of temperature and use of temperature sensitivity relationships either provided by the instrument manufacturer or developed by users for their site-specific instrument-structure interaction. It also requires ensuring that the temperature transducer is in good thermal contact with the instrument to be corrected.
Installation for Measurement of Temperature
When planning installation of transducers for measurement of temperature, care must be taken to ensure good thermal contact between the transducer and the material to be sensed.
When installing transducers in a borehole to measure subsurface ground temperature, the bore hole must be backfilled completely, and the back filling procedure should depend on the required measurement accuracy. When maximum accuracy is required, for example, when measuring the depth of frost penetration or when measurements are used to make decisions on limiting roadway traffic in the spring, great care must be taken to minimize disturbance to the thermal regime and to the pathway for moisture movement. In such cases an accuracy of about ± 0.1oc is required, and the borehole should be backfilled with material removed from the bore hole. Either the material can be placed and tamped in layers, or it can be mixed with water and poured or tremied into the borehole. When a lesser accuracy is acceptable, the borehole can be backfilled with cement grout or other material that ensures complete backfilling.
When transducers are attached to a structural surface, thermal contact is sometimes obtained simply by taping the transducers to the surface. Some manufacturers supply special rugged versions that can be welded to a metal surface for more permanent contact. Good thermal contact can also be created by drilling a small hole in the structure and using a heat-conducting epoxy to cement the transducer in place. The same epoxy can also be used to cement transducers directly to a structural surface, but this arrangement is less rugged.
Importance of Temperature monitoring in bridge constructions
In modern bridge construction, the applications of long span continuous rigid frame bridge become more common. Its construction monitoring techniques have also been paid more and more attention by bridge builders. When the long span continuous rigid frame bridge is in special section that the temperature changes very much, the temperature monitoring is especially worth notice. Temperature changes have influence in carrying the capacity of pile cap and pier, as well as geometric shape of segments. Reasonable construction monitoring technology is an important guarantee to ensure the stability of the bridge structure.
Why mass concrete temperature monitoring is important?
In order to ensure the quality and durability of larger projects, it is important to monitor and control concrete mix temperature, ambient temperature, and differential temperature in mass concrete elements. Temperature differences can cause stress that leads to thermal cracks, as well as loss of structural integrity, thus shortening the life and decreasing the strength of the mass concrete element. If the temperature goes below a certain number, the hydration of water can slow or stop meaning the concrete won’t set properly and won’t achieve optimal strength. When concrete temperatures are properly monitored, it allows for appropriate adjustments to be made when needed.
Heat generation depends on many factors such as the compound composition, the initial temperature of the concrete, the ambient temperature, the shape of the mass concrete element, the volume to surface ratio, as well as other surrounding conditions. Generally, the higher the cement content, the more heat will be produced.
There are a number of ways in which you can regulate the temperature of mass concrete elements during a project starting with an initial cooling of the concrete mixture. This can be done with the use of chilled water, ice, or liquid nitrogen. Temperature regulation can also be done throughout the project by running cool water through cooling pipes which are installed before the concrete placement. In order to know when it is appropriate to undertake cooling processes, temperatures must be monitored in real-time.
Different type of temperature sensors for different applications are available as:
a) Vibrating wire temperature meter is used for the measurement of internal temperature in concrete structures or water. Generally, the VW temperature sensor is used to monitor the heat of hydration in mass concrete.
It has a resolution better than 0.1°C. The vibrating wire temperature meter is designed on the principle that dissimilar metals have different linear coefficient of expansion with temperature variation.
b) Resistance temperature probe is a low mass water proof temperature probe for measurement of temperature between -20 to 80°C. Due to its low thermal mass it has a fast response time. Resistance temperature probe is specially designed for measurement of surface temperature of steel and measurement of surface temperature of concrete structures. Resistance temperature probe can be embedded in concrete for measurement of bulk temperature inside concrete and can work submerged under water.
c) PT RTD temperature probe can also be used for similar applications. It has excellent stability and accuracy that makes it well suited for long term installation where high accuracy and reliability is required.
Encardio-rite manufactures all type of above mentioned Temperature meters.
If one gets Unstable Reading:
Set your handheld multimeter to a high range (10 or 20 M ohm).
§ Measure the resistance between a VW wire and an RTD/Thermistor wire. The reading should be infinite or out of range.
§ Measure the resistance between any of the colored wires and the drain (shield) wire. The reading should be infinite or out of range.
§ Measure the resistance between the shield wires of two installed VW sensors. Wires must be disconnected from data logger or terminal box to make this test. The reading should be very high or infinite. A lower reading indicates the presence of a ground loop.
§ Other sources of unstable readings are electrical noise from nearby power lines, radio transmitters, or motors. Also, over ranged or shocked instruments can exhibit this problem.
