Dynamically Self-Validating Contact Temperature Sensors

Transcription

1 Dynamically Self-Validating Cntact Temperature Sensrs Daniel A. Barberree AccuTru Internatinal Crpratin Abstract. Thermcuple and RTD technlgy is the wrkhrse f the temperature measurement industry. It has been refined and etended t cver a brad range f temperature measurement needs. Hwever, it is well dcumented, thugh nt widely advertised, that these measurement devices eperience drift r de-calibratin while in service. Fr varius reasns the sensr utput can drift away frm representing the true temperature. The magnitude f the drift depends n sensr type, cnstructin, installatin and prcess cnditins. The real prblem is that there has been n way t tell when drift begins r t determine its magnitude, r even its directin. Nw, Dynamically Self-Validating Sensrs have been invented that eliminate unreliable readings and warn in advance f the nset f drift. In this paper, the technlgy f Self-Validating Sensrs is eplained and data is prvided shwing the perfrmance f a Self-Validating sensr. INTRODUCTION Temperature is ne f the mst imprtant variables measured in industry. Frequently temperature measurements are critical t the utput f the prcess. Thrughput r capacity, quality, yield, energy efficiency, and emissins depend n reliable temperature measurements. In practice we never really measure temperature directly that is. Instead the sensrs we emply generate signals that depend n their temperature. The mst cmmn f these is the vltage prduced by a thermcuple when a temperature gradient eists alng its length. Als cmmn is the measurement f the resistance f a material that varies with temperature as in the case f RTDs. It wuld be nice if these signals all had a linear relatinship with temperature, but fr the mst part they d nt. Therefre we emply signal cnditiners that take the measured signal and cnvert it int an estimate f the temperature we are trying t determine. Thermcuples and RTDs cver a wide range f temperatures and prvide different degrees f reliability and accuracy. We have becme very gd at crrelating the vltage utput r resistance change f thermcuple and RTD designs with temperature using carefully frmulated standard materials and sphisticated plynmial relatinships imbedded in the signal cnditiners. Hwever, it is well knwn that the sensrs themselves are subject t change r de-calibratin when placed in service. This is smetimes called drift. While sme causes f drift r de-calibratin have been identified and are smewhat predictable like the psitive shift f type K r E thermcuples abve a certain temperature mst are nt. In fact all knwn attempts t predict the nset, magnitude and directin f drift in thermcuples have been unsuccessful. Drift in degrees C Drift f K and N Thermcuples Elapsed time in hurs Figure 1 reprts data frm Oak Ridge Natinal Labs [1] illustrating drift f varius types f thermcuples under cnditins f thermal stress. Dr. Richard Andersn and his clleagues wrked with many different sensr designs cnducting landmark eperiments that have demnstrated the prblems f 1

2 drift. The sensrs in this particular test are type K and type N thermcuples in Incnel 6 and 316SS sheaths held at 12 C. The spikes in the curves are the result f in place inhmgeneity testing where the tips f the sensrs are held at cnstant temperature while the temperature gradient is temprarily shifted alng the length f the sensr and then returned t its riginal psitin. Dr. Andersn and his clleagues cncluded that all thermcuples can begin drifting, anytime after being placed in service, and the directin and magnitude f the drift depends n many factrs and cannt be quantified r predicted. IMPORTANCE OF THERMOCOUPLES AND RTDS AND A NEW PARADIGM Industrial prcess sensrs tell the prcess cntrl center, the peratr r autmated prcess cntrller if the prcess is perating as designed, the health f the equipment, the cnditin f the emissins and mst imprtantly is the system in prper cntrl. There are many eamples where sensrs prviding misleading infrmatin have lead t cstly misfrtunes eplsins, fires, and unwanted releases. Misleading sensrs dn t always result in catastrphes, but inaccurate sensr data causes prcess inefficiencies csting industry millins if nt billins f dllars every year in less than ptimum perfrmance. Further advancements in sphisticated mdel-based cntrl and prgress tward plant management with minimal interventin requires highly reliable sensr data as cmpanies try t squeeze the mst ut f their investment with the fewest peple and perate as clse t equipment cnstraints as pssible. We have had t live with and cmpensate fr the uncertainties f sensrs fr s lng that many peple cnsider it a way f life that cannt be imprved upn. Sensrs that are truly Self-Validating will create a new paradigm. STRENGTHS AND WEAKNESSES OF THERMOCOUPLES AND RTDS Current thermcuple technlgy has a wellprmted set f strengths. Thermcuples are rugged, have a wide range f cnfiguratins, wide temperature ranges and are relatively inepensive. Majr advancements have been made in signal cnditiners used t prcess the measurements int an estimate f the temperature. Hwever, current thermcuple technlgy has a nt-s-well prmted set f prblems. One prblem is installatin errrs. If care is nt taken with hkups, cnnectins, insertin depths and eliminating utside interferences, significant errrs can be intrduced int the system that can g undetected. Even diligent users wh peridically check calibratins can be fled by stem effects, insertin depths and ther installatin issues [2]. Anther majr prblem is gradual de-calibratin f the sensr elements at unknwn time, rate and directin. Decalibratin cannt be quantified r remved by even the mst sphisticated signal cnditining units and smart transmitters that are available fr cnventinal sensrs. RTDs are prmted as having very high accuracies but they are als subject t de-calibratin. While the causes f de-calibratin f RTDs are different, they t are unpredictable and cannt be detected ecept by grss cmparisns and reasnableness checks versus ther sensrs. The range f temperatures in which RTDs can be used is als mre restricted than thermcuples. THE NEXT GENERATION OF SENSORS In a 1998 survey f industry prcess cntrl specialists and managers [4] the tw mst imprtant imprvements being lked fr in the net generatin f temperature sensrs were: Get rid f the drifting signals while the prbe is in service and Give me lnger useful sensr life. Eperts have begun predicting that ne day smart sensrs wuld be develped that wuld be capable f checking up n themselves. Prgress tward these bjectives has been accmplished by wrking n tw parallel bjectives. Objective 1: Develp means t enhance the stability and life f eisting thermcuples. This has led t the develpment f a new mineral insulatin material fr thermcuples and RTDs that in sme tests has demnstrated 3-4 times the life and greater signal stability in thermcuple sensrs versus thse made with magnesium ide (MgO). This new mineral insulatin material was discussed in a paper presented at ISA in San Dieg in 22 [5]. Objective 2: Devise a credible calibratin reference and implant it inside a sensr prbe that 2

3 is capable f prviding cntinuus calibratin while the prbe is in service. This has led t the develpment f the Self-Validating Sensr. SELF-VALIDATION Over the years many imprvements in thermcuple and RTD technlgy have been made with the gal f increasing the reliability and stability f the measurements. Imprved materials and strict standards have been develped fr perfrmance and als fr the cnstructin f sensrs. Advances in electrnics have led t better signal cnditiners that can even check themselves t be sure they are perating prperly. Despite these refinements, we have nt been able t reliably predict the behavir r perfrmance f sensrs when in service especially when they are used under cnditins that stress the materials. The challenge then was hw t cnstruct a sensr s that the perfrmance and health f its internal cmpnents culd be mnitred cntinuusly while in service. Self-Validatin, then, is the ability t measure the prcess variable, in this case temperature, with a high degree f cnfidence while at the same time mnitring whether the cmpnents prducing the signal representing that variable are stable r are shwing signs f impairment. We have termed it Dynamically Self-Validating, r simply a Self- Validating Sensr. SELF-VALIDATING SENSOR Like many inventins, the Self-Validating Sensr has evlved frm a set f knwn technlgies cmbined in a new way. psitined. In recent years, cnsiderable effrt has been devted t studying the effects f inhmgeneities in the materials. The primary mechanism fr thermcuple decalibratin is the inhmgeneity caused by a change in cmpsitin f the wires. This is frequently due t migratin f impurities within the sensr frm wire t wire r sheath t wire r impurities left in the mineral insulatin separating the wires frm ne anther. Changes in cmpsitin cause changes in the emf signal generated and adversely affect the estimate f the temperature. We knw frm Andersn s wrk [1] cited earlier that all thermcuples can drift when placed in service. We knw als that thermcuples made with different materials will drift in different ways. In fact, the prbability f tw thermcuples f different types in the same service drifting in eactly the same way at the same time in the same directin is very remte. As it turns ut this prblem can be very useful t us. The mechanisms fr RTD impairment are quite different. The measured variable in an RTD is resistance usually f a lng length f wire ciled up in the tip f the sensr. Here the temperature prfile alng the wires cnnected t the cil at the tip has a minr effect n the resistance and can in fact be eliminated by using etra wires and measuring and eliminating the small effects. The crrelatin f the resistance f a material and temperature is ften mre linear and easier t use in accurately estimating the temperature. RTDs with very high initial accuracies can be made and they can be quite stable if treated carefully. Hwever, RTDs can becme impaired as well. Fr eample, if current leakage ccurs acrss the leads errrs in the resistance measurement will ccur. This is primarily what limits the temperature at which they can be used. Relative t thermcuples, RTDs are FIGURE 2. Patent diagram shwing Sensr Schematic (Fig. 3) and Signal Cnditiner (Fig. 4). Thermcuples derive a temperature measurement using Seebeck s bservatin that when tw dissimilar electrically cnductive materials are jined at ne end and that end is maintained at a different temperature than the pen end, a vltage r emf is generated acrss the pen end. Further Seebeck bserved that that vltage culd be crrelated with the magnitude f the difference in temperatures f the tw ends. We nw knw that the emf is nt generated at the junctin f the tw materials, but rather alng the length f the tw materials. This makes it very imprtant t have materials that have a cnsistent cmpsitin frm ne end t the ther s that the same signal is generated regardless f where the temperature gradient is 3

4 smewhat slwer in respnse and als smewhat fragile and therefre susceptible t impairment frm damage due t vibratin. The Self-Validating Sensr [3] cmbines these tw technlgies, taking advantage f the strengths and weaknesses f each. Figure 2, taken frm the riginal patent, shws that there are really tw parts t the system a sensr prbe and a signal cnditiner with special capability t diagnse the sensr. THE SVS SENSOR PROBE The first cmpnent f this new technlgy is the uniquely designed SVS prbe. Currently there are three SVS mdels. The SVS/411 is designed fr lw temperature peratin ( 2 C t 4 C) The SVS/2311 replaces thermcuples in the type K range (ambient t 126 C), and the SVS/3212 replaces thermcuples in the range up t 175 C. The SVS prbe is available in bth metal and ceramic sheaths. Eternally the SVS prbe lks and feels like a thermcuple r RTD and can be a direct replacement in mst prcesses. It is cnstructed using similar techniques. It is rugged and rbust. In metal sheaths it bends, welds and cnfigures just like a typical thermcuple. Hwever, imbedded in the tip f the SVS prbe is a specially designed Calibratin Reference Matri (CRM). The CRM prvides the infrmatin needed t bth develp an accurate temperature estimate and cntinuusly mnitr the health f the prbe while in service. FIGURE 3. SVS Prbe Tip. Figure 3 depicts the sensr Calibratin Reference Matri (CRM) in the tip f the prbe. The primary sensr is actually a part f the CRM. The CRM is a cmbinatin f thermally sensitive materials r thermelements chsen s that they can generate multiple independent signals indicating the temperature at the tip f the prbe and als prvide mnitring f the cnditin f each element. Refer t the referenced Patent [3] fr mre n prbe cnstructin. Measurement f a thermelectric prperty f ne pair f materials such as a vltage r impedance is said t be ne channel f data. As nted earlier, there are a number f thermelement pairs that are cnsidered standards. A typical SVS prbe can have ne r mre standard pairs f thermelements. Further, because f the ability t stre multiple crrelatins in the SVS transmitter, sme nn-standard pairs can als be crrelated and used. An SVS prbe can generate 15 r mre different channels f temperature and health data. This is accmplished by reading the thermelectric vltage r impedance prperties f the varius thermelement cmbinatins. Only 3 r 4 independent channels are actually needed fr sme degree f self-validatin and health mnitring, but in a typical SVS Prbe 6 t 12 channels are used. Using a larger number f independent channels t develp the temperature estimate imprves the accuracy f the estimate and the life f the prbe. Each measured signal frm a pair f materials is calibrated acrss the entire temperature range f interest and crrelated using up t 9 th rder plynmials. Each cmbinatin r pair f materials is als characterized r ft-printed acrss the temperature range using mre than ne f its electrical prperties, such as vltage and impedance. During calibratin, the relatinships f these signals t ne anther fr each cmbinatin f materials and t the signals frm ther cmbinatins at any given temperature are captured. This is saved as a cnfiguratin file fr the particular sensr and is stred in the flash memry f the signal cnditiner electrnics. After being placed in service, if the relatinships f these prperties remain the same as the day the prbe was calibrated, its perfrmance will be the same as the day it was calibrated. Any change in the relatinships f these prperties indicates a change frm its calibratin day cnditin. As the sensr ages the behavir f the thermelements may begin t change. It is the change in relatinships f these prperties that is used t determine the nset f decalibratin. That is, ne r mre f the elements f the sensr prbe are beginning t shw signs f decalibratin. The bttm line is that n element in the sensr can de-calibrate nr signal degrade withut detectin. In practice, as lng as the changes are minr and within selected tlerances f accuracy the cnfidence level in the measurement remains very high. Since n tw thermelements in the sensr are the same, when de-calibratin ccurs it is pssible t tell which f the elements is shwing signs f impairment because each f the elements is being mnitred in cmbinatin 4

5 with multiple ther elements. Thse signals generated by the cmbinatins which include that particular element will begin t deviate frm their initial ft prints and frm the established relatinships with ther signals. It is then pssible t either de-weight the cntributin f signals using that element r eliminate it altgether while cntinuing t generate the temperature estimate with the remaining healthy elements. Statistically the cnfidence in the estimate f the temperature has then been reduced but can still be very high depending n the design f the prbe and the particular remaining healthy elements. Mre imprtantly, the peratr can be warned f the nset f de-calibratin while the prbe is still yielding reliable readings. THE SVS TRANSMITTER Signal measurement, multipleing and cnditining are dne in the Self-Validating Sensr (SVS) Transmitter/Signal Cnditiner. Figure 4 illustrates part f the faceplate f an SVS Transmitter shwing shwn that the nrmal user des nt want a lt f statistical infrmatin but is primarily cncerned that the reprted temperature is reliable within certain limits r if the sensr needs t changed ut. Therefre, t quantify and reprt the Health cnditin f a sensr, a statistically based calculatin similar t a 95 % cnfidence interval was develped and translated int an verall scalar value representing the level f cnfidence in the measurement and its Health. Tlerance bands arund each individual signal measurement and arund the verall cnfidence level can be chsen that give the user apprpriate advance warning f the need t replace the prbe. Health is cmmunicated using three clrs. An unimpaired sensr is said t be in the Green r likenew cnditin. When the nset f de-calibratin has been detected, the health indicatr is changed t a Yellw r cautinary cnditin. The cnfidence level in the measurement will still be very high and cnfidence interval still small, hwever smething inside the sensr has shwn sufficient signs f impairment t becme cncerned. A Red cnditin indicates significant impairment and the sensr shuld be replaced as sn as pssible. The SVS System prvides anther benefit in that any ther impairment f the signals being measured can als be detected. Fr eample, a crrding r lse cnnectin causing impairment f the signals will be detected by the system. Als unfiltered electrical nise r anything else that degrades the signals and changes their relatinships t ne anther utside the prescribed tlerances will be detected by the validatin electrnics and be reflected in its Health indicatin. FIGURE 4. Face plate f SVS Transmitter. the temperature display. The transmitter cntains analg t digital signal prcessing electrnics, a specialized validatin mdule and a sensr cnfiguratin file. It cntinuusly prvides bth the temperature and the health f the prbe. Tgether with the SVS Prbe, this is a new class f temperature device. SENSOR HEALTH Diagnstics perfrmed by the transmitter prduce the reprt f Sensr cnditin r Health. The Health f Self-Validating Sensrs will be n dubt be defined and displayed in many clever ways as they becme mre cmmnplace. Elegant statistical and mathematical analysis is pssible. Eperience has Temperature and health are nrmally sent t a remte peratr interface using varius digital transmissin prtcls. A simple way t display the temperature and health data simultaneusly n an peratr interface is by displaying the temperature n a green, yellw r red backgrund. LEDs n the frnt f the signal cnditiner prvide lcal health indicatin alng with the lcal temperature readut. The breakpints fr Green/Yellw/Red cnditins can be chsen fr any specific applicatin depending n the degree f variatin the user wuld like t allw. But f curse there is a trade ff f sensr life with the tightness f the tlerances. Once the tlerances bands are defined, the Green cnditin indicates the temperature reprted is reliable within the prescribed tlerances f true and the same as if it were measured the day the SVS prbe was calibrated. A Yellw cnditin is a cautinary cnditin indicating that the SVS prbe is still reprting a temperature within the 5

6 prescribed tlerances but smething inside the sensr is shwing sufficient signs f impairment t be cncerned. The system shuld be investigated fr pssible cnnectin prblems r electrical interference and finding nne, it is time t begin planning t replace the sensr prbe. A Red cnditin indicates that the SVS transmitter n lnger has enugh gd data t validate the temperature within the prescribed tlerances. It is nw functining like a typical thermcuple r RTD. The sensr might still be giving a temperature indicatin but the measurement cannt be validated. In summary: Green = Healthy NIST Traceable Temperature Yellw = Cautin NIST Traceable Temperature, but signs f impairment have been detected. Red = Warning Temperature reprted cannt be validated. This infrmatin is bviusly very useful fr predictive maintenance f sensr prbes. Deviatin frm 12 deg C SVS 2311 vs. Ordinary Type 12 C 1 Type K #3 Standard Limits 5 Special Limits SVS Type K #2 Type K #1 SVS 2311 Health Status Indicatr Green Time at 12 C, Hrs FIGURE 5. SVS Sensr Cmparisn Data. Yellw COMPARISON DATA Figure 5 shws an eample f perfrmance f an SVS2311 sensr prbe (range f peratin t 125 C) in an accelerated stress test versus three type K sensrs which are recmmended fr abut the same temperature range. Test cnditins were 12 C, in air, cntrlled by a NIST traceable type S reference. The test samples were all ¼ OD in Incnel 6 sheaths. Three type K special limits thermcuples frm three different manufacturers and an AccuTru SVS 2311 prbe were used in the test. The test prtcl stated that tw cnsecutive daily readings utside f standard limits f errr are grunds fr rejectin f the type K thermcuples. The SVS measurement was t be rejected when a Yellw Cnditin (the nset f de-calibratin) was detected. The results frm this test shw that the SVS sensr indeed warned in advance (yellw cnditin) befre deviating and actually eceeded the life f the cnventinal type K sensrs in the test. Furthermre, the SVS reprted temperature was within ±1.5 C fr the life f the sensr at 12 C. This is remarkable perfrmance fr a metal-sheathed sensr at these cnditins. Additinal research and plant test data n SVS prbes are available frm the authr. SUMMARY AND CONCLUSIONS A new class f sensrs that can be dynamically validated while in service and can warn in advance f their wn de-calibratin r impairment has been develped. The implicatins f this technlgy fr prcess peratrs are significant. As this field epands t encmpass many types f sensrs, prcess peratrs f the future will nt have t accept the measurement uncertainties f the past and will be psitined t achieve anther significant step frward in prcess ptimizatin. REFERENCES 1. Andersn, R.L., et al., Decalibratin f sheathed thermcuples, in Temperature: Its Measurement and Cntrl in Science and Industry, edited by J. F. Schley, Vl. 5, American Institute f Physics, N.Y., 1982, pp Nichlas, J. V., and White, D. R., Thermcuple Thermmetry, Traceable Temperatures, Jhn Wiley & Sns Ltd., Chichester, England, US Patent # 5,713,668 issued Feb 3, Survey f Prcess Cntrl Prfessinals, AccuTru internal survey, Barberree, D. A., The Net Generatin f Thermcuples fr the Turbine Engine Industry, in Prceedings f the 48 th Internatinal Instrumentatin Sympsium, ISA Vl. 42, May, 22. 6