Determination of Concentration
Associated sections are:
- determination of the moisture of grain,
- Determination of bulk density, called mass per hectolitre,
Measuring devices for these determinations are grouped together as grain grading measuring devices.
Other sub-sections of concentration determination are:
- Measurement of the concentration of substances in solutions using a quantity proportional to the concentration (density, refractive index), see "Density"
- Measurement of breath alcohol concentration
The sub-sections in detail:
Moisture of grain:
In the case of grain, as a biological material, the moisture can only be determined using a standardized determination method (representation method):
The crushed and ground grain sample is exposed to a temperature between 130 °C and 133 °C in a drying cabinet for two hours and the mass loss is determined under these conditions. All grain moisture meters that are subject to legal metrology are connected to this method of measurement. Due to the long determination time, the measuring devices that are in practical use mostly use other measuring principles than the above-described method of direct drying.
Examples are:
Measurement of electrical conductivity:
The electrical conductivity increases considerably with increasing moisture of the grain and can be measured as a resistance of the grain sample, for example by measuring the current while keeping the voltage constant according to Ohm's law or by means of a bridge circuit.
Measurement of the dielectric constant:
The high dielectric constant of water represents a useful measure of the moisture of a grain sample. The capacitance of a measuring cell designed as a capacitor with the grain sample to be measured as the dielectric, is measured.
Measurement of absorption of electromagnetic radiation:
In the NIT method (near infrared transmission), the absorption of electromagnetic radiation is determined in the near infrared range as it passes through the uncrushed grain sample to be measured. Absorption is related not only to moisture, but also to a number of other ingredients (for example protein).
Determining the moisture of grain is a requirement for:
- Assessment of its storage suitability
- Price determination at the grain takeover points
Measurement of breath alcohol concentration:
The metrological testing and calibration are based on the static wet gas method. That means, test air is passed through a calibrator filled with a standard solution of a specific water-alcohol concentration at a constant blowing pressure and constant flow rate. The test air is saturated according to Henry's basic principle.
The test air thus has a calculable water-alcohol concentration. The verification and calibration are based on a comparison of the displayed values of the measuring device for determining the alcohol content in the breathing air with this calculated water-alcohol concentration of the standard solutions.