Radar level measurement

Radar Level

Radar level transmitter are used to measure level in industrial applications. Radar level measurement method is an advanced method.

In radar level transmitter time of flight method is used to measure the level. By calculating this time of flight we measure the level of liquid or solids.

RADAR level have a transmitter which transmit microwave or radar waves after colliding with material they return back to the receiver.

The time from the transmitter to the echo return to the receiver, is calculated, which is proportional to the level.

This time is calculated by the formula:

Distance = (Speed of light x time delay) / 2

This calculation is done by a built in microprocessor and the level is displayed on screen or lcd, a signal proportional to level in 4 to 20 mA is output of level transmitter , this standard current signal of 4 to 20 mA is for plc or dcs.

Time of Flight technology

Time of flight technology is used in devices that are much newer are advance than hydrostatic devices.

Time of flight technology is used in ultrasonic and radar devices (non-contact and guided wave). Here we will discuss Radar levels.

Radar is an abbreviation for Radio Detection and Ranging.

Radar devices are used for level measurement, this type of level transmitter operate with electromagnetic radiation having a short wavelength of range 1.5 to 26 G Hz (gigahertz). This short wave length electromagnetic radiation are called microwaves.

Types of radar level transmitter

Two main types of radar level transmitter are used.

1. Non contact level transmitter

2. Contact type level transmitter

Non-contact radar and contact type (guide wave radar) work on the same principle of microwaves.

Non contact type level transmitter

In Non-contacting pulse radar level transmitter, no physical contact of level transmitter and material happens. Non contact pulse radar sends out a microwave signal that echos back to the product surface and returns to the receiver antenna.

For non-contacting radar level measurment two main modulation techniques are used.

1. pulse radar

2. FMCW (Frequency Modulated Continuous Wave)

Pulse wave radar level

In pulse radar level measurement method microwave pulses of short duration are tramitted and echos are received at receiver so by calculating the time of flight level is measured.

The transmitter and receiver are manufactured as a single unit or seprate unit, one for transmitter and another unit as receiver.

Pulse radar level transmitter are being used widely for distance measurement since the very beginnings of radar technology invention.

The basic form of pulse radar is a pure time of flight measurement. Short time pulses of millisecond or nanosecond duration, are transmitted and echo is received back and the transit time to and from target (or sensed material) is measured.

The pulses of a pulse radar are not discrete monopulses with a single peak of electromagnetic energy, but are in fact a short wave pulses packets .

The number of waves and the length of the pulse depend upon the pulse duration and the carrier frequency used.These tranmitted packets have a relitively long time delayed packet to packet transmission. So after receiving back echo of one packet, then second pulse packet is transmitted.

Speed of light is approximately 300,000 kilometres per second. So time taken for a radar signal to travel one metre and back takes approximately 6.7 nanoseconds.

This transit time is very short so to measure such a short time, a special time transformation procedure is required to enable theses short time periods to be measured accurately. Pulse radar has a regular and periodically repeating signal with a high pulse repetition frequency (PRF).

A method of sequential sampling is used, the extremely fast and regular transit times can be readily transformed into an expanded time signal.

a small part of the pulse radar transmission pulse is used as a reference pulse.This provides automatic temperature compensation.

The echoes derived from pulse radar are discrete and separated in time. So this easily Handle false echos and accuracy is better .

FMCW (Frequency Modulated Continuous Wave)

In most of the radar level measurement FMCW ( frequency modulated continous wave) method is applied.

The FMCW radar transmits microwaves towards the product surface, but the frequency of transmitted signal is continuously varying. When the signal has travelled down to the liquid or solid surface in the tank and echos back to the receiver antenna, it is compared with the signal that is being transmitted at that time.

The difference in frequency between the received and transmitted signal is directly proportional to the distance to the liquid or solid. The level or distance is measured with high precision.

Advantages of non contact radar level

As level transmitter has no physucal contact with material so there is bo susceptibility of corrosion.

Non contact level transmitter is choice for level measurement of viscous, sticky, and abrasive fluids.

Non-contacting radar provides a top-down installation option, it directly measures the distance from sensor to the surface of material being sensed.

Non-contacting radar can frequently be used in vessels with agitators.

High frequency devices can be completely isolated from the process with PTFE seals and can be used with valves.

Non contact level transmitter are used for a wide range of level measurement upto 150 meters.

No effect of material density change in case of bob contact level transmitter.

No effect of pressure change in vessel.

No moving parts so maintenance is minimum.

Limitation of non contact radar level

A proper frequency of microwave is necessary to maintain because the frequency of the non-contacting radar can impact its performance.

A lower frequency reduces sensitivity to vapor, foam, and contamination of the antenna, while a higher frequency keeps the radar beam narrow in order to minimize influence from nozzles, walls, and disturbing objects.

Beam width is inversely proportional to antenna size. The beam width of a given frequency will decrease as the antenna size increases.

For non-contacting radar proper installation is so important because good installation is the key to success.

The gauge needs a clear view of the surface with a smooth,

unobstructed, unrestricted mounting nozzle.

Obstructions in the tank, such as pipes, strengthening bars and agitators can cause false echoes, but most transmitters have updated software algorithms to ignore these false echoes.

Non-contacting radar gauges can handle agitation, but their success will depend on a combination of the fluid properties and the amount of turbulence.

Dielectric constant of the medium beingbeinand sensed and the surface conditions will impact the performance of the level transmitter measurement.

The measurement may be disturbed by the presence of foam. Energy tends not be reflected by light and airy foam while a dense and heavy foam typically reflects the energy.

Contacting type radar level

Guided wave radar level transmitter

Contact type radar level transmitter are also called guided wave radar level transmitter.

In guidde wave radar level system there is a direct contact of the device with contents of the vessel.

Basic principle

Guided wave radar is also called time domain reflectometry (TDR) .

Guided wave radar are also called micro-impulse radar (MIR).

In a Guided Wave Radar installation, the GWR is mounted on the top of the tank , and the probe of the level transmitter usually extended to the full depth of the tank.

A low energy pulse of microwaves, travelling at the speed of light, is sent down through the probe of the level transmitter.

At the point of the liquid level (in case of water level measurement, air / water interface) on the probe, a significant proportion of the microwave energy is reflected back to the receiver.

The transmitter measures the time delay between the transmitted and received echo signal. A bulit in microprocessor calculate the level of material proportional to the transit time.

This characteristic makes guided wave radar a good choice for measurement of the liquid level.

Guided wave radar can also measure interfaces of two different liquids such as oil and water and measuring through some foams.

Advantages

Guided wave radar can be used in vessels with tight geometry, in chambers, and in tanks of all sizes.

Guided Wave Radar can be used for naterials of low dielectric and turbulent applications.

Guided Wave Radar does not dependent on reflecting off a flat surface.

Fuuded Wave Radar can be used tofor measure level of powders and grains as well as liquids with slanted surfaces caused by vortices.

Guided wave radar (GWR) provides an accurate and reliable measurement for both level and interface.

A key advantage of radar is that no compensation is necessary for changes in the density or dielectric constant.

No effect of conductivity of the fluid on measurement accuracy.

Changes in pressure, temperature, and most vapor space conditions have no impact on the accuracy of radar measurements.

GWR is easy to install.

GWR are easy to replace.

GWR is suitable for level measurement in unstable Process Conditions.

GWR can be used for Boiling surfaces, dust, foam, vapor containing materials.

Limitation of Guided wave radar

GWR extreme temperature limit is 600ºF .

GWR extreme pressures limit is 540 Bar.

Unless a coax-style probe is used, probes should not be in direct contact with a metallic object, as that will impact the signal.

If the application tends to be sticky or coating, then only single lead probes should be used.

Applications of radar level

Radar level transmitters are used in all industries including chemical, oil and gas, power plants

Radar level are used for powder or solid and liquids level measurement.