Industrial and laboratory thermometry including the definition of the InternationalTemperature Scale between the oxygen and antimony points, from
−
297 to 1167
°
F(
−
183 to 631
°
C)
Alpha Curve:
The relationship between the resistance change of a resistance temperature detector(RTD) vs. temperature. In the European alpha curves, the alpha value is 0.00385
Ω
/
°
C; in American curves it is 0.00392.
Ranges:
−
328 to 1532
°
F (
−
200 to 850
°
C) is IEC standard for platinum RTDs and
−
330 to600
°
F (
−
200 to 320
°
C) for nickel RTDs, Practical applications are usually limited to
−
328 to 1000
°
F (
−
200 to 537
°
C) (refer to Tables 4.10b and 4.1c).
Spans:
10 and 1100
°
F (6 and 610
°
C) for platinum, and 20 and 600
°
F (12 and 340
°
C) fornickel. For differential temperature transmitters: 10 and 50
°
F (6 and 28
°
C) for plat-inum, and 20 and 100
°
F (12 and 56
°
C) for nickel.
Stability:
Zero and span drift is usually within 0.1% of span for a 6-month period (and frequentlylonger).
Standard Resistances:
100
Ω
at 32
°
F (0
°
C) is most common, but elements are available from 10 to 1000
Ω
.
Linearity:
Platinum and copper are more linear; nickel and nickel/iron (Balco
TM
) are less so.The use of gold and silver RTDs is limited to cryogenic temperatures.
Sensitivity:
0.1 to 10
Ω
/degree
Inaccuracy:
The error just for a Class A sensors is from
±
0.06
°
F (
±
0.03
°
C) or 0.01% of span andfor a Class B sensor it is from
±
0.6
°
F (
±
0.3
°
C) or 0.12%. In determining the totalsystem error, one must also consider the error contribution of the signal conditioningelectronics. The total system error is usually 0.15% of span for platinum and 0.25%of span for nickel RTD elements in standard transmitters. Intelligent transmittersreduce that to 0.05% of span or to about 0.18
°
F (0.1
°
C), whichever is higher.
Costs:
Elements alone range from about $35 to $80; RTD assemblies, including thermowells,are from $100 to $250. An RTD sensor with an integral temperature transmitter maycost anywhere from $300 to $2000 depending on its level of intelligence and otherfeatures.
Partial List of Suppliers of
ABB (www.abb.com)
Intelligent Transmitters:
Foxboro/Invensys (www.foxboro.com/temp/)Honeywell (www.iac.honeywell.com/ichome)Kamstrup (www.kamstrup-process.com)Moore Industries-International, Inc. (www.miinet.com/products/ca)Rosemount/Emerson (www.rosemount.com/products)Siemens (www.sea.siemens.com)Yokogawa (www.yokogawa.com)
Thermo Electric (www.thermo-electric.com)Transcat (www.transcat.com)Transmation (www.transmation.com)TSI Inc. (www.tsi.com)TTI (www.ttiglobal.com)Truck (www.truck.com)United Electric Controls (www.ueonline.com)Validyne (www.validyne.com)Watlow (www.watlow.com)Weed Instrument (www.weedinstrument.com)Wilkerson Instrument (www.wici.com)Winters Instruments (www.winters.com)Yokogawa Corp. of America (www.yca.com)Vendor list: (www.temperatures.com/rtdvendors.html)Most popular: Rosemount-Emerson, Pyromation, Omega
In 1821 Sir Humphry Davy discovered that the resistivity ofmetals depends on their temperature. The application of thisproperty using platinum was first described by Sir WilliamSiemens at the Bakerian Lecture of 1871 before the RoyalSociety in Great Britain. The necessary temperature limita-tions and methods of construction were established by Cal-lendar, Griffiths, Holborn, and Wein between 1885 and 1900.
RESISTANCE TEMPERATURE DETECTOR BASICS
Resistance thermometry is based upon the increasing electri-cal resistance of conductors with increasing temperature(Figure 4.10a). Resistance temperature detector (RTDs) areconstructed of a resistive material with leads attached andusually placed into a protective sheath. The resistive materialmay be platinum, nickel, or copper, with the most commonby far being platinum. Platinum resistance thermometers arenow the international standard for temperature measurementsbetween the triple point of hydrogen at 24.86
°
R (13.81 K)and the freezing point of antimony at 1167.35
°
F (630.75
°
C).The laboratory application of platinum resistance thermom-eters recognizes the unsurpassed stability and repeatabilityof this noble metal sensor. Platinum resistance thermometersfor rugged industrial applications also retain their advantageover other conductors.
Detectors and Their Applications
The conductors used for resistance thermometry includeplatinum, nickel of various purities, 70% nickel/30% iron(Balco), and copper, listed in order of their decreasing tem-perature range (Tables 4.10b and 4.1c). These conductorsare all available as fine wire for sensor winding. For manyyears wire-wound construction, where a fine highly purifiedplatinum wire was wound around a ceramic mandrel to
FIG. 4.10a
The temperature coefficient of platinum is 0.00392 ohms/ohms
°
C at0
°
C (the alpha value in the USA) and 0.00293 at 800
°
C. The alphavalue used in Europe is the average value between 0 and 100
°
C,which is 0.00385. Therefore (as shown), a platinum element of 1ohm resistance at
−
200
°
C will increase its resistance to approxi-mately 4.0 ohm as the temperature rises to 800
°
C.
2
−200 0 200 400 600 800 °C
4Nickel
Copper
Platinum
6
8
Ω/Ω
TABLE 4.10b
RTD Material Properties of 0
°
C and Available Temperature Ranges
MetalTemperature Coefficient of Resistance (Ohms/Ohm
°
C)Electrical Resistance Per Circular Mil Foot (Ohms)
produce the element, was the most common type. Wire-wound sensors are still the better choice for very high, verylow, or widely cycling applications.
Platinum is also available as a deposited film sensor, whichis much less expensive, and platinum, nickel, and Balco are allavailable in foil-type sensors. Thin film construction uses aprecision laser to cut the sensor elements from a layer of plat-inum deposited over a stable ceramic base material. The sensorhas very low mass and is therefore highly resistant to vibrationand has a much faster speed of response than a wire woundsensor. All share in varying degrees the characteristics of repeat-ability, high temperature coefficient, long-term stability, andlinearity over a useful temperature range. While no sensor mate-rial surpasses platinum in overall performance, each has at leastone characteristic that may encourage its selection.
The application of the standard platinum RTD as a tem-perature standard (SPRT), differs considerably from industrialpractice.
1
The temperature standard is 25.5
Ω
at the ice pointto stay within the range of practical Mueller bridges whileproviding a nominal 0.1
Ω
/
°
C sensitivity. Wire-type platinumthermometers were constructed in a manner to be almosttotally strain-free, using very lightly supported wire of largersize (and cost) than typical in an industrial thermometer(Figure 4.10c).
Temperature coefficients near the theoretical maximumfor pure platinum and maximum thermal stability can beobtained at the expense of fragility and large size. The indus-trial platinum RTD offers ruggedness at a negligible loss intemperature coefficient compared to the standard SPRT,
while retaining the best stability and repeatability availablein industrial thermometers over a wide temperature range.
INDUSTRIAL RTD CONSTRUCTION REQUIREMENTS
All resistance thermometers require the following consider-ations in their manufacture. Wire-wound sensors must besupported on mandrels closely matching the wire in thermalexpansion to minimize strain effects. Additional assemblymaterials, such as cements, should not introduce additionalstrain in the operating temperature range. The final assemblymust be in a stable, annealed condition, trimmed to therequired resistance tolerance. Only high-purity materials andclean assembly methods should be used to avoid sources ofcontamination that might degrade the sensor.
All internal connections should be welded, and connect-ing leads should be chosen for the required temperature capa-bility and avoidance of thermoelectric junctions. To realizethe ruggedness of fully supported elements in the total sensorassembly, all internal connections should be anchored andisolated from effects of thermal and mechanical strains,including shock and vibration. The same requirements applywhen deposited film or foil-type resistance elements are used(Figure 4.10c).
For equivalent performance in their respective tempera-ture ranges, base metal RTDs cost the same as 100 ohmplatinum RTDs. Construction requirements and materials costare similar. Base metal RTDs have a materials cost advantageat higher resistance values compared to wire-wound platinumsensing elements. Thin film platinum elements erase thisadvantage.
Platinum RTDs
In the case of platinum RTDs, the full supported ruggedconstruction using “reference grade” wire has a temperaturecoefficient (alpha) over the interval 32 to 212
°
F (0 to 100
°
Cof between 0.00387 and 0.003915
Ω
/
Ω°
C, depending on themanufacturer. Compared to 0.003927
Ω
/
Ω°
C on a SPRT, thereduction in sensitivity is insignificant. For best accuracy,the user should be aware of or specify the actual temperaturecoefficient. One common value available from most manu-facturers is 0.003902
Ω
/
Ω°
C. This is the result obtained forwindings on pure alumina mandrels.
The relationship between the resistance change of an RTDvs. temperature is referred to as its alpha curve. The most com-monly used is the European curve that has an alpha of 0.00385
Ω
/
Ω°
C. The American curve has an alpha of 0.00392
Ω
/
Ω°
C.The instrument used with the RTD must be configured to usethe same alpha curve as the RTD or serious errors will occur.
Standard platinum industrial alpha curves (based onslightly doped platinum wire) have been adopted by mostEuropean countries. These curves are all substantially basedon an alpha of 0.00385
Ω
/
Ω°
C. This result is reproducibleby manufacturers everywhere, and the so-called interna-tional grade platinum curve is the most widely used curve,
Fig. 4.10c
RTD elements can be made by winding platinum wire on a glass orceramic bobbin and sealing with molten glass (A), by threading aplatinum helix through a ceramic mylinder (B), or by depositing aplatinum film on a ceramic substrate. (A) is limited by the straininduced at higher temperatures, (B) is not suited for extreme vibra-tion, and (C), while small, fast, and inexpensive, is also less stablethan the others.
even in the United States. An exception is temperaturesbelow
−
320
°
F (
−
196
°
C) (LN2), where only reference gradewire RTDs are well characterized.
Platinum RTDs using thick or thin films are also avail-able to the same curve as international grade wire-woundsensors. Performance is often equivalent to wire-wound sen-sors except maximum temperature may be reduced. Wire-wound platinum RTDs are most common at 100 ice pointresistance, with 200 and 500
Ω
available at additional cost.Using thin films, ice point resistances of 100 and 1000
Ω
are available at the same cost with slightly lower alphaspecified at 1000
Ω
.
Base-Metal RTDs
Second in usage to platinum is high-purity nickel, whichoffers the highest temperature coefficient, second-highesttemperature range, and lower assembled cost than wire-wound platinum at high resistance values. Nickel RTDs havedeclined in use over the years primarily due to their limitedrange vs. the more popular platinum RTDs.
The useable range is
−
112 to 608
°
F (−80 to 320°C). Mosttransmitters and alarm trips still offer the capability to acceptnickel RTD inputs. The most common are 120 and 500 Ωresistance, with 1000 Ω available. Nickel is not linear, becauseit increases its temperature coefficient as temperature rises(Figure 4.10a). Nickel is also highly strain-sensitive andrequires great care by the manufacturer to obtain good inter-changeability. The temperature coefficient of nickel is highlyinfluenced by both purity and state of anneal. In addition, lowerpurity nickel, such as 99% nickel (no longer available) or ballastnickel, has been used and provides a somewhat lower temper-ature coefficient.
The maximum temperature of nickel sensors should notbe much more than 500°F (260°C). There is no internation-ally recognized standard curve for nickel sensors, althoughthere are national standards, and several manufacturers in theUnited States can provide sensors to a common curve char-acterized by an alpha temperature coefficient between 32 and212°F (0 and 100°C) of 0.00672 Ω/Ω°C.
Balco Third in usage is the 70% nickel/30% iron alloy tradenamed Balco. The sole basis for its use is a very high specificresistance, which makes possible the production of very highresistance windings without increasing size. Ice point resis-tances are commonly 200 or 10,000 Ω. It has the second-highest temperature coefficient, third-highest temperaturecapability, and, like pure nickel, is nonlinear with an upwardbending R vs. T curve. There is no recognized standard curvefor Balco sensors.
Copper Last in usage is pure copper, which is generallyavailable only at 10 or 100 Ω ice point resistance values dueto the low specific resistance of the winding wire. CopperRTDs are most commonly used to sense the winding tem-perature of motors, generators, and turbines. Connectingthem to an alarm trip provides an over-temperature shut down
function. Historically 10 Ω copper RTDs were the norm, butmany users have now opted for 100 Ω or even 1000 Ω unitsto get higher resolution. They have a useful range of −58 to482°F (−50 to 250°C).
Copper’s temperature coefficient is almost the same asplatinum and it is very linear above the ice point. Copper inbifilar windings (Figure 4.10c) is used in electrical machin-ery due to very low inductive or capacitive reactance, butplatinum can also be used. Some traditional applicationshave also exploited the linearity of copper sensors in makingnarrow-range temperature-difference measurements wheretwo sensors are connected on opposed arms of a bridge.There is no internationally recognized standard curve forcopper, although some national standards do exist.
Relative performance of industrial temperature sensorsof all types is shown in Table 4.10d. RTDs, especiallyplatinum RTDs, are clearly superior for temperature mea-surement requirements below 1000°F (540°C).
MEASURING THE RTD RESISTANCE
By measuring the resistance of the RTD element one candetermine the process temperature if the change in totalresistance measured is affected by nothing but the processtemperature. In actual installations, the RTD element is con-nected by two, three, or four lead wires to the readout ortransmitting instrument.
Two-Wire RTDs
There are very few applications for a two-wire RTD sincethe error introduced by the leads can cause significant error.If a null-balance bridge is used, the circuit can be as shownin Figure 4.10e. Here the temperature coefficients of resis-tors R1, R2, and R3 are nearly zero, and the value of R3 isadjusted until the current flow of G or the voltage read bya digital voltmeter (DVM) is zero. Under these conditionsthe value of R3 equals the unknown resistance of the mea-suring leg (A + B + RTD).
Assuming that the resistance of the RTD (at 0°C) is 100Ω and assuming that the element is platinum, the resistanceof the 100 Ω RTD elements will change by 0.385 Ω/°C. If500 ft (152 m) of 20 gauge copper lead wire was used toconnect the RTD to the bridge, this adds 10 Ω to the totalresistance (5 Ω/leg). At null balance R3 = A + B + RTD =10 Ω + RTD. With a temperature coefficient of 0.385/°C theseextra 10 Ω will contribute a measurement error (as anincrease in process temperature) of 10/0.385 = 26°C.
The previous example illustrates the relatively large lead-wire error in a two-wire RTD installation, and for this reasonsuch installations are not used if accurate temperature mea-surement is desired and the length of the lead wires is morethan a few inches. When the transmitter is mounted directlyon the thermowell (Figure 4.10f), the lead-wire length is sosmall that the resulting error is not very significant. Yet even
in these configurations most suppliers will provide a three-wire RTD to minimize the lead-wire error.
Three-Wire RTDs
Measurement circuits that accept three-wire inputs minimizethe effects of lead wire resistance as long as the outer legsare equal. However factors such as terminal corrosion andloose connections can still create significant differencesbetween the lead resistances seen by the measurement circuit,because only one ohm of difference between the legs resultsin an error of 4.7°F (2.6°C).
As shown in Figure 4.10g, the lead wire C acts as asense lead and is part of both halves of the bridge andtherefore cancels out at balance. The lead wires A and Bare in different halves of the bridge and therefore at nullbalance R3 = B − A + RTD. Therefore, now the lead-wireerror is no longer the total lead resistance (A + B), but onlythe difference between their resistances (B − A).
This is a major improvement in reducing the lead-wireerror and is sufficient for the needs of most industrial appli-cations where the lead-wire lengths are short. However, it isnot a complete solution because wire resistances are guaran-teed only within a 10% tolerance; therefore, if A and B areidentical wires of identical lengths, their resistances can stilldiffer within the 10% tolerance. So if nominally they bothare 5 Ω, in reality one could be 4.5 and the other 5.5 Ω. Ifthis were the case, the difference of 1 Ω would still introducean error. With a 100-Ω platinum RTD that error would cor-respond to 1/0.385 = 2.6°C.
If the purpose of the temperature measurement is to cal-culate the exothermic heat release of a batch reactor, thiserror might still be too much. In that application the temper-ature rise through the reactor jacket is about 5°F and the spanusually selected for the differential temperature transmitteris 10°F (5.6°C). In order to identify the end point accurately,the total heat release must be determined to within 0.5%maximum error. Because the total heat release is calculated
TABLE 4.10dComparison Chart of Various Temperature Sensors
Evaluation Criteria
Platinum RTD100 Ω Wire Wound andThin Film
Platinum RTD 1000 Ω
Thin Film
Nickel RTD 1000 Ω
Wire Wound
Balco RTD 2000 Ω
Wire Wound Thermistor Thermocouple
Semi- conductor Devices
Cost-OEM Quantity High Low* Medium Medium Low* Low* Low*
Temperature Range
Wide −400°F to +1200°F (−240°C to +649°C)*
Wide –320°F to +1000°F (−196°Cto +538°C)*
Medium −350°F to +600°F (−212°C to+316°C)
Short −100°F
to +400°F (−73°C to +204°C)
Short to Medium−100°F to +500°F(−73°C to +260°C)
Very Wide−450°F to +4200°F (−268°C to +2316°C)**
Short −57°F to +257°F(−49°C to+125°C)
Interchange-ability
Excellent* Excellent* Fair Fair Poor to Fair Good* Fair
Long-Term Stability
Good* Good* Fair Fair Poor Poor to Fair Good to Fair
Accuracy High* High* Medium Low Medium Medium Medium
Repeatability Excellent** Excellent** Good* Fair Fair to Good Poor to Fair Good*
Sensitivity (Output)
Medium High* High* Very High** Very High** Low High*
Response Medium Medium to Fast*
Medium Medium Medium to Fast*
Medium to Fast*
Medium to Fast*
Linearity Good* Good* Fair Fair Poor Fair Good*
Self-Heating Very Low to Low*
Medium Medium Medium High N/A Very Low to Low*
Point (End) Sensitive
Fair Good* Poor Poor Good* Excellent** Good*
Lead Effect Medium Low* Low* Low* Very Low** High Low*
by multiplying coolant flow with its temperature rise, theflowmeter itself will contribute 0.25% in error and thereforeone must measure the temperature rise within 0.25%.
An error of 0.25% over an actual measurement of 5°F is0.0125°F (0.007°C). This is such a small error limit that even
three-wire RTD transmitters may not meet it (their usual errorlimit is about 10 times higher). For this reason, in laboratorysituations or for other high-precision measurements, onemight consider the use of four-wire systems, which com-pletely eliminate the lead-wire effect.
Four-Wire RTDs
Using a four-wire measuring circuit eliminates the aboveproblem. The design engineer should consider any of theleading brands of temperature transmitters that accept four-wire RTD inputs. Direct connection to remote devices withthree-wire extension cable will often produce errors that canbe significant and will vary with environmental conditions.
Four-wire RTDs can be connected either to a null-balancebridge or to a constant current source. Both will be describedhere. Figure 4.10h illustrates a four-wire null-balance bridge.It operates by switching a triple-pole double-throw switch andmaking alternate null-balance measurements in the two con-figurations. In one configuration, lead A is measured togetherwith the RTD resistance, while in the other configuration it is
FIG. 4.10e Null-balance bridge-type two-wire RTD installations showing agalvanometric (G) readout in a balanced condition (left) and a DVMreadout in an unbalanced condition (right) where the DVM readsV0 volts.
FIG. 4.10f RTD transmitter mounted directly on thermowell. (Courtesy of TheFoxboro Co.)
Wheatstone Bridges(R1= R2)
Vs
VsA
B RTD
A
B RTD
G+−
Balanced Bridge: Galvanometer I = 0R1 + R3 = R2 + A + B + RTDR3 = A + B + RTD
Unbalanced Bridge = Digital Voltmeter Reads VoVo = Vs(R3/R3 + A + B + RTD) - Vs/2R3(Vs-2Vo)/(Vs+2Vo) = A + B + RTD
+−
DVM
Vo
R1
R2
R3
R1
R2
R3
Allow 150 mm (6 in.) forCover Removal
Basic TransmitterPackage
1/2 NPT for TerminalConnections. Two Holes180° Apart. Plug Unused Hole.
Coupler
Insertion “U”
3/4 NPT, 1 NPT,R 3/4 or R1
mmin1214.8
26710.5
893.5
1044.1
26710.5
Max.251.0251.0
642.5
11.1 mm (0.44 in.) OD with6.60 mm (0.260 in.) Bore
FIG. 4.10g In a three-wire null-balance bridge, the lead-wire effect is reducedto the difference between the resistances of the two lead wires (B − A).
FIG. 4.10h If the leads of a four-wire null-balance bridge are switched as shownand the resulting two equations are added up, the lead-wire effectsare eliminated and the resistance of RTD = (R3a + R3b)/2.
Vs
A
B
When bridge is balanced: R1 + R3 + A + C = R2 + B + RTD + CIf R1 = R2 this becomes: R3 = RTD + B + A = RTD
RTD
DVM
Vo = 0 C
R1 R3
R2
Vs
Vs
A
B
RTD
DVM
Vo = 0
Vo = 0
CD
R1 R3a
R3b
R2
R1
R2
Switch in Position “A”R1 + R3a + A + C = R2 + B + RTD + CWhen R1 = R2:R3a + A = B + RTD
Switch in Position “B”
R1 + R3b + B + D = R2 + A + RTD + DWhen R1 = R2:R3b + B = A + RTD
lead B, so they cancel out completely and the actual value ofthe RTD resistance is determined as (R3a + R3b)/2.
Microprocessors and advanced electronics make it fea-sible to provide this level of sophistication, but complexitystill costs money, so these designs are relatively expensive;in addition, they are still limited by contact resistanceconsiderations. Even the best (gold-plated) switching con-tacts contribute some contact resistance. The differencebetween these resistances does introduce some minisculeerrors whenever one uses a switching configuration to makea resistance measurement.
Another way to eliminate the lead-wire error is to use aconstant current source (CCS) in a four-wire RTD configu-ration. These miniaturized CCS packages are available atrelatively low costs and provide an accurately constant cur-rent flow of about 2 mA or less to avoid self-heating errors.As shown in Figure 4.10i, in this configuration the bridgeitself is replaced by a DVM, which measures the resistanceof only the RTD and is insensitive to lead-wire effects asthere is no current flow through the connecting wires. Thesource lead resistances (A and B) contribute no error becausethe voltage drop is not measured along them.
For the maximum in precision, it would be prudent to makesure that the current flow (Ic) through the RTD is constant andthat the DVM does draw any current (i = 0), and also to cancelout the thermocouple (TC) junction voltages at points #1 and#2. This is necessary because as the two wires (platinum RTDand copper lead) at #1 and #2 form TC junctions, the milli-voltages they generate will also be registered by the DVM.This effect is eliminated by offset compensation. The offsetvoltage generated by the unintended TC junctions is measuredby the DVM when the CCS circuit is opened and, therefore,Ic = 0. The smart RTD readout memorizes the voltage sensedwhen no current is flowing and corrects the total reading bythat amount when the CCS is connected and Ic is about 2 mA.
In general, two-wire RTDs are only used in heating,ventilation, and air conditioning (HVAC)-type secondaryapplications, three-wire RTDs are still used in some process-ing industries, and four-wire RTDs are used in most high-precision services or in the laboratory.
SENSOR CONSTRUCTION
Figure 4.10c illustrates some of the RTD sensor elements.The most popular industrial designs are fully encapsulated.In these units a 0.001 in. (0.025 mm) diameter or smallerplatinum wire is wound into a coil and is inserted in themultiple bores of a ceramic tube or is wound directly on theoutside of a ceramic tube. The ceramic material is usually99.7% pure aluminum oxide, and the winding is completelyembedded and fused onto or into the tube. The RTD ther-mometers are in direct competition with TCs and thereforeare available with the same features.
The elements are manufactured with a protective sheaththat provides a hermetic seal to protect the sensor from mois-ture and/or contamination. These protective sheaths are offeredin a variety of lengths to provide the proper insertion into theprocess to obtain a representative measurement. The sheathedelements are often installed into a protective well to isolate thesensor from the process. One manufacturer offers a universalmodel that has a 1-in.-long sheath but is provided with longleads within a spiraled spring (Figure 4.10j). The leads can becut in the field to fit the length of the thermal well, reducing therequirement for stocking a variety of sensors in varying lengths.
FIG. 4.10i Offset compensated four-wire RTD measurement using CCS andDVM as readout.
A
#1
#2
i=0
RTD
DVMCCS
Ic
B
FIG. 4.10jRTD transmitter with a 1 in.-long sheath element and with longleads within a spiraled spring, which can be cut to match the ther-mowell length. (Courtesy of Moore Industries-International, Inc.)
Figure 4.10k illustrates a conventional industrial RTD sensorprovided with a protective thermowell and fixed length leads.Figure 4.10l shows two designs of surface-temperature-sensingRTDs. Other surface mounting configurations are illustrated inFigures 4.1t and 4.1v. The packaging of the RTD sensordepends on the application. For example, the measurement oftemperature inside glass-lined chemical reactors requires spe-cial configurations. Figure 4.10m shows an RTD element thatcan be installed either through the bottom discharge valve ofthe reactor or at the tip of the reactor baffle. The 63.2% responsetime to a step change is about 7.5 s; the 95% response time isabout 28 s.
One limitation of the early RTDs (relative to TCs) wassize. RTDs tended to be relatively bulky, because in order toobtain the required resistance (usually 100 Ω), the length ofthe sensor wire must be relatively long, frequently severalfeet. This limitation has been overcome by the film-typedesigns (Figure 4.10c), which are suited for miniaturization.
The 63.2% time constants of different RTDs (withoutthermowells) are given in Figure 4.10n. One might approxi-mate the time constant of the final assembly by doubling thetime constants for each layer of material added in the processof installation.
Installation
In terms of installation, RTDs require the same precautionsas TCs. The best installation practice is to place all electronicsdirectly on top of the thermowell (Figures 4.10f and 4.10j)
FIG. 4.10k Industrial RTD/thermowell assembly.
RTDLead Seal
RTDProbe Sheath
Insulated LeadsPacked in MgO
RTD SensingElement
Subassembly
ConnectionHead
TerminalBlock
RemovableRetainer
Spring LoadedMounting Fitting
Thermowell
FIG. 4.10l RTD surface temperature sensors.
0.25"
Coiled PTWire Element inCeramic Case
Bare 30 AWGPlatinum Leads
0.18"
0.31"
Thin Film PTElement in
Ceramic Case
Insulated28 AWG Stranded
Copper LeadsFIG. 4.10m RTD sensor for glass-lined reactors. (Courtesy of Pfaudler.)
and thereby eliminate lead-wire and noise effects. If for somereason this cannot be done, the lead wires should be twistedand shielded; the wires should also not be stressed, strained,or made to go through steep gradients. The extension wireshould be low resistance (large diameter), and the readoutinstrument should be guarded.
For further discussions on the general topics of surfaceand solids temperature measurement, refer to Section 4.1 andFigures 4.1t and 4.1v.
TRANSMITTERS
The typical performance capabilities of different RTD trans-mitters are summarized in Table 4.10o. In general, platinum
sensors provide better accuracy and less span shift than donickel elements. Also, while intelligent transmitters givebetter performance than do standard ones, they too are lim-ited to a minimum error of about 0.18°F (0.1°C). Table 4.10oalso shows that the smart transmitters that have digital out-puts provide better performance.
Intelligent Transmitters
In addition to having improved performance, intelligent trans-mitters are capable of working with any one of eight types ofTC or two types of RTD elements. This increases their flex-ibility and reduces the need for spare parts. The intelligenttransmitters are also provided with continuous self-diagnositcsand with automatic three-point self-calibration, which is per-formed every 5 s and does not interrupt the analog or digitaloutput of the unit.
A transmitter includes an input circuit referred to as ananalog-to-digital (A/D) converter that converts the sensorinput signal from its analog form into a digital representationfor presentation to the microprocessor. The microprocessorperforms all of the mathematical manipulations of ranging,linearization, error checking, and conversion. The outputstage accepts the resultant digital representation of the currentvalue of the measurement and converts the signal back to ananalog signal (D/A) that is typically a 4–20 mA DC current.For some special applications, 0–1 or 0–10 V DC signalsmay be used and in others the signal is transmitted digitallyusing either an open or proprietary protocol. Some countrieshave adopted 020 mA as the standard transmitted signal.
The intelligent RTD transmitter can also be furnishedwith dual RTD elements that can be used to measuretemperature differentials, averages, or high/low sensors,or used as redundant backup elements. This capability alsoallows for automatic RTD sensor switchover if the primarysensor fails in a redundant installation. Due to terminal
FIG. 4.10n Response time of typical resistance temperature sensor.
Probe or Element Outside Diameter — in. (mm)
0.08(2.0)
0.10(2.5)
0.12(3.0)
0.14(3.6)
0.16(4.0)
0.18(4.6)
0.20(5.1)
0.22(5.6)
0.24(6.1)
0.26(6.6)
0.28(7.1)
0.32(8.1)
0.30(7.6)
Tim
e C
onst
ant —
Sec
63.2
% in
Wat
er a
t 3 F
t/S
ec
0.4
0.6
0.8
1
2
4
6
8
10
20
High Temperature Probes
Probes To 250˚C or Metal Clad Elements
Ceramaclad Elements
TABLE 4.10o Performance Capabilities of Standard and “Smart” RTD Transmitters*
STANDARD SMART
Performance Criteria Platinum Element Nickel Element Digital OutputAnalog Output (4−20 mA DC)
Inaccuracy ±0.15% or ±0.15°F (0.08°C)
±0.25% ±0.035% or±0.18°F (0.1°C)
±0.05% or ±0.18°F (0.1°C)
Repeatability ±0.05% ±0.05% ±0.015% or ±0.18°F (0.1°C)
±0.025% or ±0.18°C (0.1°C)
Zero Shift/6 mo. ±0.1% ±0.2% ±0.06% R or 0.18°F (0.1°C)
±0.1% R or 0.18°F (0.1°C)
Span Shift/6 mo. ±0.1% ±0.4%
Supply Voltage Variation ±0.2% or 0.02°F (0.01°C) — (0.005%)/Volt
Ambient Effect (100°F or 55°C)
±0.75% Included above ±0.1%
*When two values are given the error is the higher of the two. When % is given it refers to % of span or % of calibrated span, except if %R isshown, which means % of actual reading.
limitations, these models can only accept dual three-wireRTDs. Caution must be used to minimize lead resistancedifferences to reduce the error. Another convenient featureof smart transmitters is their remote reconfiguration capa-bility, which can change their zero, span, or many otherfeatures without requiring rewiring.
The common features of the leading temperature trans-mitter models are: universal inputs from any TC, RTD, mV,resistance or potentiometer; loop-powered with 0–20/4–20mA output; digital outputs; and configuration with push but-tons, personal computer (PC) software, or a handheld con-figurator. Choices must be made for which protocol isrequired: Highway Addressable Remote Transducer (HART),Foundation Fieldbus, Profibus, vendor proprietary, Ethernet,or just 420 mA.
A/D Converters, Digital Protocols
Just as microprocessors have evolved in sophistication, sohave A/D converters. Eight-bit resolution devices commonin the 1960s provided a resolution of about ±0.4%. In theyear 2000 the first 21-bit resolution A/D was used in a tem-perature transmitter providing a resolution of ±0.00005%.D/A converters have also evolved with resolutions increasingfrom 8-bit up to the 18-bit versions used in the better trans-mitters beginning in 2000.
The result of combining these technologies is a univer-sal transmitter that accepts inputs from any TC, RTD, mV,resistance or potentiometer signal; checks its own calibrationon every measurement cycle; has minimal drift over a wideambient temperature range; incorporates self-diagnostics;and is configured using push buttons or simple PC software.The reconfiguration process is quick and convenient, and ittends to allow for lower inventories by making the transmit-ters interchangeable.
In the 1980s Rosemount developed the HART protocolto enable detailed information about the set-up and operationof the device to be superimposed onto the 4–20 mA signal.During this same time period a variety of proprietary protocolsemerged supported by many of the larger manufacturers thatprovided comparable benefits of remote setup and diagnos-tics. Unlike the HART protocol, these products were limitedto use within the manufacturer’s system. In the 1990s a trendemerged for more open protocols and Foundation Fieldbusand Profibus evolved the leaders of these groups.
ADVANTAGES AND LIMITATIONS
RTDs are among the most accurate, reproducible, stable, andsensitive thermal elements available. Some of the precisionplatinum RTDs can measure within a few thousandths of adegree, and this precision is the reason such instruments areused to define parts of the International Temperature Scale(ITS-90.)
Other advantages include their relatively good sensitivity(0.1 to 10 Ω/°F) and their ability to use conventional copperlead wire (instead of more expensive TC wire). An advantageof copper RTDs is that since both the element material andthe lead-wire material are the same, the TC effect is mini-mized at their junction. Another advantage of RTDs is theconvenience of using a single bridge to measure the temper-ature difference between two RTDs.
A concern common to all RTDs is the error produced byself-heating. Measuring the voltage across an RTD producedby passing a precise current flow through the RTD producesheat that will appear as a positive offset over the actualprocess temperature. The size of this error rises with RTDsize and its resistance. It can be reduced by improving heattransfer and by minimizing or eliminating (null-balance) thecurrent flow through the RTD. The lower is the measuringcurrent; the less is this self-heating effect. It is reduced bygood thermal contact with the process fluid and when mea-suring higher temperatures. Circuits in better transmitters useabout 250 µA.
Other disadvantages of RTDs include their higher cost,more fragile construction, and larger size, relative to TCs.Because of their size, their thermal response time is alsorelatively slow (Figure 4.10n). Errors can be introduced if theRTD insulation resistance is affected by moisture beingsealed in the sheath or by contact between element andsheath. Some RTDs are more vibration-sensitive than others.RTDs are also dependable for their precision on stable (insen-sitive to temperature changes) and constant resistances andpower supplies in the associated bridges.
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