Capillary transducers

The viscosity of a polymer solution as compared to the viscosity of the pure solvent is measured by the pressure drop AP across an analytical capillary-transducer system. The specific viscosity is obtained from AP/P, where P is the inlet pressure of the system. As the concentrations in SEC are usually very low, [q] can be approximated by qsp/c. 

Many capillary transducers are filled with mercury. Since the diaphragm of the transducer is quite thin, there is a danger of the diaphragm rupturing and leaking mercury into the plastic and into the workplace. Unfortunately, many transducers do not carry a label indicating that the transducer is filled with mercury, so adequate safety precautions are not always taken. It is important to make sure that mercury-filled transducers are not used in the extrusion of medical products and food packaging products. In these situations, other liquids can he used inside the transducer the most common alternative to mercury is NaK (sodium-potassium). 

Another type of transducer is the pneumatic pressure transducer. It has good robustness, but poor temperature sensitivity, poor dynamic response, and average measurement error. The capillary transducer has fair robustness, fair temperature sensitivity, and fair dynamic response. The total measurement error varies from 0.5 to 3% depending on the quality of the transducer. The pushrod is similar to the capillary transducer, except that it tends to have poor temperature sensitivity and poor total error. The piezo-resistive transducer has good robustness because of its relatively thick diaphragm, good temperature sensitivity, good dynamic response, and low measurement error. A comparison of different pressure transducers is shown in Table 4.1. 

To accommodate smaller liquid flows of about 10 pl/min, micro-ultrasonic nebulizers have been designed. Although basically similar in operation to standard ultrasonic nebulizers, in these micro varieties, the end of a very-small-diameter capillary, through which is pumped the sample solution, is in contact with the surface of the transducer. This arrangement produces a thin stream of solution that runs down and across the center of the face of the transducer. The stream of sample... 

Fig. 27. Abrupt contraction cell for flow visualization, birefringence and degradation measurements A inlet (from a peristaltic pump of a pressurized reservoir B outlet (atmospheric pressure or partial vacuum) C interchangeable metallic nozzle with a sapphire tip D capillary flow meter E glass window for flow visualization AP pressure drop (from pressure transducers)...

The on-line viscosimeters currently available are adaptations of the classical dilute solution capillary viscosimeters. They work on the principle of measuring the pressure drop across a capillary with a differential pressure transducer. The pressure drop can be related to the reduced or inherent viscosity of the sample via Poiseuille s law.84 Intrinsic viscosity is determined using the equation ... 

The oil-water dynamic interfacial tensions are measured by the pulsed drop (4) technique. The experimental equipment consists of a syringe pump to pump oil, with the demulsifier dissolved in it, through a capillary tip in a thermostated glass cell containing brine or water. The interfacial tension is calculated by measuring the pressure inside a small oil drop formed at the tip of the capillary. In this technique, the syringe pump is stopped at the maximum bubble pressure and the oil-water interface is allowed to expand rapidly till the oil comes out to form a small drop at the capillary tip. Because of the sudden expansion, the interface is initially at a nonequilibrium state. As it approaches equilibrium, the pressure, AP(t), inside the drop decays. The excess pressure is continuously measured by a sensitive pressure transducer. The dynamic tension at time t, is calculated from the Young-Laplace equation... 

Recently, a very practical bubble pressure tensiometer was developed using elegant pressure transducer mechanics which only needs one capillary made from a high-tech polymer [51, 52]. The tensiometer is able to measure at different immersion depths but needs calibration in order to make the resulting data comparable to surface tension values from other sources. It was shown in a series of measure-... 

FIGURE 14.18 Flow diagram of split flow capillary LC system. 1. Solvent reservoirs. 2. Model 5000 syringe pump (Varian, Walnut Creek, California). 3. Static mixer. 4. Injection port. 5. Column. 6. Detector. 7. Pressure transducer. 8. Pulse dampener. 9. Purge valve. 10. U-flow controlling device. 11. Waste. 

Figure 6. Diagram of our 1-atm ion mobility spectrometer (IMS) apparatus (a) stainless steel source gas dilution volume, (b) septum inlet, (c) needle valve, (d) Nj source gas supply, (e) source and drift gas exhaust, (f) flow meter, (g) pressure transducer, (h) insulated box, (i) drift tube, (j) ion source, (k) Bradbury-Nielson gate, (I) Faraday plate/MS aperture, (m) drift gas inlet, (n) universal joint, (o) electrostatic lens element, (p) quadrupole mass filter, (q) 6"-diffusion pump, (r) first vacuum envelope, (s) channeltron electron multiplier, (t) second vacuum envelope, (u) 3"-dif-fusion pump, (v) Nj drift gas, (w) leak valve, (x) on/off valves, (y) fused silica capillary, (z) 4-liter stainless steel dilution volume, (aa) Nj gas supply.

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 ... 

The new viscometer design utilizes two sets of the capillary and pressure transducer assemblies like the one shown in Figure 1. 

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... 

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... 

As commented in Section 2.1, the vertical capillary type of apparatus requires considerable care to set up and operate. A horizontal capillary results in a little more simple apparatus compared to a vertical capillary but in either case there is the extra necessity in the constant pressure method to accurately calibrate the capillary. Generally, the most convenient procedure is to use the constant volume method with an apparatus equipped with modern pressure transducers. 

Electrochemical detectors for liquid chromatography have reached a level of maturity in that thousands of these devices are used routinely for a variety of mundane purposes. Nevertheless, the technology is advancing rapidly in several respects. Multiple electrode and voltammetric detectors have been developed for more specialized applications. Small-volume transducers based on carbon fiber electrodes are being explored for capillary and micropacked columns. Recently, electrochemical detection has also been coupled to capillary electrophoresis [47]. Finally, new electrode materials with unique properties are likely to afford improved sensitivity and selectivity for important applications. 

The controlled drop tensiometer is a simple and very flexible method for measuring interfacial tension (IFI) in equilibrium as well as in various dynamic conditions. In this technique (Fig. 1), the capillary pressure, p of a drop, which is formed at the tip of a capillary and immersed into another immiscible phase (liquid or gas), is measured by a sensitive pressure transducer. The capillary pressure is related to the IFT and drop radius, R, through the Young-Laplace equation [2,3] ... 

The size of the drop is varied by using a computer controlled microsyringe attached to the capillary and the output of the transducer is also fed into a computer. The volume and radius of the drop at any instant are determined by the position and speed of the microsyringe plunger. For the measurement of equilibrium IFT, a drop is formed at the capillary tip and maintained at that size. After sufficient time, chemical equilibrium is achieved and the equilibrium thermodynamic IFT can be calculated from the measured, steady state capillary pressure and drop radius by Eq. 1. 

Steady shear viscosities can be measured with two different instruments. The System IV can measure polymer viscosities from about 0.001 to 10 sec 1 while the Gottfert Capillary Rheometer is capable of obtaining viscosities from 0.1 to 100,0001/s. In steady shear, the strains are very large as opposed to the dynamic measurements that impose small strains. In the capillary rheometer, the polymer is forced through a capillary die at a continuously faster rate. The resulting stress and viscosity are measured by a transducer mounted adjacent to the die. A schematic of the system is illustrated in Figure 5. 

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