U-tube manometer

U-tube manometer at the same pressure. Not deflected.

U-tube manometer in case of pressure difference. Deflected by height h

U-tube manometer for monitoring the overpressure of a breathing air supply

A U-tube manometer is a pressure measuring device that can be used to measure and display pressure differences. There are closed and open U-tube pressure gauges.

Open U-tube manometer

The pressure difference between two measuring points is displayed by moving a column of liquid. A U-shaped glass tube that has been partially filled with a barrier liquid is used for this purpose. If the pressure is higher on one side, the barrier fluid moves to the side with the lower pressure.

The following applies:

${\displaystyle {\Delta p=p_{2}-p_{1}=\rho _{SF}\cdot g\cdot h}}$.

From this it follows through transformation:

${\displaystyle h={\frac {p_{2}-p_{1}}{\rho _{SF}\cdot g}}={\frac {\Delta p}{\rho _{SF}\cdot g}}}$.

${\displaystyle \rho _{SF}}$is the density of the sealing liquid, the acceleration due to gravity and the height by which the pressure gauge is deflected. and are the pressures at both ends of the manometer. If these are the same, the deflection is zero and the liquid levels are at the same height. ${\displaystyle g}$ ${\displaystyle h}$ ${\displaystyle p_{2}}$ ${\displaystyle p_{1}}$ ${\displaystyle h}$

Most open U-tube pressure gauges work with the ambient pressure ( ) on one side. In the literature it is usually defined as the so-called standard pressure at 101325 Pa (= 1.01325 bar), but fluctuates worldwide and depends on the weather. ${\displaystyle p_{amb}}$

This type of construction is rarely used nowadays, as the liquids used are either toxic or evaporate easily. This measuring method is also only suitable for low pressures, depending on the density of the barrier liquid. A U-tube manometer for 1 bar pressure would be over 10 m high with water, and 760 mm with mercury. The most common uses were blood pressure monitors, which used mercury as a liquid. Therefore, the unit of measurement for blood pressure is also mmHg for "millimeters of mercury".

The U shape

The height difference is independent of the shape of the pressure gauge as long as there is a closed connection between the two sides. It should be noted, however, that the ratio of the deflections at the ends depends on the area ratio. With the same cross-sectional areas that are constant in the reading area, there is an upward or downward deflection on each side . ${\displaystyle h/2}$

If the areas are constant, but differ ( and ), the displacements are related according to the continuity law (constant volume) . On one side with twice as large an area as on the other side, the deflection is therefore a third of the total height difference. A scale for reading the open U-tube manometer on one side only depends on the shape (cross-section) and the density of the barrier medium. ${\displaystyle A_{1}}$ ${\displaystyle A_{2}}$ ${\displaystyle \rho h_{1}A_{1}=\rho h_{2}A_{2}}$ ${\displaystyle h_{1}/h_{2}=A_{2}/A_{1}}$

Closed U-tube manometer

The closed U-tube manometers can be divided into two groups: One is filled with a gas bubble, the other remains unfilled. With these, a vacuum (negative pressure) is then created on the closed side. Closed U-tube manometers with vacuum are well suited for determining the absolute pressure.

U-tube pressure gauges, which are filled with a gas bubble, are used to determine a system pressure in the chemical industry. The operating principle is as follows: When the pressure rises in one container, the gas in the other leg of the manometer is compressed. Therefore, the scale is not provided with the same intervals from print to print, the intervals always decrease by half.

You have to imagine that if there is a pressure of one bar per 1 liter volume and the volume halves, the pressure doubles to 2 bar. If the volume of half a liter is halved to a quarter liter, the pressure doubles again ( Boyle-Mariott's law ):

See also

• U-tube manometer on the Venturi nozzle

literature

• Norbert Weichert, Michael Wülker: Measurement technology and measurement data acquisition. Oldenbourg, Munich 2010, pages 67 f.