Differential pressure transducer capillary viscometer

Earlier experiments involved the collection of SEC effluent aliquots to measure solution viscosity in batches with the very time consuming Ubbelohde drop-time type viscometers. A continuous capillary type viscometer was first proposed for SEC by Ouano. Basically, as shown in Figure 1, a single capillary tube with a differential pressure transducer was used to monitor the viscosity of SEC effluent at the exit of the SEC column. As liquid continuously flows through the capillary (but not through the pressure transducer), the detected pressure drop (AP) across the capillary provides the measure for the fluid viscosity (h) according to the Poiseuille s viscosity law ... 

Details of the SEC/Viscometer detector system have been described previously.(16) The key component of the viscometer detector is a differential pressure transducer (CELESCO Model P-7D, Canoga Park, CA) with a jf25 psi pressure range. The transducer monitors the pressure drop across a section of stainless steel capillary tubing (length 2 ft., I.D. = 0.007 in.). Pump pressure fluctuations... 

A better approach is the use of two capillaries (Cl and C2) in series, each of which is connected to a differential pressure transducer (DPI and DP2), and a sufficiently large holdup reservoir (H) in between. With this approach, the sample viscosity q is measured from the pressure drop across the first capillary, and the solvent viscosity q0 from the pressure drop across the second capillary. Pulsations are eliminated in this setup because they appear in both transducers simultaneously. Another design is the differential viscometer, in which four capillaries are arranged similar to a Wheatstone bridge. In Fig. 10, both designs are shown schematically. 

The single-capillary viscometer (SCV) is represented in Fig. la. Its design is a direct extrapolation of classical viscometry measurement. It is composed of a small capillary, through which the solvent flows at a constant flow rate, and a differential pressure transducer (DPT), which measures the pressure drop across the capillary. SCV obeys PoiseuiUe s law and the pressure drop AP across the capillary depends on the geometry of the capillary, on flow rate Q, and on viscosity of the fluid 7j according to... 

The last differential viscometer design is the Waters Corporation detector [9], which is in the Alliance GPCV2000 high-temperature instrument. It is composed of three capillaries, two differential pressure transducers, and two holdup reservoirs it is represented in Fig. Id. The pressure transducers are connected flow-through this eliminates the need for frequent purges. This detector provides, at the same time, relative viscosity information and relative flow information. This design does not require a perfect matching of the capillaries. 

The automatic relative viscometer is ideally suited for measuring dilute polymer viscosities. It provides faster analysis and greater precision than is obtainable with conventional glass tube viscometers (Ubbelohde or Cannon-Fenske), which it replaces. The principle of operation is based on measurement of pressure drops due to the continuous forced flow of solvent and sample through two stainless steel capillary tubes placed in series. The pressure drop across each capillary tube obeys Poiseuille s law. The pressme drop is measured by a differential pressure transducer. The sample solution is loaded into a sample loop via a syringe pump and then pushed into one of the two capillaries. A steady-state condition is reached when the sample solution completely fills capillary 2, solvent remaining in capillary 1 at all times. The relative viscosity of the sample solution is determined simply and directly by the ratio of the pressure drops. From the measured relative viscosity, all other solution viscosity measurements can be calculated. Solution viscosities are determined by the viscosity of the sample relative to the reference solvent. The relative viscometer measures the solvent and sample viscosity simultaneously, so errors due to temperatme fluctuation and solvent variations are avoided. The main advantages of this approach are ... 

Another automatic capillary viscometer which directly detects the pressure difference AP (eqn (5.42)) has recently come on the market. The Relative Viscometer manufactured by the Viscotek Co. includes two capillaries (capillary 1 and 2) connected in series, with the sample injection valve located between the two capillaries. Differential pressure transducers (DPT) are connected in parallel to each capillary. The sample is injected into capillary 2 and a pressure change is detected by the DPT. The viscosity ratio v r is determined as the ratio of the pressures divided by the instrument constant. 

Single capillary viscometers are remarkably easy and inexpensive to construct using a modem differential pressure transducer with a double T-network and capillary of given L and/ . 

Another device that simplifies the measurement of dilute solution viscosity is the differential viscometer. One design [31] consists of two capillary tubes in series (Figure 7.9). When fluid is pumped at a constant rate through the two tubes, the pressure drops are sensed by differential pressure transducers, DPT-1 and DPT-2. With... 

Sample solution is introduced into the solvent flow stream from a sample loop via a sample injection valve. The solvent flow pushes the sample solution through the analytical capillary vdiere the viscosity of the sample is detected. The AP signal from each pressure transducer is fed to a differential logarithmic amplifier as shown in the Figure. The viscometer output is the In signal of the sample solution. A pump is used to... 

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