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A HYDRODYNAMIC WHEATSTONE BRIDGE FOR USE AS A TEACHING TOOL IN INSTRUMENTATION LABORATORY COURSES

Abstract

Undergraduate engineering students often find systems composed of electrical circuits difficult to grasp because variables such as current, voltage, resistance, capacitance, and inductance are not easily visualized as their analogs in mechanical systems. Thus, a Hydrodynamic Wheatstone bridge, using the analogy of flow in pipes, was developed to serve as a teaching tool in the classroom. A series of tests were performed to simulate ¼, ½, full, and shunted bridge circuits, where the increase or decrease in resistance in the strain gage is analogous to partially closing or opening a valve in a pipe network. The difference in head potential (i.e., ∆V) was measured with manometers located between the valves. The results agree with the ¼, ½, full, and shunted Wheatstone bridge circuits. Future enhancements include the addition of flow meters to relate water flow to current flow, scaled full turn valves to more accurately represent changing resistances, and a flexible tube section, in place of a valve, to replicate a strain gage in tension.

Introduction

In 1994, Civil Engineering professors at the University of Florida developed an undergraduate course, "INSTRUMENTATION FOR ENGINEERS", in which students are exposed to the fundamentals of circuitry through basic analysis of DC and AC circuits. Exercise problems are routinely performed in the classroom and given to the students for reinforcement through independent practice. Accompanying the course is a weekly two hour lab which provides the students hands on practice with civil engineering instrumentation: e.g., load cells, displacement transducers (LVDT), accelerometers, and pressure transducers. Since the Wheatstone bridge circuit is frequently used in these instruments, analysis of it is paramount to the students’ success in the course. Unfortunately, this topic is one that causes the most angst (perhaps second only to Thevenins!), and thus we felt it would be helpful to develop a device that would allow the students to observe what happens to water pressures when flowing in a “bridge circuit” pipe network.

Hence, using the analogy of flow in pipes, a bridge circuit was developed as a teaching tool. The similarity between hydrodynamics (flow, pressure, and valve position) to electricity (current, voltage, and resistance) provides a useful analogy. For example, a strain gage in a bridge circuit can be modeled with a needle valve in a pipe network so that an increase or decrease in resistance (via tension or compression) in the strain gage is analogous to closing or opening a valve in a pipe system. This will, in turn, create a ∆V (assuming a ¼ bridge) that can be represented by a difference in water heights in manometers located at the mid-points of the “bridge”.

Similarly, ½, full, and shunted bridge circuits can be modeled depending on the valve settings. For example, the students can 'balance' the bridge by adjusting the valves until the water heights in the manometers are equal. Then, by opening (less resistance = compression of a gage) one or

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Bloomquist, D., & McVay, M., & Wasman, S., & Slatton, C. (2006, June), A Hydrodynamic Wheatstone Bridge For Use As A Teaching Tool In Instrumentation Laboratory Courses Paper presented at 2006 Annual Conference & Exposition, Chicago, Illinois. 10.18260/1-2--1331