The calibration of the moisture measuring probes are effected by means of two potentiometer controls „0“ and „%“.
The controller labeled "0" is for adjusting the offset value.
The controller labeled "%" is for adjusting the amplification or slope of the calibration curve.
The probes can be delivered with built-in 0 and % adjustment potentiometers. For measurements with sands these adjustment potentiometers are not necessary because the probes are normally pre-calibrated for sand at the works.
For use with other materials it is very much recommended to obtain probes with these adjustment potentiometers.
There are no standard calibration adjustments for all materials, and also no absolute moisture measurements are possible. The calibration of the moisture measuring sensors is specific to a material for a given range. One must make certain that the sensor signal voltage is within the permitted range of 0-10V (or. within the 0/4-20mA range).
To ensure this, it is necessary to have two samples of the material for calibration purpose: one with the minimum possible amount of moisture in the material and the other with the maximum possible amount of moisture in the material.
The signal voltage is to be measured directly at the output terminals of the probe (for current outputs, the current is to be measured in series at the output terminals).
For the moisture signal processor FMP the moisture curve 0 can be selected. The displayed value in per cents (%) then corresponds to the probe output voltage of 0-10V.
In case the probe is not calibrated earlier for the 0 and % values, a following processor electronic can no longer correct it, especially if the probe signal has already entered the saturation region.
Preparations for the calibration
The moisture measuring probes are calibrated with previously prepared samples of the moisture containing material. The moisture values should lie within the desired range of measurement. One sample should be in the lower part of the scale and the other in the upper part of the desired range.
For preparing the samples one should have at least 1-2 liters of the moisture containing material per sample ready. Metal containers should not be used. Likewise no metallic support or underlay are allowed. The material height should be at least 8-10cm. This height should be the same in all containers for the samples.
In case of very moist samples there is a tendency for the water to settle down in the bottom part of the container. Likewise, evaporation from the surface may change the moisture content of the upper part of the material sample.
For these reasons the samples should be well stirred and mixed prior to every calibrating measurement.
When not being used, the containers should be kept well covered or sealed. These should also not be placed under direct sun light.
Calibration for the moist sand:
Switch on the probe approx. 30 min. before calibration (stable operating temperature).
The calibration of the probe is done in two steps:
The reason for the two steps is because of the dependency of the results on density variation of the material, and also due to the influence on the high frequency field of the sensor from the installation. Attention must be paid to maintain a relatively constant density of the material on the measuring surface.
Under static calibration we understand measurements done outside of the sand silos, where the samples of the moist materials are placed in plastic buckets.
For calibrations with sands a sample of approx. 5-10 liter of the material is collected through the dosage opening. The container should be kept covered during the calibration adjustments in order to prevent the moisture content of the material from changing.
The same amount of sand from the same source is collected, and then dried so that the moisture content goes down to a value of < 0.5%. This sample should also be stored in a cool place and kept covered. The dry sand must have cooled down before the calibration measurements.
As the preparation of the dry sample takes more time, the procedure can be initiated at an earlier stage.
Determination of the moisture content of the samples
Before starting the static calibrations the moisture content of the sample is determined in the laboratory.
The moisture content of the dried sand can be calculated according the well known formula:
Moisture content = ((Moist weight – Dry weight) / Dry weight) x 100%
Carrying out the static calibration
The sand in the plastic bucket is first loosened somewhat at the top and distributed evenly.
Static calibration with dry sand:
The probe is first placed on the top surface of the sample and then pushed in with light twisting motion of the hand. A light pressure is applied on the probe with the hand during this procedure.
The probe is then released from the hands without lifting it out or moving it.
Adjustments for the dry sand sample:
The 0-potentiometer is then varied till the laboratory value is shown. Note the indicated moisture content on the measuring instrument during this procedure.
Static calibration for the wet sand:
The probe is placed on the sample, and as before is pushed into the sample.
Adjustments for the wet sand sample:
The %-potentiometer is then varied till the laboratory value is shown. Note the indicated moisture content on the measuring instrument during this procedure.
Further checks of the calibrated values
The calibration procedure should then be repeated with eventual minor adjustments of the 0 and % potentiometers. The indication of the moisture contents on the measuring instrument for dry and wet sands should now be correct.
The measuring surface of the probe must be in full contact with the material. Care should be taken to avoid inadvertent motions (e.g. any axial movements), which might cause uneven contacts or air gaps at the measuring surface.
Always clean the sensor surface when moving from wet sand to dry sand.
Sample material should be frequently stirred – moisture tends to settle down at the bottom, thus leaving the upper surface drier.
Dynamic calibration at the sand silo
The dynamic calibration is necessary because it is not possible to vary the moisture content at will once inside the silo. The earlier static calibration enables one to obtain only an approximation to the calibration curve and fix the null point. The dynamic calibration primarily corrects the slope of the calibration curve so that the correspondence of the actual moisture values and the indicated values are more or less in agreement.
Procedure for dynamic adjustments
The probe is now mounted in the silo and switched on. Moist sand is then allowed to flow across the probe, and the indicated moisture content is noted. At the same time a sample of the sand is collected, and as before, the moisture content is determined in the laboratory using the drying method. If there is a difference between the measured and the indicated value, then this is corrected using the % potentiometer.
Important: Error in measurement during laboratory tests (due to drying etc.).
As it is possible for the moisture values in the samples for the laboratory measurements to vary due to the inhomogeneity of the source sand, the calibration curve should only be minimally corrected for the slope. The required precision of the measurement is reached only after repeated dynamic calibrations, possibly with sands with different moisture contents.
The null point is usually set only once, and normally no further corrections are necessary.
During the operational phase, usually the slope of the curve varies due to density variations. That is why it is only necessary to correct the slope with the % potentiometer as necessitated by the varying conditions.
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