eFunda: Introduction to U Tubes
engineering fundamentals U Tubes: Introduction
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The U Tube contains water or mercury in a U-shaped tube, and is usually used to measure gas pressure. One end of the U tube is exposed to the unknown pressure field and the other end is connected to a reference pressure source (usually atmospheric pressure), shown in the schematic below.

Typical U Tube

By comparing the level of the liquid on both sides of the U tube, the unknown pressure can be obtained from fluid statics,

If fluid C is the atmosphere, fluid B is the liquid in the U tube (e.g. water or mercury), and fluid A is a gas, then we can assume that rB » rA, rC. The pressure contributed by the weight of gas within the U tube can therefore be neglected.

The gage pressure of the gas can be approximated by,

Further Information

To automate the pressure measurement in a mercury-filled U tube, a Wheatstone Bridge can be fabricated by connecting two external resistance to a high-resistance wire threading the interior of the U tube, as shown in the schematic below.

U Tube Pressure Sensor

The resistance of the U tube wire is proportional to its current-carrying length. The two parts of the wire external to the mercury will carry current and therefore will impart resistances to the circuit. However, the immersed portion of the wire carries no current, since the current will instead travel through the highly-conductive mercury. The U tube wire is effectively separated into two separate resistances, each resistance dependent upon the wire length above the mercury. As a result, the difference in the resistance of these two wire segments will be proportional to the pressure difference across the U tube,

where c and k = cRw are factors that can be obtained during calibration.

For an initially balanced Wheatstone Bridge, the voltage output is given by,

where r = Rw / RRef is the efficiency of the bridge circuit.

Thus, the unknown gas pressure (with respect to the reference pressure) is proportional to the output voltage,

Pros and Cons

  • Pros:
  - Low cost
  - Simple and reliable
  • Cons:
  - Low dynamic response rate. Requires time to damp out oscillations.
  - Measurement accuracy dependent on precise leveling of U tube.
  - Cannot be used in weightless (0 g) environments.
  - The liquid in the U tube must NOT interact with measured fluid (be it gas or liquid).
  - Mercury or water vapor contamination can occur, especially in low pressure measurements.

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