Knowledge from the field of analytical calibration

A gas analyses is carried out in many industries and sectors to investigate the composition of gases and gas mixtures. Here, various gases are not only used for the value added process but are also produced as a by-product by various other processes, such as the combustion process. Corresponding measuring instruments are used here and must be calibrated regularly for safe gas analysis in order to protect people from possible harmful gases. The maintenance of the devices thus serves to prevent damage to the measuring cells, which can be caused, for example, by dirty filters in the measuring system. In addition, the measuring cells are adjusted to ensure that only minimal measuring errors occur.

Flue gas measuring instruments are calibrated with the aid of reference gases in the calibration gas test stand. Here, the flue gas measuring devices are connected to a semi-automated system. The system precisely regulates the purging times between the individual media as well as the admission of the gases. For calibration, the measured values are now only read out.

The measuring cells are adjusted/replaced during maintenance/repair, as this represents a calibration relevant change to the device, the flue gas measuring device must be calibrated again.

At the heart of every analyser are substance specific sensors or sensor systems, so-called measuring cells. Their function is based on physical or chemical principles such as absorption, transmission, ionisation, heat tinting or paramagnetic or electrochemical properties. The sensors react to a change in the measured variable with a corresponding change in its property (e.g. increased light absorption or reduction in conductivity), from which a measurement signal can be formed.

Electrical conductivity, also known as conductivity, is a physical quantity that indicates how strong the ability of a substance is to conduct the electric current. The conductivity is a measure of the total ion concentration of a test sample. The more salts, acids or bases are present in the measured solution, the higher the conductivity.

The conductivity of a solution depends on:

• The temperature: with increasing temperature the conductivity in solutions increases. The conductivity measured at any given temperature is therefore usually converted to a reference temperature. The reference temperature is usually 25 °C. The conversion is done with the help of the temperature coefficient α.
• The number of ions:  The more ions a solution contains, the greater the conductivity of that solution.

The calibration of measuring systems is carried out using reference solutions which are tempered to 25 °C in the circulation tank. The measurement of the electrical conductivity is determined by the resistance value. Ohm's law can be used to calculate the electrical conductance. The electrolytic conductivity is calculated from the electrical conductance G and the geometric constant of the electrode pair C (cell constant). The cell constant (also called electrode constant or resistance capacity) is the ratio of the electrode area to their distance from each other. The cell constant has the unit of measurement cm-1. For very simple geometries such as that of a plate capacitor, the cell constant can be calculated by dividing the electrode spacing by the electrode area. The cell constant is preferably determined by calibration. Due to inter-cell variations, cell constants are determined individually and indicated on the cell.

In pH measurement, the number of hydrogen ions is measured in order to obtain information about their concentration. The pH value is a measure of the acidic or basic character of an aqueous solution. It is the negative decadic logarithm of the hydrogen ion activity. The pH value of a solution depends on the temperature. The reference temperature is usually 25 °C. As the temperature rises, the slope of the measuring electrode also increases. If measurement or calibration is not carried out at a stable 25°C ambient temperature, this must be adjusted by setting the temperature compensation.

The calibration of measuring systems is carried out using reference solutions which are tempered to 25°C in the circulation tank. The voltage which arises at the measuring electrode and which can be of different magnitudes is compared with the voltage which arises at the reference electrode due to diffusion potentials and which, independent of the ion concentration in the water, is constant. The difference between the two voltages, the actual measurement signal, then provides information about the ion concentration in the water. Calibration determines the zero point voltage and the slope of a pH electrode and stores it in the meter. Since the zero point and the slope can change due to external measuring conditions as well as the natural aging process, a pH electrode must be calibrated regularly.

TPM is the abbreviation for "Total Polar Materials": Total polar materials). The content of total polar compounds is expressed as % TPM or sometimes also as TPC ("Total Polar Compounds or Components"). If the oil is too old, it has an increased TPM value. The threshold in Germany for spoilage was set at 24 % TPM. The capacitive measurement is based on the measurement of the dielectric constant. For this purpose, a voltage is applied to the two capacitor plates. The capacitor plates are charged until a certain amount of electrical charge is reached. As the charge increases, the polar fractions in the fat become more and more aligned. The red positive ends of the shares point to the blue negative plate and the blue negative ends point to the red positive plate. If the capacitor is charged, it has a certain capacity. It depends on the dielectric, in this case oil. The more polar fractions there are in the frying oil, the greater the capacitance of the capacitor. This change in capacity is converted and then appears, for example, as TPM content in percent on the display of the frying oil tester.

Calibration of frying oil testers is carried out using reference oils which are tempered to 50°C. The temperature of the reference oil is determined by the calibration method. The grease sensor is immersed in the tempered reference oil which has a defined TPM value. If the value displayed on the measuring instrument deviates too much, it will be adjusted to the setpoint value at 50°C and 180°C.