Nguyễn Công Phương

Sensors and Analytical Devices

Some Basic Measurement Methods, Level Measurement

Contents

A. Introduction B. Sensors Characteristics C. Some Basic Measurement Methods D. Measurement Systems

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Some Basic Measurement Methods

I. Sensor Technologies II. Temperature Measurement III. Pressure Measurement IV.Flow Measurement V. Level Measurement VI.Mass, Force, and Torque Measurement VII.Translational Motion, Vibration, and Shock

Measurement

VIII.Rotational Motion Transducers

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Level Measurement

1. Introduction 2. Dipsticks 3. Float Systems 4. Pressure-Measuring Devices (Hydrostatic

System)

5. Capacitive Devices 6. Ultrasonic Level Gauge 7. Radar (Microwave) Sensors 8. Nucleonic (or Radiometric) Sensors 9. Other Techniques 10. Choice between Different Level Sensors

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Introduction

• Required in a wide range of applications. • Can involve the measurement of solids in the form of powders or small particles as well as liquids.

• Some applications require a high degree of accuracy. • Others only need an approximate indication of level. • Simple devices such as dipsticks or float systems are

relatively inexpensive.

• Devices offer good measurement accuracy: pressure- measuring, capacitive, ultrasonic, radar, radiation.

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Ordinary Dipstick

• A simple means of measuring the level

of liquids approximately.

• The least expensive device available. • Consist of a metal bar on which a scale

is etched.

• The bar is fixed at a known position in

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the liquid-containing vessel. • A level measurement is made by removing the instrument from the vessel & reading off how far up the scale the liquid has wetted.

• A human operator is needed to remove & read the dipstick  used in relatively small & shallow vessels.

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Optical Dipstick

• Allows a reading to be obtained without removing the dipstick from the vessel.

• Applicable to larger, deeper tanks. • Light from a source is reflected from a

mirror, passes round the chamfered end of the dipstick, & enters a light detector after reflection by a second mirror.

• When the chamfered end comes into contact with liquid, its internal reflection properties are altered & light no longer enters the detector.

• By using a suitable mechanical drive system

to move the instrument up & down & measure its position, the liquid level can be monitored.

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Level Measurement

1. Introduction 2. Dipsticks 3. Float Systems 4. Pressure-Measuring Devices (Hydrostatic

System)

5. Capacitive Devices 6. Ultrasonic Level Gauge 7. Radar (Microwave) Sensors 8. Nucleonic (or Radiometric) Sensors 9. Other Techniques 10. Choice between Different Level Sensors

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Float Systems (1)

• Simple & inexpensive. • Provide an alternative way of measuring the level of liquids approximately.

• Consist of a float on the surface of the liquid whose position is measured by means of a suitable transducer. Inaccuracy: ±1%.

• • Widely used in

monitoring the level in motor vehicle fuel tanks.

• Maintenance

requirements are high.

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Float Systems (2)

• An alternative system: the float-and-tape gauge. • Used in greater number. • It has a tape attached to the

float & passes round a pulley situated vertically above the float.

• The other end of the tape is

attached to either a counterweight or a negative- rate counterspring.

• Popular in many applications, but maintenance requirements are high.

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Level Measurement

1. Introduction 2. Dipsticks 3. Float Systems 4. Pressure-Measuring Devices (Hydrostatic

System)

5. Capacitive Devices 6. Ultrasonic Level Gauge 7. Radar (Microwave) Sensors 8. Nucleonic (or Radiometric) Sensors 9. Other Techniques 10. Choice between Different Level Sensors

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Pressure-Measuring Devices (1)

h

• Measure the liquid level basing

P g

Pressure transducer

•

on the principle that the hydrostatic pressure due to a liquid is proportional to its depth & hence to the level of its surface. In the case of open-topped vessel, the level can be measured by inserting a pressure sensor at the bottom of the vessel.

• P: the measured

• One source of error: imprecise

•

pressure ρ: the liquid density • • g: the acceleration of

knowledge of the liquid density. If the liquid density varies with temperature, temperature measurement may be needed.

gravity

http://www.pressuresensor- knowhow.com/blog/2012/12/10/hydrostatic -level-measurement-2/

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Pressure-Measuring Devices (2)

•

If liquid-containing vessels are totally sealed, the liquid level can be calculated by measuring the differential pressure between the top & the bottom of the tank.

h

P g

 

• The differential pressure

transducer used is normally a standard diaphragm type. • One source of error: imprecise

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•

•

knowledge of the liquid density. If the liquid density varies with temperature, temperature measurement may be needed.

δP: the differential measured pressure ρ: the liquid density g: the acceleration of gravity

• •

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Pressure-Measuring Devices (3)

• The bubbler unit: uses a dip pipe that reaches to the bottom of the tank & is purged free of liquid by a steady flow of gas through it. • The rate of flow is adjusted until gas bubbles are just seen to emerge from the end of the tube.

• The pressure in the tube, measured by a pressure transducer, is then equal to the liquid pressure at the bottom of the tank.

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• Advantage: avoidance of the large maintenance problem associated with leaks at the bottom of the tanks at the site of pressure tappings required by alternative methods.

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Level Measurement

1. Introduction 2. Dipsticks 3. Float Systems 4. Pressure-Measuring Devices (Hydrostatic

System)

5. Capacitive Devices 6. Ultrasonic Level Gauge 7. Radar (Microwave) Sensors 8. Nucleonic (or Radiometric) Sensors 9. Other Techniques 10. Choice between Different Level Sensors

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Capacitive Devices

• Widely used for measuring the level of both liquids & solids in powdered or granular form.

• Perform well in many applications,

but become inaccurate if the measured substance is prone to contamination by agents that change the dielectric constant.

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• Suitable for use in extreme

C

A  d

ε: the dielectric constant

conditions measuring liquid metals (high temperatures), liquid gases (low temperatures), corrosive liquids (acids, etc.), & high- pressure processes. Inaccuracy: ±1 – ±2%.

•

• • A: the area • d: the distance

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Level Measurement

1. Introduction 2. Dipsticks 3. Float Systems 4. Pressure-Measuring Devices (Hydrostatic

System)

5. Capacitive Devices 6. Ultrasonic Level Gauge 7. Radar (Microwave) Sensors 8. Nucleonic (or Radiometric) Sensors 9. Other Techniques 10. Choice between Different Level Sensors

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Ultrasonic Level Gauge

• One of a number of noncontact

techniques available.

• Used primarily to measure the

level of materials that are either in a highly viscous liquid form or in solid (powder or granular) form.

• Operating principle:

– Energy from an ultrasonic transmitter above the material is reflected back form the material surface into an ultrasonic receiver.

– Measurement of the time of flight allows the level of the material

surface to be inferred.

• Alternative version: transmitter & receiver are at the bottom

(for liquids only).

• Has a small sensitivity to temperature, humidity, air pressure.

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Level Measurement

1. Introduction 2. Dipsticks 3. Float Systems 4. Pressure-Measuring Devices (Hydrostatic

System)

5. Capacitive Devices 6. Ultrasonic Level Gauge 7. Radar (Microwave) Sensors 8. Nucleonic (or Radiometric) Sensors 9. Other Techniques 10. Choice between Different Level Sensors

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Radar (Microwave) Sensors

• Use microwave radar as an alternative technique for noncontact measurement.

• Very expensive. • Provide successful level measurement in applications that are otherwise very difficult: – In closed tanks, where the liquid is

turbulent,

– In the presence of obstructions & steam

condensate.

• Also used for detecting the surface of solids in powder or particulate form.

• Operating principle:

– Direct a constant amplitude, frequency-modulated microwave signal

at the liquid surface.

– A receiver measures the phase difference between the reflected signal

& the original signal transmitted directly through air to it. – The phase difference is linearly proportional to the liquid level sites.google.com/site/ncpdhbkhn

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Level Measurement

1. Introduction 2. Dipsticks 3. Float Systems 4. Pressure-Measuring Devices (Hydrostatic

System)

5. Capacitive Devices 6. Ultrasonic Level Gauge 7. Radar (Microwave) Sensors 8. Nucleonic (or Radiometric) Sensors 9. Other Techniques 10. Choice between Different Level Sensors

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Nucleonic (or Radiometric) Sensors

• Use a radiation source & detector system

Radiation detector

located outside a tank.

Radiation source

• This is a noninvasive technique. • The absorption of both β & γ rays varies with the amount of material between the source & the detector  a function of the level of the material in the tank.

x

• Used commonly for measuring the level

of hot, liquid metals & also solid materials in a powdered granular form.

• •

•

• Very dangerous. •

•

If very low activity radiation sources are used (to overcome safety problems), the system is sensitive to background radiation.

•

I e  I 0 I: the intensity of radiation I0: the intensity of radiation in the absence of any material μ: the mass absorption coefficient of the measured material ρ: the mass density of the measured material x: the length of the material in the path from source to detector

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Level Measurement

Introduction

1. 2. Dipsticks 3. Float Systems 4. Pressure-Measuring Devices (Hydrostatic System) 5. Capacitive Devices 6. Ultrasonic Level Gauge 7. Radar (Microwave) Sensors 8. Nucleonic (or Radiometric) Sensors 9. Other Techniques

a) Vibrating Level Sensor b) Reflective Level Sensor

10. Choice between Different Level Sensors

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Vibrating Level Sensor

• The tube vibrates at its resonant

frequency.

• The resonant frequency of the tube varies according to the depth of its immersion in the liquid.

• The liquid level measurement is obtained in terms of the output frequency.

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Reflective Level Sensor

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Level Measurement

1. Introduction 2. Dipsticks 3. Float Systems 4. Pressure-Measuring Devices (Hydrostatic

System)

5. Capacitive Devices 6. Ultrasonic Level Gauge 7. Radar (Microwave) Sensors 8. Nucleonic (or Radiometric) Sensors 9. Other Techniques 10. Choice between Different Level Sensors

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Choice between Different Level Sensors

• The first consideration: whether it is a liquid or a solid. • The second consideration: the degree of measurement

accuracy required – Low accuracy: dipsticks & float systems are ok. – High accuracy: a number of different devices.

• Contact or noncontact

– Contact: all pressure-measuring devices. – Noncontact: capacitive, ultrasonic, radiation, microwave. – Both are relatively expensive.

• Measurement of the level of solids: capacitive,

ultrasonic, microwave, radiation.

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