A4.2 - Modern Technologies for Level Measurement

SENSOR+TEST Conferences 2009
2009-05-26 - 2009-05-28
Congress Center Nürnberg
Proceedings SENSOR 2009, Volume I
A4 - Mechanical Sensors II (Level and Flow Sensors)
D. Brumbi - University of Applied Sciences Deggendorf , Germany, I. van Zyl - Siemens Milltronics Process Instruments, Peterborough, Canada
97 - 102


The measurement of level in a tank, bunker or reservoir is one of the most essential measurements in the material and process industry. There are many applications using level measurements:
- Chemical and petrochemical
- Food and beverage
- Water and waste-water
- Pharmaceubcal
- Mining and offshore
- Cement
- Transportation

The physical methods are various and can be divided as in the VDI-VDE recommendation "Level measurement of liquids and solids" as follows:
- Visual - Sight glasses also with parallel vessel, diptape, dipstick.
- Float - Determining the position of a body floabng on the liquid's surface.
- Displacer - Measuring the buoyancy force of a body partly dipped in the liquid.
- Ground pressure - Measuring the hydrostatic pressure at the bottom of the vessel.
- Weighing - Measuring the weight of the vessel and considering the product's density and the horizontal crossection of the vessel
- Measunng by retarding and restricting movements - Level switches by damping mechanical vibration or retarding rotary motions, also plumbing method
- Measunng by electrical resistance - Measuring the resistance of the conductive liquid between two electrodes in the tank, also potentiometnc method or using a resistor chain activated by the filled-in product.
- Capacitive and admittance measuring - Measuring the capacitance of a dielectric product between two electrodes or the admittance d an isolated electrode system surrounded by the filling liquid if it is electrically condudve For details see section 4.
- Measuring by heat dissipation - Method based on the different thermal conductivities of the gas phase above the liquid and of the liquid itself, detected by a heated sensor.
- Radiometric - Method detecting the absorption of gamma rays transmitted through the tank or its backscattening, influenced by the filling liquid that gives a higher absorption than the gas phase The measuring equipment is usually mounted completely outside the tank, therefore it is a real non-contact measurement.
- Sonic and ultrasonic measuring - Measuring the empty space above the product's level by the propagation time of acoustic waves reflected on the surface. Or by transmission of waves from the tank bottom travailing through the liquid.
- Measuring by microwaves - Here electromagnetic waves are propagated against the product's surface, reflected there and the time-of-flight measured.
- Measuring by time domain refractometry - Similar as microwave measurement, but the waves are guided by a transmission line dipped into the product to be measured it has advantages over the "free-space" microwave method on low reflective products or for interface measurements
- Optical measuring - Here also a propagation time measurement with optical waves is possible in clean environments or a triangulation method, but mostly optical methods are used as level switches by absorption, reflection or refraction of light at the sensor in contact with a liquid.

Some of these physical principles have been very well established for a long period, like the mechanical methods such as visual, float, displacer, ground pressure, weighing, radiometric and measuring by retarding and restricting movements They still have their place where special or difficult process situations exist, like easy handling, missing electrical energy, high pressure etc. In the following the "more modem" methods will be handled that are in continuous technical progress, mainly by advances in electronic components.