Process measurement and transducers--part 2

Sensor dynamics

Process dynamics have been discussed , and these same factors will apply to a sensor making it important to gain an understanding of sensor dynamics. The speed of response of the primary measuring element is often one of the most important factors in the operation of a feedback controller. As process control is continuous and dynamic, the rate at which the controller is able to detect changes in the process will be critical to the overall operation of the system.

Fast sensors allow the controller to function in a timely manner, while sensors with large time constants are slow and degrade the overall operation of the feedback loop. Due to their influence on loop response, the dynamic characteristics of sensors should be considered in their selection and installation.

Selection of sensing devices

A number of factors must be considered before a specific means of measuring the process variable (PV) can be selected for a particular loop:

• The normal range over which the PV may vary, and if there are any extremes to this

• The accuracy, precision and sensitivity required for the measurement

• The sensor dynamics required

• The reliability that is required

• The costs involved, including installation and operating costs as well as purchase costs

• The installation requirements and problems, such as size and shape restraints, remote transmission, corrosive fluids, explosive mixtures, etc.

Temperature sensors

Temperature is the most common PV measured in process control. Due to the vast temperature range that needs to be measured (from absolute zero to thousands of degrees) with spans of just a few degrees and sensitivities down to fractions of a degree, there is a vast range of devices that can be used for temperature measurements. The five most common sensors; thermocouples, resistance temperature detectors or RTDs, thermistors, IC sensors and radiation pyrometers have been selected as they illustrate most of the application, range, accuracy and linearity aspects that are associated with temperature measurements.

Thermocouples

Thermocouples cover a range of temperatures, from -262 to +2760 °C and are manufactured in many materials, are relatively cheap, have many physical forms, all of which make them a highly versatile device.

Thermocouples suffer from two major problems that cause errors when applying them to the process control environment.

1. The first is the small voltages generated by them, for example a 1 °C temperature change on a platinum thermocouple results in an output change of only 5.8 µV = (5.8 × 10^-6 V).

2. The second is their non-linearity, requiring polynomial conversion, look up tables or related calibration to be applied to the signaling and controlling unit.

Ranges of six types of common thermocouples:

Metal Composition Temperature Span Seebeck Coefficient

K Chromel vv alumel -190 to +1371 °C 40 µV/°C J Iron vv constantan -190 to +760 °C 50 µV/°C T Copper vv constantan -190 to +760 °C 50 µV/°C E Chromel vv constantan -190 to +1472 °C 60 µV/°C S Platinum vv 10% rhodium / platinum 0 to +1760 °C 10 µV/°C R Platinum vv 13% rhodium / platinum 0 to +1670 °C 11 µV/°C

Thermocouple types, temperature range and value of the seebeck effect.

Principles of thermocouple operation:

A thermocouple could be considered as a heat-operated battery, consisting of two different types of homogeneous (of the same kind and nature) metal or alloy wires joined together at one end of the measuring point and connected usually via special compensating cable, to some form of measuring instrument. At the point of connection to the measuring device a second junction is formed, called the reference or cold junction, which completes the circuit.

The Peltier and Thomson effects on thermocouple operation:

The Peltier effect is the cause of the emfs generated at every junction of dissimilar metals in the circuit. This effect involves the generation or absorption of heat at the junction as current flows through it and temperature is dependent on current flow direction.

The Thomson effect, where a second emf can also be generated along the temperature gradient of a single homogeneous wire can also contribute to measurement errors. It’s essential that all the wire in a thermocouple measuring circuit is homogeneous as then the emfs generated will be dependant solely on the types of material used. Any thermal emfs generated in the wire when it passes through temperature gradients will also be canceled from one to the other.

Additionally, if both junctions of a homogeneous metal are held at the same temperature, the metal won’t contribute additional emfs to the circuit. It follows then that if all junctions in the circuit are held at a constant temperature, except the measuring one, measurement can be made of the hot, or measuring, junction value against the constant value or cold junction reference value.

++++ Thermocouple cold junction and reference junction circuit examples

Reference or cold junction compensation:

As described, we have to ensure that all the junctions in the measuring circuit, with the exception of the one being used for the actual process measurement, must either:

• Be held at a constant known temperature, usually 0 °C, and called a 'Cold Junction'

• Or the temperature of these junctions should be measured and the measuring instrument takes this into consideration when calculating its final output.

Both methods are commonly used; the first one, the cold junction, utilizes an isothermal block held at a known temperature and in which the connections from the thermocouple wires to copper wires are made. The second method is to measure the temperature, usually by a thermistor, at the point of copper to thermocouple connections, feeding this value into the measuring system and have that calculate a corrected output.

Resistance temperature detectors or RTDs

In the same year as the discovery of the thermocouple by Thomas Seebeck, Sir Humphry Davy noted the temperature/resistivity dependence of metals, but it was H C Meyers who developed the first RTD in 1932.

Construction of RTDs:

RTDs consist of a platinum or nickel wire element encased in a protective housing having, in the case of the platinum version a base resistance of 100 ohm at 0 °C and the nickel type a resistance of 1000 ohm, again at 0 °C. They come packaged in either 2, 3 or 4 wire versions, the 3 and 4 wire being the most common. Two wire versions can be very inaccurate as the lead resistance is in series with the measuring circuit, and the measuring element relies on resistance change to indicate the temperature change.

++++ Construction of RTD ; RTD sensing element sub-assembly Insulated leads packed in MgO RTD probe sheath Spring-loaded mounting fitting Removable retainer Terminal block Connection head RTD lead seal Thermowell

Range sensitivity and spans of RTDs:

RTDs operate over a narrower range than thermocouples, from -247 to +649 °C. Span selection has to be made for correct operation as typically the sensitivity of a PT100 is: 0.358 ohm/ °C about the nominal resistance of 100 ohm at 0 °C. This corresponds to a single resistance range of (100-88 = -247 °C to 100 + 232 ohm = 649 °C ) resulting in 12-332 ohm, which is outside the range of a single transducer.

Example of RTD application in a digital environment:

++++ configuration of a 3-wire RTD used in a digital process control application. Modern digital controllers use these 3-wire RTDs in the following manner: A constant current generator drives a current through the circuit [A-C] consisting of 2RL + RX. A voltage detector reads a voltage, VB, proportional to RX + RL between points [B and C] and a second voltage VA which is proportional to RX + 2RL between points [A and C].

++++ 3-Wire RTD configuration for a digital system -- Platinum element; Internal lead wires; Ceramic insulator; External leads; Hermetic seal; Protective sheath; Ceramic powder; Current source

As VA - VB is proportional to RL so VB - VA - VB is proportional to RX where:

• RL = The resistance of each of the three RTDs leads

• RX = The measuring element of the RTD

• VA = The voltage supply to the RTDs measuring element RX from the constant current source

• VB = The final measured voltage, or output from the RTD (3-wire version).

The measurements are made sequentially, digitized and stored until differences can be computed. RTDs are reasonably linear in operation, but this depends to a great extent on the area of operation being used within the total span of the particular transducer in question.

++++Characteristics of thermocouples, RTDs IC and thermistor temperature sensors -- RTD Temperature Resistance Thermocouple Temperature Voltage Thermistor Temperature Resistance Temperature Voltage or current IC sensor

Self-heating problems associated with RTDs:

RTDs suffer also from an effect of self-heating, where the excitation current heats the sensing element, thereby causing an error, or temperature offset. Modern digital systems can overcome this problem by energizing the transducer just before a reading is taken.

Alternatively the excitation current can be reduced but this is at the expense of lower measuring voltages occurring across the transducers output, and subsequently induced electrical noise can become a problem. Lastly the error caused by self-heating can be calculated and adjustment made to the measuring algorithms.

Thermistors

These elements are the most sensitive and fastest temperature measuring devices in common use; unfortunately the price paid for this is terrible non-linearity and a very small temperature range.

Thermistors are manufactured from metallic oxides, and have a negative temperature coefficient, that is their resistance drops with temperature rise. They are also manufactured in almost any shape and size from a pin head to disks up to 25 mm diameter × 5 mm thickness.

Thermistor values, range and sensitivity:

Most thermistors have a nominal quoted resistance of about 5000 ? and because of their sensitivity, this base resistance is quoted at a specific temperature, reference having to be made to the relative type in the manufacturer's published specifications.

Thermistor values can change by as much as 200 ohm/°C which, in this case would give a maximum range of only +25 °C from the quoted base temperature.

IC sensors

Integrated circuitry sensors have only recently began to make their presence felt in the process control world. As such they are still limited in the variability of shape, size and packaging that is advisable. Their main advantages are their low cost (below $10.00) along with their linear and high output signals.

IC sensor ranges and accuracy:

As these sensors are formed from integrated silicon chips, their range is limited to -55 to +150 °C but easily have calibrated accuracies to 0.05-0.1°C.

Cryogenic temperature measurements:

An exception to the normal operating temperature range of IC sensors is a version that can be used for cryogenic temperatures -271 to +202 °C by the application of special diodes designed exclusively to operate at these sub-normal temperatures (absolute zero =-273.16 °C). 2.7.5 Selection of temperature transducer design and thermowells

Temperature measurement transducers, in particular thermocouples, need different housings and mountings depending on the application requirements.

Sensing devices are usually mounted in a sealed tube, more commonly known as a thermowell; this has the added advantages of allowing the removal or replacement of the sensing device without opening up the process tank or piping. Thermowells need to be considered when installing temperature-sensing equipment. The length of the thermowell needs to be sized for the temperature probe. Consideration of the thermal response needs to be taken into account. If a fast response is required, and the sensor probe already has adequate protection, then a thermowell may impede system performance and response time. Note that when a thermowell is used, the response time is typically doubled.

Thermowells can provide added protection to the sensing equipment, and can also assist in maintenance and period calibration of equipment.

Thermopaste assists in the fast and effective transfer of thermal dynamics from the process to the sensing element. Application and maintenance of this material needs to be considered. Regular maintenance and condensation can affect the operation of the paste.

++++three typical designs of thermocouple probes:

1. Open ended; subject to damage and should not be used in a hostile environment

2. Sealed and both thermally and electrically isolated from the outside world

3. Sealed but with thermal (and/or electrical) connection to the outside world.

++++Sectional views of three typical thermocouple probes

Radiation pyrometers

At the other end of the scale is the requirement to measure high temperatures up to 4000 °C or more. Total radiation pyrometers operate by measuring the total amount of energy radiated by a hot body. Their temperature range is 0-3890 °C. The infrared (IR) pyrometer is rapidly replacing this older type of measurement, and these work by measuring the dominant wavelength radiated by a hot body. The basis of this is in the fact that as temperature increases the dominant wavelength of hot body radiation gets shorter.

Developments in infrared optical pyrometry

Two recent developments in the world of pyrometry that should be mentioned are the utilization of lasers and fiber optics.

Lasers are used to automatically correct errors occurring due to changes in surface emissivity as the object's temperature changes.

Fiber optics can focus the temperature measurements on inaccessible or unfriendly areas. Some of these units are capable of very high accuracy, typically 0.1% at 1000 °C and can operate from 500 up to 2000 °C. Multiplexing of the optics is also possible, reducing costs in multi-measuring environments.

NEXT: Process measurement and transducers--part 3: Pressure transmitters

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