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Realizing that moisture measurement is more of an art than a science, in 2005, Germany’s Bauhaus University in Weimar was researching methods using microwave transmission lines and Time Domain Reflectometry (TDR) techniques to measure moisture in soil, which varies immensely in density. Soil (and aggregates) can be compacted and the density changes as a result. Moist sand, for example, can have a density range from 50 lb/cu. ft. when “fluffed”, up to about 130 lb/cu. ft. when fully compacted. Since all microwave moisture sensors measure the water in a specified volume, they are sensitive to the density of the product. Consequently, to accurately measure moisture percentage, the density must first be controlled, either by compacting it in the bin or by other methods when free-falling and on conveyors. AquaSense and all other microwave technologies use the compaction method to stabilize their readings.
The method
A German company, Imko, realized this and collaborated with the researchers at Bauhaus to produce a range of moisture sensors based on the TDR principle. They inserted a transmission line with two parallel conductors into the material. They send a pulse of energy from one end and the returned composite reflection varies according to the moisture of the surrounding material. The picture at top shows Sono-Vario, where the transmission line is embedded in very hard ceramic and the material is only on one side. In the Pico-64 below, the transmission line is two prongs that are inserted into a pile of aggregates or soil. In both cases, the method of measurement is the same. To be clear, Imko does not claim that the sensor is insensitive to density. They do claim that accuracy is better than standard microwave techniques, however. Because of this, the technique measures the larger aggregate sizes with greater accuracy. Imko claim 0.2% accuracy for large aggregates, up to 1-1/4″ in size. Most microwave sensors have difficulty measuring anything larger than 1/4″ with adequate accuracy for concrete production.
Oven-Dry or Bake-Out Tests
Plant personnel no longer do “bake-out” moisture tests twice every day to set the moisture values. And that’s good because moisture can change with every batch. Your front end loader picks a load of aggregate or sand from the pile and dumps it into the bin. Depending on where in the pile he takes his load from, the moisture can be quite different from the previous load. Even in the bin, the gate can draw from one side of the bin or the other, depending on which side slips first.
As a result, the moisture, even in the same batch, can vary as the material flows through the gate. The moisture sensor tries to average based on its “field of view”, the volume within which the sensor measures. This is small compared with the gate area. In consequence, there will be errors when the sensor does not measure parts of the flow. The sensor does do a good job of averaging the moisture during the flow period, however. Usually this is still a good measure of the average moisture in the batch.
Test techniques In comparison, an oven-dry or bake-out test on samples taken manually during the batch feed is from the front edge of the gate. This is far from the moisture sensor and will show errors. This makes the moisture sensor inherently more accurate than the sample for bake-out. Other articles consider the difference between surface and internal moisture and differences between moisture metering techniques. These are beyond the scope of this article.
I commented in a previous article that during tests at a concrete plant we found an expert in oven-dry testing who had an ingenious way to ensure SSD conditions. He used a piece of ½” thick plate glass held over the pan for a few seconds. If it misted, it indicated there was still surface or free water. When no mist occurred, he shut off the heater. As a result, he got a very precise SSD condition for weighing the sample.
Sono-Vario TDR Sensor Features
Precalibrated, no need for on-site calibration.
15 choices, sand to coarse aggregates.
Auto-calibration as it wears.
High accuracy on coarse aggregates.
Measures 1-1/4″ rock with 0.2% accuracy.
Digital and analog outputs.
Face-plate is replaceable.
Read more on Sono-Vario TDR sensor.
Find out about Moisture Measurement.
Two models of Sono-Vario:
Sono-Vario Standard, alumina faceplate.
Sono-Vario Xtrem, silicon oxide faceplate
Stainless steel body. Specification.