Level switches are sensor devices that are specifically used for detecting the powder or liquid levels. These level measurements are mostly indicated with the help of an electrical switching action. Level switches are mostly point level switches that measure whether the level of container has gone over or reached the predetermined point.

The multiple point switches can be incorporated together to offer a stepped version of the constant level. These level switches can either be used as alarm devices or as control switches for turning something off or on like a pump. They can also be used for sending signals to valve actuators.

One of the special characteristics of level switches is that their output can either be solid state or electro-mechanical, either normally closed or normally open. The typical options for throws and poles are also available. Most level switches have a one or two throws or poles. However, some manufacturers do offer customised level switches for special applications.

Technologies used for measuring level include:

• Air bubbler technology
• RF or capacitive admittance
• Electrical resistivity or conductivity
• Magnetic or mechanical floats
• Differential pressure
• Radio frequency

Some important specifications of level switches

The measuring range is an important specification to consider while looking for level switches. At the same time, it is also necessary to consider the ratings for voltage and current.

Level switches can be mounted in different ways, depending on the specific needs of your application. They are mostly mounted on the bottom, top or on the side of the container that holds the substance to be measured.

The RTD temperature transmitters are used for converting the RTD resistance measurement into current signals. This eliminates problems in RTD signal transmissions that are naturally caused due to lead resistance. Errors in RTD circuits (particularly 2 and 3 wire RTDs) are mostly due to the added leadwire resistance between the instrument and the sensor.

If you are looking for RTD temperature transmitters, the sensor connections, environment, user interfaces, transmitter input, features and specifications are some important parameters you should consider. The reference resistance, reference materials, sensed temperature and other inputs are some transmitter input specifications you must consider when choosing RTD temperature transmitters.

Options for reference materials include nickel alloys or nickel, copper and platinum. For the measurement integrity of RTDs, platinum is the most commonly used metal. While nickel alloys and nickel are affordable and commonly used metals, they are not as precise as platinum. On the other hand, copper is rarely used as RTD element. This is mainly because the low resistivity of the element is quite longer compared to the platinum element.

Other considerations while looking for RTD temperature transmitters

The temperature range that is to be sensed and transmitted is an important consideration. Input, output and mounting of RTD temperature transmitters are other specifications you need to consider. Common mounting styles include thermowell or thermohead mounting, cabinet or board mounting and DIN rail mounting.

Inputs include analogue current, analogue voltage and resistance input. Similarly, common outputs include analogue voltage, switch or relay output and analogue current. User interface options include digital front panel, computer interface and analogue front panel.

Level controllers are devices that control, regulate and monitor solid and liquid levels in a process. There are three different types of control functions that can be used by level controllers – limit control, non-linear or advanced control and linear level controllers.

How different level / process controllers work

Limit controllers - The limit controllers operate by interrupting the power using a load circuit when the level falls below or exceeds the defined set limit point. A limit controller can protect both people and equipment when installed correctly with its own power lines, power supply sensor and switch.

Non-linear or advanced controllers – These types of controllers includes a dead-time compensation, adaptive gain, lead/lag, fuzzy logic and neural networks. These level controllers can also be used for powder, liquid or any other dry material application.

Linear level controllers – They are available in many different types and styles. The feedforward control offers compensation or direct control from the reference signal. It can either be in conjunction with the PID (proportional, integral and derivative) control or an open loop. The PID control is an intelligent program instruction or I/O module that offers automatic closed-loop operation to the process control loops.

There are proportional plus integral (PI) controls that come with an integrated error signal. They are mostly used to get rid of offset or steady state errors. It can also be known as automatic bias/offset/reset control.

As level controllers differ in terms of user interface, features and specifications, you must be careful in selecting the one that suits your application needs.

A sensor can be termed as a device that measures a physical quantity and converts it into a signal that is easy to read for an observer or computer.

There are a number of distance sensors available on the market. It is important to identify the right type. Mentioned below are some questions you need to ask before opting for a distance sensor.

• How much space can be allocated to these devices?
• What is the resolution that you require?
• What is your target material?
• What should be the range to your target machine?

Based on the above questions, you can choose a distance sensor that is best suited to the needs and requirements of your business.

Inductive sensors

The measuring principle of these sensors is based on the evaluation of inductive eddy currents. Therefore inductive sensors work best on electrically conductive metal targets such as steel, metallic alloys and aluminium. These sensors are known to be resistant against non-metallic environmental contaminants that include dust, oil and cutting fluids.

Ultrasonic distance sensors

Ultrasonic distance sensors offer distance measurement with the help of ultrasonic transmission. These sensors are capable of offering resolutions up to >0.3 mm based on the speed of sound through air. Ultrasonic distance sensors also measure the time required for the sound to return to the sensor.

Optical distance sensors

Laser interferometers, diffuse sensors and fibre optics are some of the different ways of optically sensing distance. Interferometers offer nanometre resolutions and long sensing ranges, but are expensive and require complex peripherals and optically efficient targets.

The aforementioned distance sensors will help you determine the type that is best suited to your needs and requirements.

You must have always wondered as to how scientists and researchers identify and measure the depth of an ocean without actually getting into it. The answer to that question is distance sensors. Distance sensors are non-contact devices that use different technologies including infrared, laser and ultrasonic. Over the last few years, most distance sensors have been using ultrasonic technology to determine the distances between these devices and their targets.

The functioning of distance sensors

It is easy to understand the functioning of an ultrasonic distance sensor. These devices send out ultrasonic pulses and then listen for returning echoes, just like bats. Distance measurements are obtained from the time of transmission till the echo returns. Most distance sensors can also pick up targets that are small or even those which are very absorptive to sounds. Distance sensors are also capable of measuring multiple targets.

Independent measurements may be necessary in some instances regarding distance sensors. This becomes necessary as the speed of sound in air differs from the speed in other mediums. For this purpose, a simple calculation is used along with the initial reading and this helps to get a precise distance reading.

Like any other sensor, there are different kinds of distance sensors available for various applications. Distance sensors are commonly used to measure water levels and snow depths. Due to their use in different applications, distance sensors have to be designed to meet the most stringent requirements.

It is important to research the market carefully and opt for a distance sensor that is best suited to the needs, requirements and budget of your company. Seek professional help if you are unsure about the type that is ideal for your machinery.

Distance sensors in the past consisted of noncontact displacement sensors that gave a discreet signal as when to turn a particular machine on or off. With the changing times, these sensors had to perform more than just giving out basic signals.

With the advent of highly developed distance sensors, measurement of distance and displacement with the help of distance proportional analogue output has become possible. These sensors can be used in a variety of applications. These include absolute distance measurement, thickness calculation, deformation control, profile logging, eccentricity measurement and linear distance measurement.

Designs available

If you are looking for distance sensors for your business, it is necessary to opt for the right design that can fit your budget and requirement. Distance sensors are available in a variety of designs and operating principles. The self contained laser sensors weigh approximately 37g and measure 12 × 37 × 34 mm. The inductive devices are 12mm tubular and 18mm flat packed. The tubular ultrasonic sensors are 20mm square and weigh 25 – 30g.

Basic functionalities

Analogue output sensors and linear sensors are commonly known as distance sensors. These devices allow the operator to determine more than the simple presence or absence while offering simplicity and ease of use. Based on inductive, ultrasonic, or optical technologies, the sensors provide an analog output signal proportional to the distance between the sensor and the object of interest.

It is important to identify the needs and requirements of your business before opting for a distance sensor. These devices are responsible for monitoring the load on your machineries and ease the pressure of an operator at the same time. Seek expert help if you are unsure about the type of distance sensor best suited to your business.

The eddy current linear position sensors utilise a reference coil for producing a magnetic field in a target. This reference coil is also used for monitoring the field in the neighbouring sensing coil to find out the distance from the target. As the target starts getting closer to the sensor, the magnetic field gets disrupted and this disruption is sensed by the other coil.

Eddy current linear position sensors also determine the variations in the magnetic field due to the presence of close conductive objects. Field detection and generation information is offered in the range of kHz to MHz. As these sensors operate by the interruption of the magnetic field, it is possible to have ferrous or nonferrous, magnetic or non-magnetic targets.

How to select the appropriate eddy current linear position sensors

The range that eddy current linear position sensors can measure and its sensing frequency are two of the most important specifications to consider. The sensing frequency is actually the frequency range on which the device operates. Static resolution of the eddy current linear position sensors is also an important consideration.

The smallest rise of the quantity that can be recognised is known as the resolution. The static resolution tells you about the change in quantity that is required for the sensor to determine the change. In order to determine the small changes in position, you will need a sensor with a very small resolution.

Electrical output offered by the eddy current linear position sensors can be computer signals (serial or parallel), digital, analogue voltage, frequency or current. By considering these things, you can easily get the right eddy current linear position sensor to suit your requirements.

Sensor transmitters are signal conditioning or measurement packages that offer a standard and calibrated output from transducers and sensors. The output types of sensor transmitters include variable voltage levels, pulse or frequency signals, current loops, counters or timers, variable resistance outputs and relays.

The sensor transmitters can also offer transistor-transistor logic (TTL) and radio frequency (RF) signals output. Current loop like 4 – 20 mA are ideal for sensing signals over long distances. The current imposed on the output circuit is usually proportional to its measurement. Feedback is also used for offering an appropriate current, irrespective of the impedance and its line noise.

Modulated frequency or frequency output of sensor transmitters include;

• Pulse trains
• Sine waves
• Frequency modulation (FM)
• Amplitude modulation (AM)

Types of input received by sensor transmitters

The sensor transmitters can receive three types of input; sensor inputs, signal inputs and from special devices. The signal inputs of sensor transmitters include AC current, DC current, AC voltage and DC voltage. They also include charge and frequency signals that require amplification and filtering.

Sensor transmitters can also receive inputs from thermocouples, resistance temperature detectors (RTD), accelerometers and thermistors. Strain gauges are another means through which sensor transmitters can receive input. Most strain gauges utilise a Wheatstone bridge from both rotary and linear variable differential transformers (RVDT and LVDT).

Signal inputs from special devices include encoders and signal per cycle counts from tachometers and counters. Switches, relays, clocks, timers can also provide special inputs to these sensor transmitters.

Level switches are often used for detecting interfaces between liquids, or to determine powder and liquid levels. These level measurements are often indicated through an electrical switching action. The point level switch measures the level of a container that has gone over or reached a pre-determined point.

The switching mechanism is vital for continuous level switches. The multiple point switches can also be incorporated together to offer a stepped version at a constant level. The level switches can be used as control switches or as alarm devices to turn something off or on, like sending signals to a valve actuator or pump.

Technologies used for measuring include;

• Air bubble technology
• RF or capacitive admittance
• Electrical resistivity or conductivity
• Differential pressure
• Pressure membrane
• Optical units
• Magnetic or mechanical floats
• Radio frequency
• Sonic or ultrasonic
• Tuning fork or vibration

Types of level switches

Level switches have a switched output that can either be solid state, electromechanical state, normally closed or normally open. Options for throws and poles are also available. Most level switches either have one or two throws or poles. Some manufacturers even produce custom level switches for specialised applications.

When purchasing a level switch, you need to consider the measuring range that it offers. The ratings for voltage or current of the level switch are also an important consideration. Depending on the application requirements, level switches can be mounted in many different ways. Level switches can be mounted on the side, top or the bottom of the container that holds the substance which is to be measured.

Liquid level sensors are mostly used for detecting interfaces or liquid levels between liquids like water and oil or solid and liquids. These sensors can be defined as a transducer, sensor, or an integrated system with control and instrumentation capabilities.

Depending on the application, you can choose between point level and continuous sensing. The point level sensors usually turn off the system or trigger an alarm depending on a specific limit. There are some applications that depend upon continuous liquid level monitoring. It can use different methods including vibration, ultrasound and pressure for continuous liquid level monitoring.

Different types of liquid level sensors

The differential and pressure membrane liquid level sensors determine the pressure or changes in pressure in a vessel like a storage or holding tank. There are sonic / ultrasonic liquid level sensors that determine the length of time taken for a reflected sound wave to return back to the transducer.

Tuning / vibrating fork liquid level sensors utilise a piezoelectric crystal or other type of technology for vibrating a probe. They also continuously monitor the absence, presence, decrease or increase of that vibration.

In some applications, it is essential to determine the liquid level without having any direct contact with the media. Applications that include highly corrosive liquid use non-contact meter technologies like sonic / ultrasonic liquid level sensors or/and microwave / radar liquid level sensors may be the most favourable options.

When it comes to choosing ultrasonic liquid level sensors, you need to consider the temperature, surface properties and media types.