For continuous level measurement in silos with powders, there are two choices for level measurement:
1. Contact level measurement method - this method is very accurate and will avoid errors which may occur during silo fill or empty cycle. A typical device used for such measurement is GWR (Guided Wave Radar). Level measurement accuracy is +/-5 mm (+/-0.2 in.). This method is cost effective.
Limitations: Dielectric constant of the measured material - if very low (<2), this may present a problem for the GWR device. Another limitation is the silo height - if silo height exceeds 15 m/50 ft, usually the advice is to check how well re-enforced is the silo roof - the pull forces on the GWR cable during material discharge can be very high and this may have an impact on the silo roof integrity.
Plum Bob - this product is a mechanical on-demand measurement device with good accuracy. Not advisable to be used during silo fill cycle.
Here we will not discuss another common method for measurement - Load Cells - as this technology comes with a higher price tag, it also measures weight instead of level and to be effective must be installed before the silo is erected.
2. Non-contact level measurement method - the advantage of this method is the easy installation and almost no need for preventive maintenance. One disadvantage is that with powders, especially during silo fill cycle, the generated dust may introduce errors and impact the measurement reliability. Many end-users, however, will look first to this method to find their level measurement solution as non-contact avoids lot of post-installation issues.
Products used: Laser, Ultrasonic, Open Air Radar. Note: it is not advisable to apply Ultrasonic products in pressurized silos (pressure exceeding 1.5 bar/22 psi)
To determine which technology/product is the most applicable, the application specifics must be reviewed.
Conclusion: If accuracy and measurement reliability are the two most important criteria, then GWR should be applied if limitations (see above) are not met.