Scale-Tron, automation, sensors and weighing systems

MIXTRON FACEPLATES - and their wear

Most manufacturers of microwave moisture sensors for concrete mixers use a ceramic known as alumina, which is aluminum oxide, Al2O3.  This material is very hard and tough, lasting months to years in concrete mixer applications, depending on the volume of concrete produced and the abrasive qualities of the aggregates.  One other manufacturer has switched to sialon, which is an aluminum oxide / silicon nitride composite giving superior wear characteristics.  In 2002 we investigated sialon and silicon nitride, settling on silicon nitride, Si3N4 for all future production.  This material is harder than both alumina and sialon:

Hardness of Alumina is 1800 Knoop.  Sialon is  2000 Knoop and Silicon Nitride is 2200 Knoop. Diamond is 6000 - 9000 Knoop.

Although the difference in hardness does not seem a lot, the difference in wear is amazing.  The reason is in the mechanism of wear;  aggregates are just hard enough to scratch and abrade the surface of alumina, but they cannot make an impression on the slightly harder silicon nitride.  Thus, it suffers practically no abrasion from even very aggressive aggregates.  The cap at right is alumina and has been in a high production concrete mixer using very abrasive aggregates for only 4 months.  Note the wear at the left hand edge.  The cap at left has been in a similar mixer that had a similar wear problem until the cap was replaced with silicon nitride.  Measured wear is only 0.010" over one year of operation, 50 hours of continuous operation per week with granite aggregates, making its projected lifetime 10 years - an increase of 20 to 30 times!  Note the high polish on its surface, showing that the aggregates cannot score this material.

Since 2002 we have eliminated replaceable caps.  The wear characteristic is so good that we don't need to replace them - they last for the life of the sensor.  And the proof is that we've only had one back, as of January 2014.

The faceplate at right has been returned in 2013,  at the end of its useful life at the same plant, A. Jandris and Sons, Gardner, MA.  After 9 years of use with granite aggregate, although worn down to the metal on the right side, the left side is almost untouched.  If it had been rotated occasionally to even out the wear, it would have lasted longer.  Call us for this reference if you would like to verify it for yourself.

Note that the faceplate is twice as thick as it seems from the outside.  It's stepped so that an equal depth of material lies inside the metal tube.  This explains its enormous strength and freeedom from breakage when used as recommended.