How do level transmitters work?
Level Transmitters measure the level of a variety of materials, including water, other liquids, bulk solids, and powders.The Level Transmitter provides a continuous electrical output interpreted by a measurement or control system connected to it. Applications for Level Transmitters include material handling, chemical process plants, water treatment plants, and the food and beverage industry.
In an industrial setting, it is often critical to know the amount of material you have in a tank to ensure your industrial process's safety and cost-effectiveness. Because you:
- Don't want to run out of material
- Do you want to avoid a tank overfill incident (hazardous spill)
- Need to know the rate of material consumption to optimize your process.
There are three main ways that Level Transmitters measure level:
- Weight of a material
- Pressure Head of a fluid
- Position of material within a container
Classifications
Level Transmitters fall within two main classifications—direct and indirect measurement.- Direct measurement is when the actual level of material is measured; whereas
- Indirect measurement provides the measurement by way of
- Tracking a magnetic structure floating on the liquid, or
- A calculation, e.g., the pressure head, indicates fluid height when the fluid and tank geometry properties are known.
Methods
A Level Transmitter will use the contact or non-contact measurement method:- Contact measurement is when the Level Transmitter comes in direct contact with the medium to be measured.
- Noncontact measurement is when the Level Transmitter is used to bounce a signal off the surface of the material or track the position of a magnetic float sitting on the surface of a liquid.
Features Tour of Level Transmitters
Liquid Level Transmitters
The liquid Level Transmitter communicates the pressure head of a liquid. This pressure head is used to calculate the liquid's height based on the density of the liquid and the tank's geometry. Typical applications include storage tanks for water and other liquids, and transportation tanks. Advantages include: easy to set up, available in materials robust against most environments.Magnetic Level Transmitters
As the title indicates, this type of Level Transmitter tracks the position of a magnet. The magnet is embedded in a float that tracks the surface of the liquid. The float is traditionally constrained from lateral movements in a standpipe connected to the main tank. The output of the magnetic Level Transmitter changes with the magnet's position, which is changing with the liquid level. Note: the standpipe and float must be constructed of a non-magnetic (magnetically transparent) material for proper operation. Advantages include: easy to install or retrofit, and simple electrical connections.Capacitance Level Transmitters
Capacitance Level Transmitters have two or more electrodes. These electrodes (conductive plates) are immersed in a dielectric liquid in the tank. The liquid's dialectic constant must be known for this technique—the capacitance changes as a function of the liquid level. Therefore the liquid level can be calculated by transmitting the capacitance. Advantages include accuracy independent of fluid movement, simplicity (no moving parts), and low maintenance.Hydrostatic Level Transmitters
This Level Transmitter type uses the pressure head produced by the liquid to create a variable electrical output. The pressure level can then be calculated by knowing the liquid's density and the geometry of the tank. Advantages include: fits into pipes, robust, reliable, and simple to install.Radar Level Transmitters
In this application, a radar beam is typically shot from above to the surface of a liquid. The time it takes for the radar pulse to hit the liquid and return indicates the liquid level. Advantages include: easy to install and service, accurate readings, and minimal contact with the measured material.Ultrasonic Level Transmitters
For an ultrasonic Level Transmitter, an ultrasonic pulse is shot from above to the liquid's surface. The time interval for the ultrasonic pulse to hit the liquid and return tells you the liquid level. Note: gases above the liquid may impact the time interval of the ultrasonic pulse. Advantages include minimal contact with the material being measured,Microwave Level Transmitters
The microwave Level Transmitter sends a microwave pulse through a sensor cable or rod to the liquid's surface. The time is taken to go out and come back directly correlates to the liquid level. Advantages include minimal contact with the material being measured,FAQs of Level Transmitters
WHAT ARE LEVEL TRANSMITTER LIMITATIONS?
- Some sensors like ultrasonic level sensors require fixed atmospheric conditions. When the atmospheric conditions are known, then the Level Transmitter can be calibrated appropriately. If the atmospheric conditions are rapidly changing, then a different type of sensor may be appropriate.- For sensors that come in contact with the liquid, it is appropriate to evaluate the liquid for the potential to have build-up or otherwise damage the sensor.
- For the types of Level Transmitters that bounce a signal off of the liquid surface, the transmitter must have a clear "line of sight" to the liquid. Anything that deflects or absorbs the signal may cause reading errors—including foam, vapors, dust, or powders. In many cases, you can:
- Define a maximum and minimum detection distance 'window' to filter out any out-of-range readings, and
- Average readings, e.g., take a running average of the last ten readings to account for the liquid's sloshing, etc.
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