Viscometry : Ask the Experts

SI Analytics (Germany) produces 90% of all the viscometry glass capillaries used in the world today. Why do their competitors buy the most important part in a viscometer from them?

The keyword is traceability. SI Analytics glass capillaries are calibrated by comparative measurements using reference viscometers of the PTB (Physikalisch-Technische Bundesanstalt, the Federal German Physical-Technical Institute in Braunschweig). The PTB is the head standards office in the European Union for viscosity, time and temperature. Hence their calibrated and referenced viscometers have direct traceability to the international standard as required in ISO 9000 ff for test equipment in quality assurance systems.

For the comparative measurement, the viscometers to be inspected and the reference viscometer are placed in the same thermostat bath. The test liquid is filled in, tempered and the flow-through times in both viscometers are measured. The difference is expressed as a constant referring to the tested viscometer, calculated according to the following equation:

K = K_PTB*t_PTB / t

K = constant of the viscometer to be tested
t = flow time of the viscometer to be tested (Hagenbach-corrected)
KPTB = constant of the PTB reference viscometer
tPTB = flow time of the PTB reference viscometer (Hagenbach - corrected)

Then, a second run is performed with another reference viscometer of the PTB and a calibration liquid of different viscosity and a second constant (K2 ) is obtained.

If both constants (K1 from the first and K2 from the second measurement) correlate within 0.1 % they will be accepted. If the constants (K1 and K2) deviate more than 0.1 %, the tested viscometer will be rejected.

Traceability is an issue not only for manufacturers as SI Analytics but also for end users. In a quality assurance system according to DIN/EN ISO 9000 ff, traceability of the measuring equipment to national or international measuring standards is demanded, and the holder of the equipment is responsible for complying with these regulations.

This can be achieved by inspecting the end users’ instruments in regular intervals at a standards office or at a manufacturer location having reference viscometers calibrated at a standards office and provided with a constant. Such service is provided by SI Analytics.