The CD12 provides a DC voltage proportional to the pressure drop measured by the DP15. In the standard application of this device, one maintains a constant flow rate for a given capillary tube. A measurement of the pressure drop along the capillary tube can then be converted into a measurement of viscosity. Further, if Eq. 1 holds and the response of the pressure transducer is linear, a direct measurement of the reduced viscosity can be effected without the need for a calibration of the device. This is a significant advantage of the single capillary design.
V is the voltage read from the pressure transducer.
Any calibration constants will cancel in the ratio. That is, we measure the step response of the system. Using the Maxwell model to interpret these results, we can obtain the relaxation time of the polymer solution to a sudden release of stress. One then assumes that the stress is the same in each element, Hooke's law is valid, and that the strain is independent of time. In the absence of an applied pressure, we assume that the volume flow rate, Q, of solution is driven by the stress and so, using Eq. 1, the time dependence of pressure drop along a length of pipe will be as in Eq. 4.