A level playing field?

Latest technologies for level measurement compliment older techniques by Peter West

Reflex-radar non-contact technology is used with Krohne’s latest BM100 level probe. It has a long pair of rigid rods which guide 1MHz pulses down into tanks or silos to simultaneously measure levels and interfaces on all liquids and granular products

In the pursuit of level measurement accuracy, a number of technologies are now available – with some new techniques, like Time Domain Reflection (TDR), among the latest to emerge.

Applications for level measurement range from liquid sulphur at 140oC to the storage of liquified gases at low temperatures; from tanks on ocean-going ships to nuclear power stations; and from grain silos to cement storage vessels.

Liquid level indicators are particularly suitable for difficult operating conditions, such as extreme temperatures and pressures or with toxic fluids.

Employing robust and well-proven technologies, these displacement (Archimedes principle), and reference chamber float types have easy-to-read local displays, and can also have limit switches installed to allow initiation of signalling or switching functions at specific levels. Electrical and pneumatic signal outputs are also an option to allow long distance data transmission.

Liquid level switches are mainly employed when continuous level measurement is unnecessary, but where signals have to be generated when specific levels are reached.

Float operated level switches can be installed at specific levels (pivoted float type), to generate an electric or pneumatic signal output when that level is reached. Since the float position is transmitted to the switching device by a magnetic follower system, pressure-tight separation is maintained between the tank interior and the switch housing at all times.

Alternatively, a range of switching levels can be provided by a single (guided float) design, where the movement of the float triggers reed switches located inside the central guiding tube, giving an electrical output signal.

Level switches

Float switching technology has its limitations: by definition it is affected by specific densities. So in applications where specific level switching has to be unaffected by the fluids or bulk materials in the vessel, capacitive level switching technology is an effective alternative – although it can only be used under moderate conditions.

The resonating probe is a more recent technology, developed for the automatic monitoring of max/min levels in a wide range of operating conditions. It’s particularly good for applications with high viscosity fluids or bulk materials – such as in the food industry.

These resonating probes consist of a number of diaphragm-couple probes which are magnetically set into resonant vibration by an oscillator built into the probe housing. As the probe comes into contact with the liquid, its resonant frequency drops, generating a binary output signal which initiates an overfill alarm.

Basic guided float technology, where a float on a non-magnetic tube follows the level of the liquid surface, also covers a wide range of general applications.

The reed switch guided float design not only operates as a level switch, it can also be fitted with an electrical remote data transmission system to provide a continuous level output signal.

A visual display version of the guided float design uses a follower magnet inside the central tube, rather than reed switches. The follower magnet transmitting the changes in level via a flexible wire cable to a measuring drum with a circular scale display.

These are frequently specified on large process vessels, since they can accommodate a measuring depth of up to 18m.

More recent (non-contact) technologies have introduced new standards of accuracy to the measurement of liquid levels (liquids, pastes, slurries and particulates). Typically, Krohne’s BM70 technology is suitable for use in metal process and concrete bins under extreme and exceptionally difficult conditions. The device generates 1GHz microwaves which are reflected from the liquid surface back to the antenna.

The transit time of the microwave signal is evaluated, and translated into a liquid level height which can be accurate to +/-5mm.

Other devices can use ultrasonic pulses which are reflected from the liquid surface. In this case, however, although the time between the propagation and reception of the pulse is directly proportional to the distance between the probe and the liquid surface, it is also influenced by the temperature and density of any gas, and the device is not suitable for pressurised vessels.

Krohne’s latest development is the non-contact Reflex-Radar BM100, as shown above.

* The Author is with Krohne.