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HydrometerTorque TubeDifferential Pressure GaugeBubbler MethodVibrating TubeVibrating CylinderGamma Radiation Density GaugeDensity MeasurementsHydrometer

The up-thrust acting on a body immersed or partially immersed in a fluid is equal to the weight of the fluid it has displaced(Archimedes principle).Hydrometer is essentially a weighted tube which floats in a liquid to a depth which depends on the density of the liquid.HydrometerThe weight of hydrometer is then equal to the weight of the fluid displaced by the hydrometer.

A scale on the stem of the instrument enables the density to be read off as the value at the liquid surface.

The instrument is simple and cheap

Torque Tube

A float is completely immersed and so because the volume of liquid displaced does not change, the up-thrust force on the float only changes if the density of the liquid changes.This up-thrust force is used to twist a tube. This can be monitored by means of a pneumatic system. Such method can be used for high temp liquids and both open and closed tanks.

Differential Pressure methodsThe difference in pressure between any two levels in a liquid or gas is equal to hg,

where h is the vertical distance between the levels, is the density and g the acceleration due to gravity.

Bubbler method

Based on the principle that the pressure in an open tube in a liquid when fed with gas is limited by the gas escaping as bubbles.

Two tubes are used with the same gas supply connected to both. The open ends of the tubes are at different depths in the liquid and so there is thus a difference in pressure produced between the two tubes.

For a constant height difference between the tubes the pressure difference is related to the liquid density.

Not suitable for closed vessels or liquids containing particles which might block the tubes.

Vibrating tubes

The tube is fixed at each end and tube oscillated by magnetic forces using ac in the drive coil and the coil being located at the mid point. The amplitude is monitored by pickup coil and the tube is maintained at in oscillation at its natural frequency and the frequency depends on the total mass of the tube and its contents. Since the tube has a constant volume the frequency is affected by changes in the density of the fluid inside. It can be used with liquids and liquid-solid mixtures. Gamma Radiation Density GaugeFor a particular gamma radiation source with the radiation passing through a constant thickness of the absorbing material, the fraction transmitted depends on the density.

This is provided the changes in material are not such as to significantly change the mass absorption coefficient.

Typically a density gauge would consist of a radioactive source on one side of containing vessel and a n ion chamber detector on the other sideGamma Radiation Density GaugeThe output from the ion chamber is related to the density of the material in the vessel.

Because the gauge measures the total density in the radiation path, problems can arise if deposits of entrained solids or bubbles occur between the source and detector.Pressure MeasurementLiquid ColumnsU-Tube ManometerIndustrial ManometerInclined tube Manometer

DiaphragmsCapacitance Diaphragm GaugeStrain Gauge Diaphragm GaugeBellows Pressure Gauge

Vibrating Wire Pressure gaugeDiaphragm GaugeBourden Gauge

U-Tube Manometer

Industrial Manometer

Inclined Tube Manometer

Capacitance Diaphragm Gauge

Strain gauge diaphragm gauge

Bellow pressure gauge

Bourden tube instruments

Bourden tube instruments

Vibrating wire pressure gauge, diaphragm gauge

Vibrating wire pressure gauge, Bourdon gauge

Displacement MeasurementDial GaugeJohansson ComparatorInterferrometerDial gauges

Johansson Comparator

interferometer

Venturi tube

Venturi tubeOrifice plate

Orifice plate

Nozzle flow meter

Variable area flow meter

33Variable Orifice