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Type

There are three types of load cell provided by Geosense

  • Hydraulic Load cells
  • Strain gauge load cells
  • Vibrating wire load cells

The things to take into account when deciding which load cell type to choose are

  • Duration of monitoring
  • Accuracy required
  • Data logging requirements
  • Robustness

Hydraulic load cells

These are low cost but they are the least accurate as they are affected significantly by temperature. They are useful for monitoring anchors on long term projects where any changes in tension are likely to be significant. They can be upgraded to be data logged by attaching VW or Analogue pressure sensors. When fitted with a Bourdon gauge they offer quick and simple readings.

Strain gauge load cells

Should be used if dynamic monitoring is required. This is because are they give a constant output and do not have to be polled for information. They are very accurate over the short term but can be prone to drift in the long term. This is down to glue drift of the attached sensors but also down to electrical drift of the sensors and other electronic components themselves. They can be easily and cheaply data logged and are arguably the easiest and cheapest to use for wireless monitoring applications.

Vibrating wire load cells

The most expensive of the load cell types. They are also the most robust and suitable for civil engineering applications. Due to this robustness this type of cell is certainly our choice for Loads of 1000 kN. The sensors used are electro mechanical sensors attached in the sensor mechanically and so are also very stable over the long term. Due to the fact that each cell has either three or six individually read gauges eccentric loading is also easily spotted. In terms of accuracy the VW load cells are the equal of the Strain gauge load cells.  VW load cells cannot be dynamically monitored. 

Sizing

The important information for deciding your load cell size is the outer diameter of the anchor or bolt being measured and the intended load that is expected to be carried within the bolt. If the diameter of the rock bolt is 38mm then it is recommended that the smallest ID load cell requirement is the diameter of the bolt/anchor plus 5mm i.e. 38 + 5 = 43mm

Using the vibrating wire load cell dimensions the load cell choice would then be the cell with the Inner diameter closest to but larger than 43mm and therefore the 500 kN load cell.

VW STANDARD DIMENSIONS

Capacity(kN)  Sensors   ID(mm)  OD(mm) Height(mm)
300  3 30 58 100
500  3 50 80 100
750  3 75 107 100
1000  3 112 142 100
1500  6 150 184 100
2000  6 150 194 100
2500  6 150 204 100

Capacity

The load carrying capacity should then be considered. It is recommended that the chosen load cell should work within 50% to 80% of the calibrated range of the load cell.  In this case the design load is 350 kN. The percentage of the calibrated load to the design load is:

(500/350) x 100= 75%

75% is between 50% and 80% is ok, so therefore the 500kN cell is ok.

If the load carrying capacity had been 550 kN then the ration would have been:

(500/550) x 100= 110%

This would be over the recommended limit and so we would move to the load cell with the next highest load which would be the 750 kN cell. The Inner diameter is 75mm and so is considerably higher than the 38mm but this should not cause any concern and will in fact just make the cell easier to install.