Level sensors

What Are Level Sensors? How to Choose One and End Tank Overflow and Pump Failure

What are level sensors? Tank overflow, or a pump running dry until the seal burns out, has a direct effect on the production line. This article looks in depth at how level sensors solve that problem. We explain how a level sensor works, along with techniques for matching the specification of a replacement transmitter or level gauge to the site. If you need to replace a level sensor, send us the details and SCMA will assess and match the model online straight away.

What Are Level Sensors?

Level sensors are electronic devices that detect the level of liquid, powder or plastic pellets inside a tank or silo. They assess the quantity of raw material in real time, and the internal circuit converts the physical value into an electrical signal sent to the PLC board. Understanding that level sensors are the brain of raw material stock management makes the work far easier.

Accurate level monitoring with a level sensor lets engineers keep the production process running continuously. Scrap caused by mixing raw materials incorrectly is detected before the main machine is damaged. Whether you use a level switch or level transmitters, correct signal transmission is the key.

The Difference Between a Level Switch and Level Transmitters

Many engineers are confused when ordering a replacement level sensor. Distinguishing between the measuring formats of a level switch and a transmitter lets the purchasing department order exactly the right specification.

Level Switch

Detects the level at a point (point level), reporting only whether the liquid has reached the set point. The status it outputs is simply on or off. A level switch is widely used to cut the water pump to prevent tank overflow, or to stop the pump running dry.

Level Transmitters

Detect the level continuously. Level transmitters report the liquid level at all times, from the bottom of the tank to the top, sending a standard 4-20 mA or 0-10 V output back to the central control system so that SCADA can plot a level sensor graph in real time.

How Many Types of Level Sensor Are Used in Plants?

Choosing a level sensor must be based on the nature of the chemical on site. Choosing the wrong type will make the PLC read the level sensor value severely differently from reality.

Magnetic Float

Relies on a float rising and falling with the water level, with a reed switch concealed inside the stem. It suits clean water with no sticky sediment. It works like a level switch based on a basic physical mechanism.

Radar Transmitter

Fires a high-frequency wave at the liquid surface which reflects back. It withstands chemical vapour and thick foam on the surface extremely well and needs no direct contact with the liquid. This is the most accurate level transmitter technology available today.

Tuning Fork

Uses the principle of resonant frequency: when liquid or powder covers the fork, the frequency changes and the system switches immediately. It suits plastic pellet silos and acts as a level switch that stands up very well to dust.

Level Gauges

The traditional sight glass fitted to the side of the tank, letting the maintenance technician see the level with the naked eye. Newer versions fit a level sensor over the level gauge so it can send an electrical signal too, combining the old mechanism with a modern transmitter.

Classic On-Site Problems When the Level Sensor Specification Is Wrong

Letting a level sensor work in an unsuitable environment shortens its life. Check these conditions at the machine to keep your level sensor going as long as possible.

• Thick foam on the surface: Tanks mixing shampoo or dishwashing liquid produce a great deal of foam. With an ultrasonic level sensor the waves are absorbed by the foam and an error value is returned; the technician must switch to radar level transmitters instead
• Sticky scale: Glue or syrup with a high viscosity clings to the probe, making a tuning fork level switch latch permanently on. The probe must be removed and cleaned regularly on the PM cycle
• Severe surface turbulence: Tanks with an agitator make the liquid surge heavily, so the output of the transmitter swings. The maintenance team must set a delay time in the controller to solve this

How to Choose a Transmitter and End Downtime

The engineering team must assess the site environment carefully to find replacement level transmitters or a level sensor more durable than the last.

• Temperature and pressure: Check whether the heat in the tank exceeds 100°C and how many bar of pressure build up. The level sensor specification must cover these conditions without the seal failing
• Wetted parts: A sulphuric acid tank requires a PTFE or PVDF plastic probe to resist corrosion. With ordinary stainless steel the level sensor will be eaten through within a few weeks
• Output signal: Check whether the control cabinet's receiving board supports IO-Link or the 4-20 mA current of the transmitter. Communicating over IO-Link allows far deeper data to be pulled into a proactive maintenance system

Summary

Level sensors are a vital line of defence protecting the pump, tank and silo systems in a plant. Understanding the differences between the measuring technologies — transmitter, level switch or level gauge — lets the purchasing team choose exactly the right level sensor specification. Using a standard brand of level sensor such as ifm decisively reduces unplanned downtime, because we know well what level sensors are: the heart of fluid control. If your maintenance department has a headache with level transmitters failing often, send a photo of the site to SCMA for assessment. We offer an online engineering consultation service and will match the specification quickly, with no waiting for a technician to visit.

Frequently Asked Questions

Can a tuning fork level switch measure every kind of liquid?

It works with almost every liquid. The only caution is a liquid so thick and sticky that it clings to the fork and will not fall away. In that condition the level switch circuit latches, so the pump will not switch off. This point must be watched carefully when choosing a level sensor.

What is the maximum distance a 4-20 mA signal can travel?

The 4-20 mA current from level transmitters is designed to resist electrical interference, so a level sensor cable can be run from an oil tank to the control cabinet over hundreds of metres without the value dropping. It suits large plants covering a wide area.

The original part is discontinued and unobtainable — what should I do?

Tell the SCMA engineers the tank depth, the type of liquid and the transmitter output signal type, and we will help. We can find an equivalent European-brand level sensor specification and advise on the wiring online straight away — no need to worry even if you are still using an old level gauge.

Looking for “Level Sensors”? Think of SCMA!

Choosing durable level transmitters or a level sensor is how you put an end to production line stoppages. SCMA is ready to serve and supply the level sensor solution that meets your plant's needs, with a deep understanding of what level sensors are. Although we work as a fully online service, we make up for it with professional speed in matching an emergency transmitter or level switch specification. Just send us the nameplate details of your existing level sensor, or a photo of the level gauge on site, and our engineering team will assess the specification and propose an engineering-sound solution immediately!