Study Unit 6 Flashcards
What is the process followed by measuring equipment?
- A sensor normally produces a signal, which can either be a mV or mA signal. (4 - 20 mA)
- A transducer converts this signal into a meaningful value, that is useful for the plant operating personnel
- The controller performs a calculation and its output is also a mA (or mV) signal
- A current to pressure transducer (I/P) (or voltage-to-pressure (E/P)) transducer will convert this mA (or mV) signal into a pneumatic pressure value that is used in the pneumatic control valve
8 criteria that need to be considered when selecting the proper sensors for control applications
- Measurement range (span)
- Performance
- Reliability
- Materials of construction
- Prior use
- Potential for releasing process material to the environment
- Electrical classification
- Invasive or non-invasive
Temperature sensors
- Thermocouple
- Resistance temperature detector (RTD)
- Filled-system thermometer
- Bimetal thermometer
- Pyrometer (total radiation; photoelectric; ratio)
- Laser
- Surface acoustic wave
- Semiconductor
Flow sensors
- Orifice
- Venturi
- Rotameter
- Turbine
- Vortex-shedding
- Ultrasonic
- Magnetic
- Thermal mass
- Coriolis
- Target
Pressure sensors
- Liquid column
- Elastic element (bourdon tube; bellows; diaphragm)
- Strain gauges
- Piezoresistive transducers
- Piezoelectric transducers
- Optical fiber
Level sensors
- Float-activated (chain gauge, lever; magnetically coupled)
- Head devices (bubble tube)
- Electrical (conductivity)
- Radiation
- Radar
Composition sensors
- Gas-liquid chromatography (GLC)
- Mass spectrometry (MS)
- Magnetic resonance analysis (MRA)
- Infrared (IR) spectroscopy
- Raman spectroscopy
- Ultraviolet (UV) spectroscopy
- Thermal conductivity
- Refractive index (RI)
- Capacitance probe
- Surface acoustic wave
- Electrophoresis
- Electrochemical
- Paramagnetic
- Chemi/bioluminescence
- Tunable diode laser absorption
Are thermocouples expensive?
No
How do thermocouples measure temperature?
Principle of measurement is based on the Seebeck effect.
Two wires made from different metals are connected at the ends. When one end of the connected wires is heated, there will be a temperature difference between the ends. A potential difference can then be measured.
Advantages of thermocouples
- Can directly measure temperatures up to 2600 degrees Celsius
- Respond very quickly to temperature changes due to high conductivities of metals
- Thermocouple junction can be grounded and brought into direct contact with the material being measured
Disadvantages of thermocouples
- Additional temperature measurement device is required at the cold junction to determine the reference temperature.
- Materials of which thermocouple wires are made are not inert and the wire may corrode.
- Relationship between process temperature and the thermocouple signal is not linear
- Calibration of a thermocouple is difficult
Why is an additional temperature measurement device required in a thermistor?
Temperature measurement with a thermocouple requires two temperatures to be measured; the junction at the work end (hot junction) and the junction where the wire meets the instrumentation copper wires (cold junction). The measurement of temperature at the cold junction is done by means of a semiconductor, thermistor or resistance temperature detector (RTD).
How should a thermocouple be calibrated?
It should be calibrated by comparing it to a nearby thermocouple.
How do RTDs work?
A thermowell is placed inside the line where the temperature is to be measured.
The sensor is fitted into a rod shaped tube within the thermowell.
Resistance sensors based on thin film technology are the most commonly used.
A very thin, platinum conductive track is deposited onto as ceramic substrate.
With an increase in T, there is an increase in electrical resistance.
Temperature transmitter converts the measured resistance value into a standardized interference resistance output signal shown on a display.
Electrical resistivity
Quantifies how strongly a given material opposes the flow of electric current (ohm.m)
What is the relationship between resistivity and resistance?
Higher resistivity = higher resistance
Resistance is directly proportional to resistivity
Describe the principle by which RTDs and thermistors work
The direct correlation between the resistivity of a certain material and the temperature
Advantages of RTDs
- High accuracy
- Low drift
- Wide operating range
- Suitability for precision applications
Disadvantages of RTDs
- Expensive
- Max temperature limited at 600 degrees Celsius
- Slower response time compared to thermocouple
Thermistors vs RTDs
- RTDs are made of pure metal whereas thermistors are made of ceramic or polymer
- Thermistors are more accurate than RTDs
- Thermistor wires can be > 1000m whereas RTD wiring cannot be longer than 30m
- RTDs need transmitters whereas thermistors work properly without transmitters.
- Thermistors can only work up to 130 degrees Celsius whereas RTS can work up to 660 degrees Celsius
Types of thermistors
NTCs (Negative temperature coefficient) (R decreases as T increases)
PTCs (Positive temperature coefficient) (R increases as T increases)
Advantages of thermistors
- Point measurements are possible
- Possible to work very accurately over a small temperature range
- Sensitivity of thermistors can be an order of magnitude greater than that of the RTD
- High resistivity negates the need for a four-wire bridge circuit
Disadvantages of thermistors
- Not very accurate at temperatures above 130°C
- Highly non-linear and not rugged, which limits application
- Self-heating effects produced by an excessive bias current when the thermistor is powered up
- To reduce this source of error, it is necessary to use a current suited to the value of the measured resistance
Pyrometers
Transducers that make use of visible or invisible radiation emitted by an object to determine temperature without any physical contact are known as pyrometers
Classes of pyrometers
- Total radiation pyrometers
- Selective radiation pyrometers
- Infrared pyrometers (special class)
How do total radiation pyrometers work?
Sense both visible and invisible electromagnetic radiation from a body.
They direct a regulated sample of the radiation from the object onto a temperature measuring device.
Output signal is electrical.
Sources of error in total radiation pyrometers
- Calibration is required at several known temperatures prior to use. Emissivity value used during calibration may differ from actual emissivity during operation
- There can be something between the test object and disc that can cause partial or full absorption of the spectrum of radiation emitted
Both these aspects are difficult to estimate in field situation.
How accurate is a total radiation pyrometer?
Does not provide very high accuracy in temperature measurement
How accurate is a optical pyrometer?
Provides good accuracy (+-1%) in temperature measurement
Measurement error (or error)
Difference between a measured value and the true value
Percentage of full scale error (% FS)
The error is expressed as a percentage of the span of the instrument
Relative error
Obtained by dividing the error by the measurement value
Resolution
Smallest interval between two numerical values that can be distinguished. If the resolution of a thermocouple is e.g. 1°C, it can distinguish between 68°C and 69°C, but not between 68.8°C and 68.9°C. This concept is important in selecting the correct equipment.
Precision
Variability of a measurement for specified conditions and a particular instrument. It is usually specified as a standard deviation of range. If the composition of a sample that was divided in four equal parts were to be measured for each part, and the values obtained were e.g.: 21.3%, 22.7%, 20.6% & 21.5%, the analyser’s precision can be expressed as the range (22.7 - 20.6) = 2.1%, or the standard deviation is calculated as 0.87%.
Repeatability
This is almost similar to precision, but refers to the variability of replicate measurements in a set of data. This can be due to random errors, e.g. measurement error, environmental factors etc. - errors from both known and unknown sources. This is also expressed as a range or standard deviation.
Bias
This refers to a constant error in a measurement due to a deterministic cause rather than random variations. The measured fluid temperature in a vessel can be constantly lower than the actual temperature of the fluid in the vessel due to e.g. the position of the thermocouple. The difference between the actual fluid temperature and the measured fluid temperature is referred to as a bias.
How is an instrument callibrated?
Is usually performed by verifying the readings in the operator control room with measurements done using portable measurement equipment
Instruments used to measure pressure
Manometer
Bourdon tube gauges
