On-Line Physical Measurements
Terry A. Ring
Department of Chemical Engineering
University of Utah
Types of Instruments
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Flow Measurement
Level Measurement
Temperature Measurement
Pressure Measurement
Density Measurement
Concentration
– Analytical Instrumentation
– Chemical Sensors
• Safety, Weight and Misc.
On-Line Connections
• Analogue Signal
– mV, mA, Volts, Amp, Watts,
other
• Amplification to Industrial Std
Scales
– 4 to 20 mA
– + 5 to -5 V
• Digitization
– Step size = Scale/(2^N)
– N = bits in a byte of A-to-D
board
– Calculation Precision 8, 10, 12
16 digits or 32 digits of binary
words are common
Flow Measurement
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Orifice plates
Pitot tubes
Rotary Flowmeters
Mass Flowmeters – Coriolis, Thermal
Magnetic Flowmeters
Sight Flow Indicators – paddle
Variable-area Flowmeters – rotameters
Venturi Tubes
V-cone Flowmeter
Many, many others
Sight Flow Meter
Rotameters
Turbine Flow Meter
Thermal Mass Flow Meter
Orifice Plate / Venturi Meter
Level Measurement
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Float Level Devices
Level Gauges
Differential Pressure Level Detectors
Slip tubes and Dipsticks
Ultrasonic Level Detectors
Laser Level Detectors
Load Cells on Tanks
Many, many others
Temperature Measurement
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Filled-bulb, Glass-stem thermometer
Thermocouples
Thermistors
Resistance Temperature Detectors (RTDs)
- platinum resistance thermometer
• Pyrometers
• Fiber-Optic Thermometers
• Others
Thermocouples
Thermocouple
Fermi – Dirac Distribution
µ=Fermi Level or 50% fill level
µ=Chemical Potential of electron
V=I*R
RTD’s
• Based on the electrical resistance of a
conductor increasing with temperature
• Most accurate, reproducible and stable
thermal element available
• Relatively expensive
• Very precise
• Fragile but can be industrially hardened
Thermistors
• Semiconductors made from mixtures of pure
oxides (Mn, Ni, Cu, Zn, Fe, etc.) sintered at >
1800 oF
• Very good for narrow span measurement
• Output highly non-linear, not good for wide span
applications, fragile unless industrially hardened
• Small size, fast response
• Lack of stability at higher temperatures
Pressure Measurement
• Bourdon pressure sensors
- C Bourdon
- Spiral Bourdon
- Helical Bourdon
• Differential pressure instruments
• Electronic pressure sensors
- strain gauge transducers
- piezoelectric sensor
• Manometers
• Diaphragm sensors
Pressure Measurement
• Absolute
• Gauge
• Vacuum
Pressure Sensor
Piezoelectric Sensor
Liquid Density Measurement
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Hydrometers
Weighing a fixed volume
Oscillating Coriolis Densitometers
Hydrostatic Densitometers
Radiation Densitometers – liquid/sludge
Vibrating Densitometers – Liq/sludge/gas
Hydrostatic Densitometers
ρ=ΔP/(g h)
Process Piping
Radiation Densitometer
Radiation β or γ
Light
X-rays
Beer’s Law
I/Io= exp(-kcL)
hυ
Chemical Sensors
• Gas Analysis
– Electrochemical Analysis
• CO sensor
• alcohol sensor
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Surface Electrodes:
Fired-On
Silver Paste
Doped
Nickel Oxide
Thin Film
0.5 um
Film
Thickness
2.5 mm
2 mm
3 mm
1 mm
2.5 mm
1 mm
10 mm
– Ion mobility
Silver Lead
Wires
Alumina Substrate
+
Micro-Weld
Drift Cell Analysis
• time the ions spend in the
drift cell td can be obtained
in terms of P/V.
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L = length of cell
V= Voltage
vd = drift velocity
Mobility
Ko= mobility
P, T = Pressure and Temperature,
subscript o is on-line conditions,
without subscript is inside instrument
(typically vacuum)
• Create Ions by Plasma
depends on MW and Charge
On-Line Analytical Instruments
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Infra Red
Gas Chromatography
Liquid Chromatography
Ultra Violet
Visible
Mass Spectrometer Mw 1 to 60 or 1 to 300.
– How would each of these work?
• Time delays
Spectroscopy
Liquid / Gas Chromatography
Mass Spectroscopy
Vac
Capillary
Mw=106.12 gm/mole
Process
Safety, Weight and Misc.
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Relief Valves
Rupture Discs
Leak Detectors Anemometers – air and gas flows
Boroscopes – inspect remote or limitedaccess locations – reactors, process lines,
etc. (fiberscopes)
• Weighing – Electronic Load Cells
On-Line Connections
• Analogue Signal
– mV, mA, Volts, Amp, Watts,
other
• Amplification to Industrial Std
Scales
– 4 to 20 mA
– + 5 to -5 V
• Digitization
– Step size = Scale/(2^N)
– N = bits in a byte of A-to-D
board
– Calculation Precision 8, 10, 12
16 digits or 32 digits of binary
words are common
Keep it Running Well
 KNOW YOUR EQUIPMENT
Accuracy and Precision
UNDERSTAND THE LIMITATIONS OF YOUR
EQUIPMENT
 CALIBRATE AS MUCH AS IS
REASONABLE
 Maintain Equipment so it will Perform
Reliably