Viscometer Calibration

Calibration of Viscometers

How much effort is required for calibration? The calibration of measuring devices is intended to ensure that the reproducibility of measurement results is guaranteed with the greatest possible accuracy and that changes are recognized in good time. However, the term calibration is understood differently and various services are offered under this term.

Here the differences are explained using the example of rotational viscometers and, above all, clarified: What is necessary and useful? Calibration of a measuring device means first of all determining the actual status. Is the meter still working properly? Are the measurement results the same for repetitions?

In-House Calibration of Viscosity Meters

Viscosity meters can be checked in-house with a substance of known viscosity, and many companies perform this calibration on a weekly or monthly basis by measuring the viscosity of a calibration oil. These calibration oils are silicone oils whose viscosity is confirmed on a calibration certificate. There are two important things to note here. The viscosity should be specified as a function of the temperature, whereby the measurement uncertainty must be taken into account. In Fig. 1 you can see the temperature dependency of a calibration oil. It can be seen that the temperature control is a major source of error, because with a temperature difference of 0.1° C in the measuring gap, the viscosity changes by around 1%.

Furthermore, the instructions for storing the oils must be strictly observed, the oil is dark and does not keep for more than 6 months in a closed container. Once used, liquid should be discarded and not reused. If you look at the costs for the calibration oils and the working time required for the measurements, this method is not exactly cheap. However, it gives the opportunity to continuously document the condition of the measuring devices. However, it in no way replaces a traceable calibration as shown below. But what is traceable calibration? For this purpose, the measurement technology of a rotational viscometer must first be briefly discussed.

Measuring technology Rotational viscometers The measuring system of every rotational or oscillating viscometer consists of a measuring body and a measuring cup. The substance to be examined is located between the measuring body and the measuring cup. Measuring bodies and cups can take on different shapes, e.g. B. coaxial measuring systems, cone-plate, anchor stirrer and much more. What they have in common is that either the measuring body or the cup rotates and the liquid to be measured is sheared in the measuring gap between the cup and the body. The viscosity is calculated from the resistance that the liquid opposes to this movement. With a cylindrical measuring body z.

COMPLIANCE: ISO/IEC 17025:2005 ACCREDITED

We are an accredited, SAMM (Skim Akreditasi Makmal Malaysia) Malaysia-based laboratory, with very well trained and experienced technicians able to accomplish your temperature and humidity calibration both on-site and in our laboratory. We calibrate all types of temperature and humidity instruments regardless the industries, for instance Food and Beverage manufacturing industry, Pharmaceutical, Chemical plants, both private and government Hospitals, both private and government Laboratory, Testing laboratory, Commercial Kitchen, Hotels, Quarry, Palm Oil Industry etc.

Coaxial measuring system

It can be seen that the viscosity is calculated from the speed n and the torque M. The calculation formula also depends on the measuring system geometry and, as shown in the previous section, on the temperature.

There are therefore the following sources of error:

Speed
Mechanics
Temperature
Torque

The influences of these different parameters must be examined separately for a meaningful calibration. This is not possible with a measurement of calibration oil. If you go e.g. assuming that the surface of the measuring system is damaged, the measured torque increases. However, this can easily be compensated for by a minimal increase in temperature, so that the measurement of the viscosity of a calibration oil is correct again. When measuring the viscosity of a non-Newtonian substance with a different viscosity-temperature gradient, however, the measurement result is influenced in a completely different way. A major weakness of the in-house calibration is also that this check is often carried out by the employees who also work with the device on a daily basis, so that errors that have crept into the daily routine are not noticed, since the does the same thing and thus always gets the same results.

Traceable calibration In contrast to checking using a calibration oil, traceable calibration examines each of the four components independently of the others and traces them back to the calibration standard. What does this mean in detail: The speed can be checked using a calibrated speedometer.

The mechanics of the measuring body is checked using a special apparatus that allows both the concentricity of the measuring system axis and the surface quality of the measuring body to be checked. The temperature measurement is compared with a calibrated temperature measuring device. Checking the torque measurement requires a special device that converts the torque into another measurable quantity. This is usually done with a calibration stand, which makes the torque measurement verifiable by measuring a weight that can be calibrated.

The rotating measuring body of the viscometer is immersed in a friction-free measuring cup that is filled with a liquid. A thread is attached to this measuring cup, which pulls a weight that stands on a calibrated scale. At a constant speed and immersion depth of the measuring body in the liquid, a balance of forces is established between the torque acting on the measuring body and the force acting on the weight. With known geometrical conditions, conclusions can be drawn about the torque. This form of calibration makes it possible to examine all influences that affect the viscosity measurement separately. Finally, the measurement of a calibration oil can be carried out in order to eliminate the last uncertainty and to check everything finally.

The viscometer calibration should be performed by 3rd party laboratories like Premier Calibration Sdn Bhd to minimize the human factor. Adjustment of the measuring device The setting or adjustment of the measuring device is not initially included as a matter of course in a calibration. It represents an additional step when calibration shows that it is required. It therefore makes sense to commission the manufacturer of the measuring device with the calibration, since in this case it is guaranteed that the measuring device will also be adjusted if necessary. In summary, it can be stated that an objectively correct and comparable viscosity measurement can only be guaranteed through a traceable calibration, including any necessary adjustment.