>> For the user of a rheometer a practical and inexpensive approach to check its precision is to measure a Newtonian fluid with defined viscosity, a socalled test liquid. [...] They are mineral or synthetic oils whose viscosity was determined with high accuracy at one or several temperatures. The viscosity, its accuracy, the measurement temperature (normally 20°C or 25°C) and the expiry date of the test liquid are marked on the attached certificate. It is recommended to use test liquids with a viscosity similar to the one of the samples usually measured.
Impurities lead to premature ageing of the test liquids, which generally becomes obvious via a slight increase in viscosity. For this reason, any test liquid used once may not be returned into the original container. Moreover, they should be kept in a cool, dark place. However, ageing of test liquids cannot be entirely prevented. Despite proper storage, experience shows that the viscosity increases by 1 % after 8 months. Taking into account the time between measuring the oils viscosity and delivery, the customer can rely on the certified values for at least 6 months.
For testing viscotesters, silicon oils are also used since they exhibit a lower temperature dependence of their viscosity and many viscosity testers are not provided with temperature control. When using silicone oils it has to be taken into account that silicones with  higher molecular weights exhibit a shearthinning effect at higher shear rates; i.e. the viscosity decreases from a specific shear rate on. Moreover, there are areas of application, such as foods, pharmaceutical products or paints, in which silicone oils may not be used at all. Depending on the result of the test-oil measurement, one decides whether the initially measured viscosity values are correct and whether the measuring method or the rheometer must be checked. That is the reason why the measuring process must be carried out with the greatest possible care. <<

_{From: Application note V-218 "Test liquids", Dr. Klaus Oldörp, Thermo Fisher Scientific, Process Instruments, Karlsruhe, Germany
Accentuation: PSL Systemtechnik}