Differential pressure transducers

P. G. Dargie, S. T. Hughes. A thick-film capacitive differential pressure transducer. Measurement Science and Technology, 5, 1994, pp. 1216-1220. 

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 ... 

Foam created in the foam generator is fed directly to the inlet of the test unit in the test cell, and the pressure drop developed by its flow through the test unit is measured by means of a differential pressure transducer, not shown in the figure, connected between inlet and outlet, and provided with by-pass and isolation valves for protection against accidental overloading. 

Figure 4. Differential pressure transducer output, as a function of time, during a typical experiment at atmospheric pressure.

Pressure measurements are made with four differential pressure transducers which produce a 0- to 5-VDC output signal from an unregulated 24-VDC Input. The range of three of the transducers 1s 0 to 500 psi and the other 1s 0 to 320 psl. The signal from these transducers 1s connected to an analog Input card 1n the HP 3497. 

The flow of fluids is most commonly measured using head flowmeters. The operation of these flowmeters is based on the Bernoulli equation. A constriction in the flow path is used to increase the flow velocity. This is accompanied by a decrease in pressure head and since the resultant pressure drop is a function of the flow rate of fluid, the latter can be evaluated. The flowmeters for closed conduits can be used for both gases and liquids. The flowmeters for open conduits can only be used for liquids. Head flowmeters include orifice and venturi meters, flow nozzles, Pitot tubes and weirs. They consist of a primary element which causes the pressure or head loss and a secondary element which measures it. The primary element does not contain any moving parts. The most common secondary elements for closed conduit flowmeters are U-tube manometers and differential pressure transducers. 

FIGURE 3.18 Response to selected detectors as a function of retention volume for myoglobin (dissolved in PBS buffer at a pH of 6.9). The three detectors are the RI = refractive index signal, LS = lightscattering signal, and DP = differential pressure transducer (viscosity signal). (Courtesy of Viscotek, Houston, TX. With permission.)... 

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... 

The differential pressure transducer is a model PID-0.1PSID with model CD11 carrier demodulator (Pace Engineering Co., North Hollywood, Calif.). It is operated at 0.4 maximum sensitivity and is connected to a standard potentiometric recorder using the 200-mv. range. The reference side of the transducer is connected to an empty flask submerged in the thermostat (Figure 2). 

A Reaction vessel with syringe B Differential pressure transducer to recorder C Piston buret (synchronous motor)... 

The value for [BMIM][PF6] corresponds to about 5700 bar, which is consistent with our measurements. However, it should be noted that Berger et al. [16] used a constant volume stoichiometric technique with a 50 cm vessel, pressurized to 50 atm. and containing 10 cm of IL. The resulting pressure drop when the gas is absorbed into the liquid would only be on the order of 0.005 atm. The authors do not report the uncertainty of their Henry s constants, nor the accuracy of their pressure gauge. Unless a highly accurate differential pressure transducer was employed, it is Hkely that these values are good order of magnitude estimates only. 

Total nitrogen of TNT in wastewater was determined by preliminary digestion with K2S203 in alkaline soln and the N03 released was reduced by an automated Cd reduction method (Ref 133), Trinitrobenzoic acid was dissolved in water and titrated with NaOH without an indicator until a weak pink color appeared and persisted even in the presence of TNT and TNB (Ref 34). A gas comparison pycnometer used in conjunction with a Velidyne D15 differential pressure transducer with He yields a density accuracy of 0.002g/ml (Ref 134). 

Gas permeabilities were measured in a Skirrow-Barrer gas diffusion apparatus operating at a partial vacuum (7). The asymmetric membrane separated a large upstream gas chamber and a small downstream gas chamber. The gas pressure change in the downstream chamber was monitored with a differential pressure transducer (MKS Baratron 586/386). 

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