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Density Measurement: The tip of the Iceberg!!!

A real time solution to Radiometric Density Measurement in Mining

Currently in Mining applications radiometric measurement has been the only way to measure the density of a slurry. Until recently and due to the harsh and abrasive environment of slurry no other measuring principle could work. This has recently changed due to the use of tuning forks (FTL 50) in conjunction with a Density computers (FML621)

The reason why radiometric density is frowned upon is not because the principle of measurement does not work, but on the contrary it works brilliantly with very few issues. The issue with radiometric density measurement is all the red tape that surrounds this measurement point. To have radiometric measurement a site needs to conform to certain local and international legal standards. A radiation officer needs to be permanently on site in case of an accident. Training around safe practices with radiation needs to be performed. Complex and costly transportation regulations that need to be adhered to. These are some of the challenges that plants face when it comes to radiometric measurement for density. This process is becoming less challenging with the introduction of a tuning fork that measure slurry density. Depending on the slurry applications correctly installated tuning forks last longer than 12 months before they require any maintenance.

The tuning fork principle

Vibration limit switches are mechanically vibrating systems, which are made to vibrate at their resonant frequency. A piezo drive, the core element of the sensor generates drive impulses that are transferred to a stainless-steel tuning fork, the only part of the system which comes into contact with the product. Due to this well-known piezo effect, it is possible to make the tuning fork resonate and measure the shift in resonant frequency. The limit switch also analyses the resonant frequency at which the fork vibrates.

Thanks to this cleverly employed technical feature, limit switches have reinvented themselves as fully-fledged density measuring instruments, which fulfil the user’s requirement for information directly from the process and is a cost-effective alternative to established measurement methods such as manual sampling. Parameters such as conductivity or dielectric constant do not interfere with the function of the sensor. However, temperature compensation is recommended to achieve highest accuracy. If a pressure change in the process is greater than 6 bar, then pressure compensation would be required too.

The measuring principle is based on the fact that the resonant frequency of the tuning fork depends on the density of the product, the process temperature and pressure. Changes to the resonant frequency are directly affected by the density of the material.

Materials with a lower density such as liquefied gas (0.4g/cm3) give rise to higher resonant frequency than materials with a greater density such as water (0.99g/cm3). The density of the medium can be accurately calculated using mathematical formulae depending on the resonant frequency, temperature and pressure. The frequency shift is, therefore, used to measure changes to density levels caused by different liquids or concentrations. Combined with a density calculator, the corresponding density value of the medium can be calculated from the resonant frequency in a reliable and reproducible manner. Fluctuating process temperature near the tuning fork should also be measured by a temperature sensor, which is then recorded and compensated in the density computer. Therefore, influences on the density value are taken into account.

Things to take not of when installing a tuning fork in a flow density application:

  •  Vertical Installations are recommended
    •  To eliminate air bubbles
  • To eliminate build up while empty    
  • Low (0- 2m/s) Velocity
  • Fine slurry particles < 0,15 mm

FML621 Density Computer Specs:

4x FEL50D / 0/4-20mA.

Output (without ethernet):

2x 0/4-20mA/pulse.

1x Rel.SPST, 1x loop power supply

Output (with ethernet):

1x Rel.SPST, 1x LPS.

4x LPS = loop power supply.

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