The criteria for evaluating shear stress measuring range and the lowest measurable shear stress of rotational viscometer.

In the Guidelines for measurement of blood viscosity and erythrocyte deformability by ICSH in 1986, it points out that the viscometer for hemorheology should ideally be able to operate over a wide range of shear conditions, and for a constant shear-rate viscometer a high-shear measurement at 200/second and a low-shear measurement at 1/second (or shear-rates approximating to these values if they are not attainable in the instrument used) should be made [1]. It also states that a high degree of instrument sensitivity is required for measuring viscosity at low shear. A suitable shear stress measuring range and a high degree sensitivity at low shear-rate are required for measuring blood viscosity. Unfortunately, many of the industrial viscometers do not meet the requirements [2]. In fact, the lowest measurable shear-stress is a critical parameter for a rotational blood viscometer. However, the criteria for evaluating shear measuring range and the lowest measurable shear-stress are not clearly described in the guidelines. We proposed double 5% criteria for the evaluation, i.e. less than 5% error in viscometry and less than 5% in coefficient of variation as “measurable” criteria. Recently, we experienced the criteria with a domestic rotational viscometer (anonym for avoiding possible conflicts in commercial interests). The viscometer is a shear-rate controlled rotational viscometer with double gap cylinder sensor designed for measuring blood and plasma viscosities. A suspending torsion strip is employed as the shearstress sensing element. The viscometer works in the shear-rate range of 1–220/second and shear-stress range of 10–2000 mPa according to the specifications described in the user’s manual. The sensor temperature is controlled at fixed 37 ± 0.5◦C. The required sample volume is 1.2 ml. We selected a number of standard oils (provided by the Chinese Academy of Metrology), including GBW(E)130251, GBW(E)130253, GBW(E)130254 and GBW(E)130255, for evaluation of the viscome-