Floats
Designed for use on liquids, the float is the most basic form of level switch available. There are many different versions that can be mounted in either the top or the side of a tank or vessel. The top mounted variety usually consists of a guide tube with reed switches placed at the desired switch levels and has one or more floats sliding up and down the tube. The floats carry magnets that operate the reed switches. Side mounted are usually simpler, relying on a pivot action as the float rises to activate the output.
Features
low cost, wide range of options for materials and mountings. Many OEM designs.
Applications
Liquids only
Paddle switches
Designed for use on solid materials, a paddle switch is an electro-mechanical device that relies upon product stopping its rotation. During normal operation and with no material present a synchronous motor rotates the paddle at 1 rpm. When the paddle rotation is impeded by the material surrounding it, the motor will stall and cause the micro-switch to change state.
Features
Low cost, simple design, corrosion resistant, adjustable probe lengths.
Applications
Powders, plastics, granular products, grains, cement, sand, fillers, pigments, wood dust, particulate materials.
Conductivity
This low-cost simple liquid level switch uses the conductivity of the product to complete a circuit so it can only be used on conductive media like water. A small voltage passes between the probes when water is present which the controller will detect and operate an output relay. When the water level drops and the probes are exposed the unit will switch back to its normal state. These are typically used in simple water tank applications.
Features
Stainless steel probes, very low cost adjustable time delay.
Applications
Simple water only level switch.
Capacitance
A solid-state level switch using capacitance to detect the level of liquids and solids. The probe consists of a sensing element and an integral grounding sleeve that is separated by an insulator. There is a fixed capacitance between these two elements and this will change when product comes into the vicinity or contacts the sensing probe. The switch will then detect this change and activate the output relay. This method of detection relies on the dielectric properties of the media. The higher the dielectric the easier it is to detect (water has a value of 80+ and air is around 1).
Features
Universal for liquids and solids, low maintenance technology, unaffected by temperature, pressure. Corrosion resistant and a wide range of mounting options. Relatively low cost.
Applications
Liquids, slurries, powders, pastes, granules, pellets and can also be used for interface detection in applications such as oil and water.
Admittance
Suitable for liquids and solids - this principle uses a very similar technology to capacitance to detect the level, but has an additional reference probe. The probe consists of a sensing element and an integral grounding sleeve that is separated by an insulator and an additional reference probe. The probe will change output when product comes into the vicinity or contacts the sensing probe. The advantage of this technology is that it has the ability to work with considerable build up on the probe without false switching as it can distinguish between a coating and product level. This method of detection relies on the dielectric properties of the media. The higher the dielectric, the easier the product is to detect (water has a value of 80+ and air is around 1).
Features
Wide sensing range for liquids and solids, stainless steel construction, no moving parts, high resistance to product build-up.
Applications
Liquids, slurries, powders, pastes, granules, pellets and can also be used for interface detection in applications such as oil, water and sticky products such as bitumen, glue and paints.
Vibrating probes
Versions for both liquids and solids are available but the operating principle is the same for all. A piezo-electric crystal is used to force a blade to oscillate at its fundamental frequency (natural resonance). When the blades come into contact with the process medium the natural frequency of oscillation is damped; the electronics sense the change in frequency that causes the unit to switch. Whilst the operating principle is the same for liquid and solids switching, the probe design for each is fundamentally different.
Features
No moving parts, simple to install, unaffected by pressure, temperature dielectric properties, wide range of mounting options.
Applications
Powders, plastics, granular products, grains, cement, sand, fillers, pigments, wood dust, particulate materials, and liquids.
Microwave level switch
his Level switch normally consists of two parts; a sender and a receiver placed on the outside walls of a tank or vessel, on opposite sides. The sender emits a burst of microwave energy towards the receiver. This burst of energy is transmitted approximately 200 times each second. If a sufficiently reflective or absorbent material interrupts the line of sight between these two units, the energy will not reach the receiver. Any loss in signal at the receiver will trigger a change of state and operate a relay to indicate high or low condition. The sensitivity is adjustable to cater for a wide range of materials.
Features
Long ranges, non-invasive, wide temperature range.
Products
Granules, flakes, stones, sands, cement, fillers, liquids, pastes.
Radar
Two types of non-contact radar are possible, Pulse and FMCW. Hycontrol's radars' use Frequency Modulated Continuous Wave. FMCW radar uses a high frequency signal (~10GHz) which increases linearly, by 1GHz during the measurement cycle. The signal emitted is reflected from the target surface and received at a time-delayed frequency. The difference between the transmit frequency and the received frequency is directly proportional to the distance measured, i.e. a large frequency difference corresponds to a large distance and vice versa. This frequency difference is transformed via a Fast Fourier Transformation (FFT) into a lower frequency spectrum and the distance is then calculated from this spectrum.
Features
High accuracy options, long range unaffected by temperature, pressure, vacuum, foam, non contact suitable for aggressive media.
Applications
Liquids and solids, pellets, granules, acids.
TDR (Time domain Reflectometry)
Often referred to as Radar on a Rope or contact radar, TDR uses pulses of low power microwaves sent along a conductor (cable or rod) immersed in the product. Where the waves meet the product surface they are reflected back. The intensity of the reflection depends upon the dielectric constant of the product. The higher the dielectric constant the stronger the reflection will be, eg 80% reflection for water. The instrument measures the time between transmission and reception that is proportional to the distance.
Features
Suitable for solids and liquids, immune to coating, long measuring range, measures interface, unaffected by dust, temperature, pressure, steam, foam, vacuum. High accuracy.
Applications
Powders, granules, flakes, pellets, liquids, slurries, paints, hot bitumen.
Ultrasonic
Non-contact ultrasonic level measurement is one of the most versatile level measuring technologies and is suitable for both liquids and solids measurement. It can be used for continuous or point level detection. The principle of operation is often referred to as 'time of flight'. An ultrasonic pulse is emitted from a transducer down towards the media and is reflected back from the surface. The time it takes to travel from and to the transducer is directly proportional to the distance traveled.
Features
Liquids and solids, low cost, wide range of applications, simple to install and non- contact, long measuring ranges up to 60m.
Applications
Powders, pellets, granules, slurries, pastes and liquids.
Thruwall
This range of instruments can be used for continuous or point level detection of liquids. A transducer attached to the outside wall of the tank bottom transmits a pulse of high frequency sound through the tank wall and the fluid until it reaches the fluid's surface where it is reflected back. The time taken for the pulse to travel from and to the transducer is proportional to the depth of the fluid. Simple level switching can be achieved by mounting the instrument onto the side of the tank wall.
Feature
Non-intrusive, unaffected by pressure or surface foam, simple retrofit mounting.
Applications
Liquids, fuel, foods, acids, spirits, chemicals, pharmaceutical.