Electronic Design

Hyperbaric Fall-Time Viscometer Is PC-Based

The measurement of the viscosity of solutions is an integral element in analytical chemistry and chemical manufacturing. It has particular value in designing and controlling the processes of polymerization, an essential step in the production of plastics. Considerable research is being conducted at the University of North Carolina toward the use of liquid and supercritical carbon dioxide (CO2) as an environmentally benign industrial solvent. This work generated the need for a viscometer compatible with use in high-pressure (approximately 1000 psi) apparatus of the sort used to contain liquid and supercritical CO2.

The style of viscometry chosen is the "sinker" method, using the fall time of a steel slug (sinker) down an accurately bored "fall-tube" containing the pressurized sample. Due to the very high (hyperbaric) pressures involved, this viscometer fall-tube is massive. It resembles a smooth-bore stainless-steel gun barrel. The use of a thick-walled metallic fall-tube posed an interesting design problem: How do you measure the motion of the sinker as it moves inside an opaque, conductive tube?

Magnetic induction is the answer chosen to take advantage of the difference in properties between a ferromagnetic sinker and the nonmagnetic tube. An air-core variable-differential-transformer (VDT1) wound on a brass tube is placed coaxially over the fall-tube. When the sinker is released by drop magnet L1, it descends through the tube and the VDT (upper right in figure). As it moves, the permeability of the sinker progressively changes the inductive coupling between the windings of the VDT. It thereby systematically disturbs the null established by trimmer R1 in a consistent pattern. This makes the accurate timing of the fall possible.

Initially, with the sinker outside the VDT, R1 is balanced for a null at A4's input. Entry of the sinker into the VDT disturbs the mutual-inductance to produce a non-null input to A4. A4 boosts this VDT signal, which is then synchronously demodulated by S2 and S3. The dc demodulated signal is then digitized by the integrating analog-to-digital converter (ADC) comprising differential integrator A1, comparator A2+U2, and U1. Because the timing for both the VDT drive waveform and the ADC conversion cycle are derived from the same 1-kHz countdown reference, demodulation ripple is effectively cancelled by the synchronism of the ADC conversion. This results in a low-noise conversion result compatible with accurate measurement of the speed of the sinker's progress through the VDT and fall-tube.

The VDT is excited with an approximately 1-kHz symmetrical triangle waveform derived via op amp A3's integration of the square-wave reference produced by the U1 8254 counter-timer. The relatively weak coupling between the VDT and the sinker (hindered by the massively thick fall-tube wall) makes the maximum amplitude of the triangular drive a desirable design goal. Because significant variation can be expected in both the 5-V rail and in Q1's saturation voltage, an automatic drive-amplitude regulation feature is incorporated in the form of Q2 and Q3. This configuration ensures that, at the waveform minima, Q1 is driven to cutoff, and at the maxima, Q1 is driven to full saturation.

Drive regulation works as follows: Q1 is enclosed in the feedback loop of integrator A3 so that, when the triangle drive is ramping positive and Q1 begins to enter saturation, A3's feedback loop opens. This causes A3's output to suddenly slew positive. This action drives Q3 into conduction, incrementally discharging the positive-slew-rate reference voltage on C1. The C1 discharge pulses continue until the peak triangle voltage causes Q1 to just "kiss" saturation, thus closing a feedback loop that maximizes the positive-going triangle-wave amplitude. A similar action takes control on the negative triangle half-cycles as the approach of Q1 to cutoff causes a rapid negative slew of A3's output. These negative spikes induce Q2 to conduct and thereby adjust the negative-ramp control voltage on C2. The resulting negative-peak-amplitude feedback loop maximizes the negative drive waveform excursion, completing the VDT drive optimization.

The rest of the circuit is a fairly straightforward ISA-bus interface. This lets elementary MSBasic control software (see the listing under Ideas For Design at www.elecdesign.com) easily compile the viscometry datasets.

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