Level measurement Cảm biến mức

The participant knows… • … the different applications for level measurement • …the different technologies for level measurement • …which technologies are being used for the training rig • …th h i i f h h l i e characteristics of the used technologies The participant understand… • … each measurement technology has strength and weaknesses • …application and process conditions are very important

Level Measurement

Learning objectives

The participant knows…

• … the different applications for level measurement

• …the different technologies for level measurement

• …which technologies are being used for the training rig

• …the characteristics of the used technologies

The participant understand…

• … each measurement technology has strength and weaknesses

• …application and process conditions are very important

Applications with Level Device

Level measurement applications in liquids including liquefied gases and bulk solids are divided into four areas:

Applications with Level Device

The overview contains the measuring principles suitable for each area

Finding the solution

• The first step in finding a solution for a level metering point is finding the best suitable measuring technology g g gy

• The selection is based on the requirements of the measuring point e.g.:

Influences on the selection of the measuring system

Following can have an influence on the device selection

Influences on the selection of the measuring system

Influences on the selection of the measuring system

Training Rig Level Measurement Technologies

• Vibrating level detection

• Capacitive level detection

Level Measurement – Vibrating Level Detection

Basics of Vibrating level detection

The instruments of the Liquiphant family

reliably monitor the point level of all

reliably monitor the point level of all

pump able liquids in tanks and pipes There are numerous applications from simple

operational point level detection (minimum and maximum control), certified leakage

monitoring and overfill prevention through

to protective equipment in plant parts

subject to Safety Integrity Levels (SIL2/3)

subject to Safety Integrity Levels (SIL2/3)

Basics of Vibrating level detection

Functional principle:

A tuning fork sensor oscillates at its resonant frequency The drive works piezoelectrically.The oscillating frequency changes as the fork enters the medium The change is analyzed and translated into a switching signal

Basics of Vibrating level detection

Self Monitoring:

Automatic self-monitoring of the oscillating system A change in frequency Beyond a permitted value indicates an irregularity in the oscillating system,

Beyond a permitted value indicates an irregularity in the oscillating system,

e g corrosion or build-up The instrument then switches in a safety-orientmanner

Influences on the measuring system

changing

mediaOperation Independent of Medium Properties

electrical properties foam

temperature change

p p

plant vibration

Advantages of Vibrating level detection

• Accurate switch-point

• Universal use – unaffected by medium properties such as

conductivity, dielectric constant, viscosity, pressure and temperature

• Free of calibration and maintenance

• Can be mounted in any position

• No adjustment is needed only choose for minimum or maximum

• Suitable for products with high viscosity

• Self Monitoring

Disadvantages of Vibrating level detection

• The only restriction to ensure good operation is the maximum size of solid particles of 10 mm in the liquid Longer particles p q g p could become lodged between the tines.

Level Measurement – Capacitance Level Detection

Basics of Capacitance level detection

Capacitance measurement is based

on the capacitance change of the

capacitor formed by the probe

and the container wall

and the container wall

Minimum probe length

is determined by analyzable capacitance change and medium properties

Basics of Capacitance level detection

Functional principle:

The capacitance level measurement principle is based on the capacity change

of a capacitor due to a change in level The probe (rod or rope) and the silo

wall form the two electrodes of a capacitor As product enters the electric field

between the probe and the silo wall, the capacity increases This capacity change

is analyzed and leads, with the appropriate setting, to switching.y pp p g g

Basics of Capacitance level detection

In level measurement, only part of the capacitor is usually filled with

Basics of capacitance level - Interface measurement

Functional principle interface

value in level measurement while media

with a high DC produce respectively

large

capacitance changes In many interface

applications, the medium with the lower

DC is on top, e g oil on water The

upper medium provides only a minimum

contribution to the overall capacitance

value – only the water level (the

interface layer) is thus issued as level.y

Influences on the measuring system - DC

εr7.8 8

LiquidsAlcohol

LiquidsAlcohol

εr3

9.32,2 3.61.2 54.6

PetrolChloroformDesmophen 5100Desmodur 44 V

PetrolChloroformDesmophen 5100Desmodur 44 V

2 35.52.278.12

2 35.52.278.12

The larger therelative dielectricconstant,

2 3

53.34.5

2 3

DeuteriumLiquid gasses

- Chlorine

°

DeuteriumLiquid gasses

- Chlorine

°

1.3

1.81

1.3

1.81

constant,the larger the capacitance

2.32.3 34.51.8

- Air -140°C

- Butane

- PropaneGlycerin

- Air -140°C

- Butane

- PropaneGlycerin

1.51.21.213.2

1.51.21.213.2

should be more than

DC1.6 for capacitance measurement

3 5

1 1 1 3

23.5

3 5

1 1 1 3

Fuel oilLatexPetroleumWater

Fuel oilLatexPetroleumWater

2.1162.181

2.1162.181

Influences on the measuring system - DC

gasoline, solvent, oil

butyric acid

10 %

Citric acid sludge

Aqueous dispersions and emulsions

1000μS/cm 10μS/cm

0,01μS/cm 0,001μS/cm

DK 2

Aqueous dispersions and emulsions

Conductivity of mediaΔC

Min ΔC

= 25pF

Advantages of capacitance level detection

• Tried and tested technology

• Universally adaptable probes

• Reliable performance also in viscous media or heavy build-up

• Reliable performance also in viscous media or heavy build up

• Independent of density changes

• Easy to install

• Accurate measurement in small vessels

Capacitance level detection

Level Measurement – Hydrostatic Level Detection

Basics of Hydrostatic level detection

Functional principle:

Hydrostatic level measurement is based on

the determination of hydrostatic pressure

d b h h h f h l f

generated by the height of the column of

fluid The pressure is calculated on basis of

the following formula:

q= Specific weight (density)

In constant medium density, the height (h)is the only variable in this equation.The pressure is thus a direct level measurement Hydrostatic pressure sensors either consist of a dry capacitive measuring diaphragm of ceramics or a

piezoresistive sensor with a metal diaphragm

Basics of Hydrostatic level detection

P air

h

Basics of Hydrostatic level detection

Hydrostatic Pressure

p = ρ y g y h

To get the hydrostatic pressure in mbar the following

units/values have to be used:

Influences on the measuring system

Advantages of hydrostatic level detection

• Established technology ,tried and tested

• Measuring principle for temperatures up to 400°C and

pressures up to 400bar

• Easy engineering

• Unaffected measurement with tank baffles or surface foam

• Unaffected measurement with tank baffles or surface foam

• Unaffected measurement with tank obstacles/tank geometries

• Hygiene instrument designs

Disadvantages of hydrostatic level detection

The system is used as a level

gauge, it actually measures

pressure, which is equal to the

height and density of the

product and specific gravity

p oduct a d spec c g av ty.

This can be an advantage in

cases where the product weight

is required rather than its

is required rather than its

volume, or can be disadvantage.

Level Measurement – Time of Flight TOF

Basics of Time of Flight TOF

• The impulses are reflected by the surface of the medium

• The reflected impulses are received and the time from sending to receiving the signal is measured

Time of Flight Measurements

• Radar level measurement

• Guided radar level

measurement

• Ultrasonic level measurement

Level Measurement – Radar Level Detection

Basics of Frequency Modulated Continuous Wave FMCW

Basics of Pulse Method

Basics of Radar level detection

Functional principle :

Radar level device uses high-frequency radar pulses which areemitted from an antenna and reflected by the product surface The Time-of-Flight “t” of the reflected radar pulses is directly proportional to the path traveled d

d = c (t/2)

c = speed of light 300 000km/s

Taking the tank geometry into consideration, the level can be calculated from this value

Influences on the measuring system

Influences on the measuring system - Dielectric Constant

Influences on the measuring system - Dielectric Constant

„ Microwaves are electromagneticwaves

„ Reflection of microwaves depends

on a change of the wave

Medium:

εr> 1

Influences on the measuring system - Microwave Frequen

• Frequency of RADAR technology is in the range of

microwaves (500MHz to approx 30GHz)

Influences on the measuring system -Microwave Frequen

„ Small beam angle

„ Ideal in small vessels

„ Less effected by turbulent

surface and foam

„ Less effected by condensate

Microwaves – Effects on Humans

Inside: approx 1 W/cm² Leakage at

Leakage at closed door: approx 0.005 W/cm²

At transmitting power of 2 W:

approx 0.0001 W/cm²

Pulse: approx 0.00015 W/cm² Average: approx 0.00000021 W/cm² /

Influences on the measuring system - Antenna Types

• Antennas are required to transmit and receive electromagnetic waves

• Depending on the application, the antenna can have various forms

Influences on the measuring system - frequency & antenna

Influences on the measuring system - Antenna Extension

P bl d Ri k

Problems and Risks:

„ Wrong MF leads to level offset

„ Changing rod diameters

„ Bended antenna extensions

Influences on the measuring system - Tank Types

Influences on the measuring system - Tank Types

„ Typically causes for multiple echoes:

Æ Multiple level reflections via the tank ceiling (e.g dome ceiled tanks)

Æ Conical outlet (e.g funnel-shaped tank) g p

Æ Signal propagation via the tank wall

„ Problem: Output signal of RADAR device ‘jumps’ between multiple echoes and actual level

„ Solution: Special software settings Æ First Echo Factor

Influences on the measuring system - Polarization

„ Electromagnetic waves are consisting out of an electrical vector E and a magnetic vector B

„ Both vectors are in phase and perpendicular to each other

„ Both vectors are in phase and perpendicular to each other

„ For level transmitters linear polarization is used

H – Field E –

Field

Influences on the measuring system - Polarization

„ False echoes and destructive interferences in case of wrong

Æ B vector parallel to conductive regions

m 0

regions

60 80

optimal position (turned by 90°)

80 100 120

original position dB

120 dB

Influences on the measuring system - Foam

!!!signal absorption!!!

„ dense foam

„ small bubbles, conductive

„ wide echoes reflections „ measurement

possible! „ better results

with 6GHz „ Increasing inaccuracy

Advantages of radar level detection

• Non-contact, maintenance-free measurement

• Non-contact measurement, free of wear and tear, that can be used in extreme process conditions

• For high temperatures up to +400°C/+752°F

• Measurement unaffected by:

• Vapors or dusty media have no affect on the measurement

• Highest process safety due to Multi-Echo Tracking evaluation

• Safe measurement in vessels with changing product

Disadvantages of radar level detection

• Strong Condensate

• Heavy, conductive foam formation

• Measurement to 0% level (tank bottom effect)

Level Measurement – Guided Radar Level Detection

Basics of Guided Radar level detection

Functional principle :

The Levelflex uses high-frequency radar

pulses

guided along a probe The characteristic

impedance changes as pulses meet the

surface of the medium and part of the

transmitted pulses is reflected The time

between transmission and

reception of the reflected pulse is measured

and analyzed by the instrument and provides

a direct value for the distance between the

a direct value for the distance between the

process

connection and the medium surface

Basics of Guided Radar level detection- Block Distance

Basics of Guided Radar level detection of the end of the m

Basics of Guided Radar - Interface Measurement

As the pulses impact the medium

surface, only part of the sending

pulse is reflected Especially in

media with a

low dielectric constant ,the other

part penetrates the medium As the

part penetrates the medium As the

signal enters the lower medium

with a

higher dielectric constant it is

reflected once more Taking the

delayed Time-of-Flight of the

pulse through the upper medium

into consideration the distance to

the interface is determined in

addition

Basics of Guided Radar - Interface Measurement

The name of the innovation in

interface measurement is FMP55

interface measurement is FMP55

Multiparameter This instrument

combines the advantages of the

capacitance and guided radar

measuring principles Emulsion

layers may cause signal losses in

interface

detection in guided radar g

measurements Only Levelflex

FMP55 Multiparameter can

guarantee safe measured values for

both the interface and the overall

both the interface and the overall

level with this unique, redundant

measuring system

Influences on the measuring system - DK

Influences on the measuring system - DK

Reflection signal of a micro impulse measurement with different media

Advantages of guide radar level detection

• Unaffected by medium surface (agitated surface, foam)

ff

• Unaffected by tank obstacles

• Additional measuring safety through End of Probe (EOP) recognition

• DK starting at 1.6 without stilling well (1.4 for coax probe)

Disadvantages of guide radar level detection

It is not Non-contact

measurement anymore,

Specially for solid

application, check grain

size etc

Level Measurement – Ultrasonic Level Detection

Basics of Ultrasonic level detection

Functional principle :

Ultrasonic pulses which are reflected from the medium surface by the density change between air and the medium The time

between transmission and reception of the pulse is measured and analyzed by the

instrument and provides a direct value for the distance between the sensor

the distance between the sensor

membrane and the medium surface

Basics of Ultrasonic level detection

Sound = mechanical wave What are ultrasonic waves?

Frequency of sound

Air compression

λ wave length (meter)

f

λ wave length (meter)

f frequency (Hertz)

Influences on the measuring system

x: adiabatic index M: molecular weight

p change in run time

Influences on the measuring system

Application limits for ultrasonic level measurement

• Strong temperature fluctuations in the measuring range can affect the accuracy

• p <–0.3bar

• Vapor pressure >50mbar

• Surface of liquid

Advantages of ultrasonic level detection

• Non-contact, maintenance-free measurement

• Unaffected by product properties, e.g DK value, etc

• Calibration without filling or discharging

• Easy and fast commissioning due to preset application parameters

• Quick and simple commissioning via menu-guided onsite operation with envelope curves on the on-site display for simple diagnosis

• Linearization function (up to 32 points) for conversion of the p p

measured value into any unit of length, volume or flow rate