Radar level transmitters work based on the time of flight (TOF) measuring principle or time domain reflectometry (TDR). To start with, we can measure the distance from the reference point to the surface of a liquid. Then the meter sends a high-frequency signal from an antenna or along a probe.
When the product surface reflects the pulse, the meter receives the reflection. Then the device calculates how long it took the pulse to return and translates that time delay into a level measurement.
Before we apply a radar meter, we need to know the dielectric constant (DC) of a product, as that has a direct impact on the quality of the reflections. In fact, products with high DC values will reflect strong, clear pulses. On the other hand, a product with a low DC value will absorb more of the pulse, reflecting less and reducing accurate readings.
Radar level measurement is a safe solution even under extreme process conditions (pressure, temperature) and vapours. Radar level transmitters can also be used in hygienic applications for non-contact level measurement. Radar level transmitters versions are available for different industries like for water/wastewater, the food industry, life sciences or the process industry. Various antenna versions for every kind of radar applications are available.
Basic radar level transmitter setup
The basic radar level transmitter setup isn’t too hard. Regardless, nowadays they come with a “setup wizard,” which makes them even easier. Usually, the setup wizard will walk us through the setup. For example, it’ll often start by asking which product we want to measure. Then it’ll go on to ask for the dielectric of the product, then the type of tank, and so on.
A radar level detector basically includes:
- A transmitter with an inbuilt solid-state oscillator
- A radar antenna
- A receiver along with a signal processor and an operator interface
The operation of all radar level detectors involves sending microwave beams emitted by a sensor to the surface of the liquid in a tank. The electromagnetic waves after hitting the surface of the fluid returns back to the sensor which is mounted at the top of the tank or vessel. The time taken by the signal to return back i.e. time of flight (TOF) is then determined to measure the level of fluid in the tank.
Yes, devices that use radar can have significant problems with radar buildup. That’s because when the buildup increases, the signal strength will drop, giving bad measurements.
Thus, proper cleaning of the antenna will fix this problem and get us back to reliable measurements. However, depending on the device, we can also control the clean up with the device itself or our programmable logic controller.
If a device doesn’t have the automated option, then we’ll have to clean it manually. Therefore, we may want to consider upgrading the radar device. Today’s transmitters can measure even with buildup and perform their own cleanings when necessary.
Also, some devices on the market have algorithms to reduce the interference caused by buildup. That means we can maintain the device’s accuracy, even with high levels of contamination, without the need for cleaning. The ability of the transmitter to control the cleanup process using compressed air will also save us money and reduce unplanned downtimes.
Types of radar level transmitters
We have two kinds of radar level transmitters:
- Noninvasive or Non-contact Systems
- Invasive or Contact System
Noninvasive radar level measurement
Radar level measurement is based on the principle of measuring the time required for the microwave pulse and its reflected echo to make a complete return trip between the non-contacting transducer and the sensed material level. Then, the transceiver converts this signal electrically into distance/level and presents it as an analogue and/or digital signal. The transducer’s output can be selected by the user to be directly or inversely proportional to the span.
Pulse radar has been used widely for distance measurement since the very beginnings of radar technology. The basic form of pulse radar is a pure time of flight measurement. Short pulses, typically of a millisecond or nanosecond duration, are transmitted and the transit time to and from the target is measured.
Everything inside the tank that conducts energy, such as level switches or heater systems, can reflect the signal. If the product has a low dielectric level, then the radar may find a false level. We may also wind up with bad readings from vapour, foam, or other product conditions.
We can find many solutions to avoid this issue – high-frequency radar level transmitters, echo analysis, stilling wells, and more. This type of radar level measurement can be very accurate.
Invasive or contact radar level measurement
The invasive method used for liquid level measurement is called Guided-wave radar i.e. GWR method. In this method, a cable or rod is employed which act as a wave guide and directs the microwave from the sensor to the surface of the material in the tank and then straight to its bottom. “The basis for GWR is time-domain reflectometry (TDR), which has been used for years to locate breaks in long lengths of cable that are underground or in building walls.
A TDR generator develops more than 200,000 pulses of electromagnetic energy that travel down the wave guide and back.”
The dielectric constant of the process material will cause variation in impedance and reflects the wave back to the radar. Time taken by the pulses to go down and reflect back is determined to measure the level of the fluid.
In this method, the degradation of the signal in use is very less since the wave guide offers an extremely efficient course for signal travel. Hence, level measurement in case of materials having very low dielectric constant can be done effectively. Also in this invasive measurement method, pulses are directed via a guide; hence factors like surface turbulence, foams, vapours or tank obstructions do not influence the measurement.
The GWR method is capable of working with different specific gravity and material coatings. However, there is always a danger that the probe or rod used as a waveguide may get impaired by the agitator blade in the fluid under measurement. A typical guided wave radar system is shown in the figure below.
If you need help choosing the right radar level sensor for your application, take a look at our new level measurement smart assistant.
To know more about radar level measurement, you can get in touch with our engineers and we will be happy to help.