*#

*# Bi-metallic ThermometersTypes of Temperature Instrument Thermocouple (T/C) Resistance Temperature Detector (RTD) Thermistor Thermowell Filled Thermal Systems

*#Various Units of Temperature MeasurementC degrees Celsius (or Centigrade)F degrees FahrenheitK KelvinR RankineRelationship between different unitsC = (F - 32)/1.8F = 1.8 x C + 32 K = C + 273.15 R = F + 459.67

Conversion tables or software can be utilized to facilitatewith converting between these units.

*#Basic TheoryIn 1821 a German physicist named Seebeck discovered the thermoelectric effect which forms the basis of modern thermocouple technology. He observed that an electric current flows in a closed circuit of two dissimilar metals if their two junctions are at different temperatures. The thermoelectric voltage produced depends on the metals used and on the temperature relationship between the junctions. If the same temperature exists at the two junctions, the voltage produced at each junction cancel each other out and no current flows in the circuit. With different temperatures at each junction, different voltages are produced and current flows in the circuit. A thermocouple can therefore only measure temperature differences between the two junctions, a fact which dictates how a practical thermocouple can be utilized.Iron (Fe)Constantan (CuNi)100C0CThermocouple CircuitThermocouples (TCs)

*#Iron (Fe)Constantan (CuNi)100C20CEquivalent to 80C readingThermocouple measuring circuitCopper (Cu)Copper (Cu)Hot Junction:In ProcessCold Junction:Needs to be held constant to give a fixed reference. ( early methods held cold junction at 0C using ice or refrigeration unit).Thermocouples (TCs)

*#Standard Thermocouple Alloy Conductor CombinationsThermocouples (TCs)

CODECONDUCTOR COMBINATIONTYPICAL OPERATING RANGE FBPlatinum-30% Rhodium / Platinum-6% Rhodium+2500 to +3100CTungsten-5% Rhenium / Tungsten-26% Rhenium+3000 to +4200DTungsten-3% Rhenium / Tungsten-25% Rhenium+2800 to +3800ENickel Chromium / Constantan0 to +1650JIron / Constantan+0 to +1400KNickel Chromium / Nickel Aluminium0 to +2300NNickel-Chromium-Silicon / Nickel-Silicon-Magnesium1200 to +2300RPlatinum-13% Rhodium / Platinum1600 to +2600SPlatinum-10% Rhodium / Platinum1800 to +2600TCopper / Constantan-300 to +650

*#Thermocouples (TCs)A graph of temperature vs. voltage shows thermocouple characteristics are not perfectly linear.

*#Thermocouple ResolutionTemperature Change From 500 deg F to 510 deg F

TYPE500 OF510 OFDIFFC4.1404.2480.108E17.94518.3710.426J14.11014.4180.308K10.56110.7890.228R2.0172.0700.053S1.9622.0120.050T12.57412.8870.313

*#Thermocouple ConstructionArc Welded Junction(some are earthed at tip For improved response time)Sheath (normally stainless steel)Conductors insulated by Magnesium Oxide PowderNormally element is in a thermowellCommonly element is 1/4 outside DiameterSheath material, normally Stainless steel but can be special material such as Inconel, Incoloy, Hastelloy etc.Duplex thermocouples have 2 elements inside one sheath.

Thermocouples (TCs)

*#Thermocouple Tip TypesThermocouples (TCs)Ungrounded For use in corrosive and pressurized apps. Slow response time. Offers electrical isolation.Grounded For use in corrosive and pressurized apps. Quicker response time than ungrounded due to improved heat transfer.Exposed For use in dry, non-corrosive, non-pressurized apps. Quickest response time of all three.

*#Thermocouples (TCs)Response time comparision among the different thermocouple tip types.

*#RTDs (Resistance Temperature Detectors) operate under the principle that the electrical resistance of certain metals increases and decreases in a repeatable and predictable manner with a temperature change. RTDs

*#Thin Film Element Metallic ink is deposited onto a ceramic substrate. Lasers then etch the ink to provide a resistance path. The entire assembly is encapsulated in ceramic to support and protect.Wire Wound Element Precise lengths of wire are wrapped around a ceramic mandrel, then inserted inside a ceramic shell which acts to support and protect the wire windings.Inner Coil Element Wires are coiled then slid into the holes of a ceramic insulator. Some manufacturers backfill the bores with ceramic powder after the coils are inserted. This keeps the coils from shorting against each other.RTD Elements

*#2-wire: Should only be used with very short runs of leadwire. No compensation for leadwire resistance.

3-wire: Most commonly used for industrial applications. Leadwire compensation.

4-wire: Laboratory use historically, moving more into industrial applications. Full compensation for leadwire resistance.RTD Leadwire Configuration

*#The most common method for measuring the resistance of an RTD is to use a Wheatstone bridge circuit. In a Wheatstone bridge, electrical excitation current is passed through the bridge, and the bridge output current is an indication of the RTD resistance. Wheatstone Bridge

R1

R2

R3

AMMETER

RTD

*#The most common material is Platinum. Its resistance is 100 at 0Celsius. Hence the term PT100Its resistance is 138.5 at 100Celsius.Hence the Fundamental Interval of 38.5Or 0.385 per 1Celsius Rise in Temperature.There are other materials available for more unusual temperature ranges such as Germanium (e.g.10 to 100 Kelvin).RTDs

*#RTDs and T/Cs

Temperature Sensor Selection GuideRTDThermocoupleTemperature Range-328F to 1562F-310F to 3308FAccuracy0.001F to 0.1F1F to 10FResponse TimeModerateFastStabilityStable over long periodsNot as stable

*# RTD Thermocouple Type J & KTemp.C Grade B Grade A StandardPremium -200 1.10C 0.47C -100 0.67C 0.30C 0 0.25C 0.13C 2.2C 1.1C 100 0.67C 0.30C 2.2C 1.1C 200 1.10C 0.47C 2.2C 1.1C 300 1.50C 0.64C 2.3C 1.2C 400 1.90C 0.81C 3.0C 1.6C 500 2.40C 0.98C 3.8C 2.0C

RTD vs T/C Accuracy

*#Temperature Element Assembly

*#ThermowellsFlangedThreadedTapered ShankStep ShankWeld-inVan StoneStraight ShankAccessoriesPlugPlug with Chain

*#ThermowellsLagging ExtensionInsertion Length

*#Thermowell InstallationPIP Flanged Thermowell Installation RequirementsElbow InstallationPerpendicular Pipe Installation

*#Considerations for Thermowell selection:

Process temperatureEnvironment / Process mediaFluid or gas pressurePipe or vessel sizeFlow velocity

Thermowell Design & Material

*#Thermowells must be carefully selected for processes where significant velocity is present.By penetrating the process flow, the thermowell is subject to the stress and friction of the flow. This may set up a natural vibration that may result in the shearing off of the thermowell into the process. This is called the Wake Frequency.ASME PTC 19-3 Thermowells This Standard establishes a mechanical design standard for reliable service of thermowells in a broad range of applications. This includes an evaluation of the forces caused by external pressure, and the static and dynamic forces resulting from fluid impingement.Wake Frequency

*#fWake = fNaturalfWakefNaturalVorticesWake Frequency (fWake)Energy AbsorbedBy ThermowellSide ViewTop ViewResonanceConditionThermowell CalculationsEnsure that:

fNaturalfWake

*#Other TW Failure Modes:Process-Induced Bending StressFlowFDragVelocityDensityDiameterAreaLength

*#Thermowell Insertion Modification

TYPICAL THERMOWELL CONSTRUCTION

SHORTENED THERMOWELL CONSTRUCTION

STEPPED THERMOWELL CONSTRUCTION

*#Signal ConditionerLow level inputs mV from thermocouples from RTDsHigh level outputs 4-20mA current Digital (i.e. Fieldbus)Transmitters

*#Thermistors are temperature sensing devices that are similar to RTDs in that their resistance changes as temperature changes.

The major difference is that for most thermistors the resistance decreases as temperature increases.

Thermistors are an inexpensive alternative to RTDs when temperature ranges are below 150C. Thermistors can be used from temperatures of 80C to 300C.

Most thermistors have base resistances, which are much higher than RTDs.

One of the greatest advantages of using a thermistor sensor is the large change in resistance to a relatively small change in temperature. This makes them very sensitive to small changes in temperature.Thermistors

*#Bimetallic ThermometersA Bimetallic Thermometer consists of an indicating or recording device, a sensing element and a means for connecting the two.Coil rotation is caused by the difference in thermal expansions of the two metals.Bimetal CoilBasic example:Two metal strips expand at different rates as the temperature changes.A pointer is attached to the rotating coil which indicates the temperature on the dial.

*#Filled Thermal Systems

*#ISA MC 96.1 Temperature Measurement ThermocouplesPIP PCETE001 Temperature Measurement GuidelinesPIP PCFTE100 Thermowell Fabrication DetailsASME PTC 19.3 Temperature MeasurementInternet websites:Sensorsmag.comOmega.comIsi-seal.comSensortecinc.comWikipedia.orgRosemount.comReferences

*#QUESTIONSAny Questions???

ISA = International Society of Automation-Here are the types of temperature instruments that will be covered today.Thermocouple, Resistance Temperature Detector, Thermowell, Thermistor, Bi-metallic Thermometers, Filled Thermal Systems[Click for each type to appear. There are 6 items]-Shown are the various temperature units typically used.-C and F are the most commonly used unit in our industry.

-Below that are the equations used to convert between the different units.

-One notable fact is that the size of a degree in the Kelvin scale is the same as the size of a degree on the Celsius scale.Similarly the size of a degree in the Rankine and Fahrenheit scales are the same.

-The use of conversion tables or software to convert between units is preferred. This reduces error.Basic Theory-In 1821 a German physicist named Seebeck discovered the thermoelectric effect which forms the basis of modern thermocouple technology. He observed that an electric current flows in a closed circuit of two dissimilar metals if their two junctions are at different temperatures. -The thermoelectric voltage produced depends on the metals used and on the temperature relationship between the junctions. (This simple circuit is made of Iron and Constantan metals.)-If the same temperature exists at the two junctions, the voltage produced at each junction cancel each other out and no current flows in the circuit. -With different temperatures at each junction, different voltages are produced and current flows in the circuit. -A thermocouple can therefore only measure temperature differences between the two junctions, a fact which dictates how a practical thermocouple can be utilised.-Here is the same thermocouple circuit connected to a meter with copper leads-This combination (J-type thermocouple) is very widely used in the industry.-The Hot Junction is in the process-The Cold Junction is at the other end. This junction needs to be held constant to give a reference. Early methods used ice or refrigeration units to keep the junction at a constant 0C.-Today, transmitters provide cold junction compensation to prevent errors induced by variations in temperature at the cold junction.Here is a Table of Standard Thermocouple combinations and their operating temperatures in degrees C.-Here is a graph of temperature versus voltage for several thermocouple conductor combinations. Notice they are not perfectly linear.-The voltages induced are in the range of a few tens of millivolts, roughly from about 0 to 80 millivolts.-Thermocouples are usually protected by placing them into a metal sheath, usually made of stainless steel.-The conductors are then mineral insulated usually with Magnesium Oxide powder.-The element is normally placed into a thermowell for protection.-Elements are commonly 6 millimeters outside diameter.-When required, the sheaths can be of special materials, like Inconel, Hastelloy or other materials.-Duplex thermocouples have 2 elements inside one sheath. This is used for the purposes of redundancy.The different thermocouple tip types are Ungrounded, Grounded and Exposed.-Ungrounded tips are used in corrosive and pressurized applications. They have slow response times, but offers electrical isolation.-Grounded tips are used in corrosive and pressurized applications too. They have quicker response times than the ungrounded types due to improved heat transfer.-Exposed tips are used in dry, non-corrosive, non-pressurized applications. They offer the quickest response time of all three types.-Here is a graph of probe diameter versus time. This study was done using water.-The exposed tip has the quickest response time. But it has limited uses in our industry. Grounded tip is next, then Ungrounded is the slowest.-Notice the bigger the probe diameter the slower the response time for all tip types.An RTD or Resistance Temperature Detector operates under the principle that the electrical resistance of certain metals increases and decreases in a repeatable and predictable manner with a temperature change. As the temperature increases so does the resistance.There are 3 types of RTD constructions.In the Wire Wound Element RTD, precise lengths of wire are wrapped around a ceramic mandrel, then inserted inside a ceramic shell which acts to support and protect the wire windings.In the Inner Coil Element RTD, wires are coiled then slid into the holes of a ceramic insulator. Some manufacturers backfill the bores with ceramic powder after the coils are inserted. This keeps the coils from shorting against each other.In the Thin Film Element RTD, metallic ink is deposited onto a ceramic substrate. Lasers then etch the ink to provide a resistance path. The entire assembly is encapsulated in ceramic to support and protect.

This slide shows the types of RTD leadwire configurations.

The 2-wire configuration should only be used with very short runs of leadwire to minimize wire resistance. This type does not provide compensation for leadwire resistance.

The 3-wire configuration is most commonly used for industrial applications and provides some leadwire compensation.

Historically, the 4-wire configuration was used in the laboratory, but is moving more into industrial applications. This type provides full compensation for leadwire resistance.The most common method for measuring the resistance of an R, T, D is to use a Wheatstone bridge circuit. In a Wheatstone bridge, electrical excitation current is passed through the bridge, and the R, T, D and bridge output current is an indication of the R, T, D resistance. The circuit uses a very stable excitation power source, three high-precision resistors that have a very low temperature coefficient, and a high-input impedance amplifier to measure the resistance change of the R, T, D with changes in temperature.

-RTDs are most commonly made of Platinum and are sometimes referred to as Platinum Resistance Thermometers or PRTs for short.

-At 0Celsius, their resistance is 100, hence the term PT100.-At 100Celsius, their resistance is 138.5, hence the fundamental interval of 38.5 or 0.385 per 1Celsius rise in temperature.

-There are other materials available for more unusual temperature ranges such as Germanium (e.g.10 to 100 Kelvin).Here is a temperature element (sensor) selection guide. It shows the different characteristics for RTDs and thermocouples which can aid in sensor selection.-Thermocouples have larger temperature ranges than RTDs, but RTDs have better accuracy.-RTDs have great stability, linearity and sensitivity, but lag behind on response time when compared to the thermocouple.-In vibration applications, thermocouples are the best choice.

-Knowing what is important for a particular installation will help you make the correct sensor choice.-This table compares accuracies between RTDs and thermocouples.-Again, RTDs have better accuracy than thermocouples.This is what a completed assembly looks like.-There are several thermowell designs. Among the most popular are: 1) Flanged 2) Threaded 3) Van Stone 4) Weld-in-The flanged type is the most common. It is used in high pressure cases where well removal is required. The Van Stone is a sub-type of the Flanged.-Threaded is preferred is non-hazardous, low temperature and low pressure applications where well removal is not required often.-Weld-in type is used in high temperature and high pressure applications, such as steam services. And used where well removal is not required

-Some shank types used are: 1) Straight 2) Tapered 3) Step-These are labelled in blue text.-The tapered and step shanks provide improved heat transfer (thus quicker response) over the simple straight shank. The tapered design also performs better than the straight type in high fluid velocity applications due to having greater stiffness than the straight.

-The plug and chain option is used in testing installations (called test wells). These installations do not have an element inside at all times, so the plug is screwed into the well to keep it free from debris.This slide shows the standard design details of a flanged thermowell. Among the most important are the T dimension and the U dimension.

The T dimension is called the Lagging Extension. It extends the instrument connection past any pipe insulation that may be present.The U dimension is called the Insertion Length. This is the portion of the thermowell that goes into the process.P, I, P Practice P, C, F, T, E 100 provides guidelines for the installation and insertion length of the different types of thermowells. The U dimension provided, takes the nozzle length into account and will place the thermowell tip from 1/3 to 1/2 of the way into the process pipe line.Note that when the diameter of the line is less than 6 inches The PIP guideline recommends the thermowell be installed in an elbow.Things to considerations when selecting thermowells are:

Process temperatureEnvironment / Process mediaFluid or gas pressurePipe or vessel sizeFlow velocityWhen fluid flows past a thermowell inserted into a pipe or duct, vortices form at both sides of the well. These vorticies detach, first from one side, and then from the other. This phenomenon is known as the Von Karmann effect. The frequency of the shedding of these vortices is a function of the diameter of the thermowell, the fluid velocity and, to a lesser extent, the Reynolds number. The vortex shedding subjects the thermowell to a periodic transverse force. As the vortex shedding frequency approaches the natural frequency of the thermowell, the thermowell will oscillate, and is liable to snap off. This effect is most often caused in gas or vapor flow but can also happen in high velocity liquid flow when the liquid is of low density.

[A series of 6 clicks]-As the fluid flows past the thermowell, vortices begin to shed off. The frequency at which these vortices oscillate from side to side is called the wake frequency.-All themowells have an inherent natural frequency. When the wake frequency equals the thermowells natural frequency, a resonance condition occurs and the thermowell begins to vibrate in a destructive manner. -For a safe design, steps must be taken to ensure that the wake frequency is less than 80% of the natural frequency.

-Industry specification ASME PTC 19.3 (Temperature Measurement) is a good reference for the equations used in these calculations.[A series of 4 clicks]-Process induced bending stress is another concern in thermowell design.-The constant flow acting on a thermowell can exert a drag force strong enough to bend or break off the thermowell.-Some contributing factors that increase the drag force are increases in velocity and density. Large thermowell surface areas and lengths also increase this force.To mitigate the impact of wake frequency or process induced stresses it is often necessary to modify the thermowell insertion. This is usually done by reducing the insertion length (U dimension). In some cases this does not provide an adequate design margin. In those cases a stepped thermowell may be required. Always work with the thermowell vendor when you suspect a thermowell needs to be modified to meet process conditions.-Transmitters serve to condition the reading received from the element.

-Input to the transmitter is either a millivolt signal from a thermocouple or an ohms reading for an RTD.-The transmitter then puts out a high level output, either a 4-20 milliamp current or a digital signal.-Thermistors are temperature sensing devices that are similar to RTDs in that their resistance changes as temperature changes.

-The major difference is that for most thermistors the resistance decreases as temperature increases.

-Thermistors are an inexpensive alternative to RTDs when temperature ranges are below 150C. Thermistors can be used from temperatures of 80C to 300C.

-Most thermistors have base resistances, which are much higher than RTDs.

-One of the greatest advantages of using a thermistor sensor is the large change in resistance to a relatively small change in temperature. This makes them very sensitive to small changes in temperature.-What is a Bimetallic Thermometer?-It is a thermometer consisting of an indicating or recording device, a sensing element (called a bimetallic thermometer bulb) and a means for connecting the two.[Click once to show graphic]-Coil rotation is caused by the difference in thermal expansions of the two metals.[Click again]-A pointer is attached to the rotating coil which indicates the temperature on the dial.[Click again to show graphic]-Here is a basic example: Two metal strips expand at different rates as the temperature changes.[Click once more to show last graphic]-Filled thermal systems are another type of temperature instrument.-Principle of operation: A sensing element (bulb) contains a fluid which changes in physical characteristics with temperature. This change is communicated to the Bourdon tube through a capillary tube. The Bourdon movement provides as essentially linear pointer motion through mechanical linkages .[A series of 3 clicks.]-Here is the P&ID symbol for the filled thermal system.Here are the references used for this presentation.This completes the Control Systems Training Module 000.270.CSE156.1 Flow Instruments. Are there any questions?