Tem­per­a­ture Mea­sure­ment for Con­di­tion Mon­i­tor­ing

The anal­y­sis of ma­chine vi­bra­tions usu­al­ly re­quires the record­ing of slow­ly chang­ing state vari­ables (or op­er­a­tional data), ad­di­tion­al­ly to the vi­bra­tion data. Only in com­bi­na­tion, these dif­fer­ent types of mea­sure­ment data  pro­vide a com­pre­hen­sive pic­ture of the cur­rent over­all ma­chine con­di­tion. Tem­per­a­ture is an im­por­tant ex­am­ple of such a state vari­able, es­sen­tial­ly for three rea­sons:

De­tec­tion of dam­age and un­fa­vor­able op­er­at­ing Points

Me­chan­i­cal fric­tion and pres­sure lead to the con­ver­sion of me­chan­i­cal en­er­gy into heat, e.g., in bear­ings, gears or hy­draulic sys­tems. This is es­pe­cial­ly true in the case of faulty or dam­aged ma­chine com­po­nents: They often lead to a lo­cal­ly in­creased heat input and thus a mea­sur­able rise in tem­per­a­ture. Hence, con­tin­u­ous tem­per­a­ture mon­i­tor­ing can help to de­tect prob­lems and pre­vent dam­age. Fur­ther­more, lu­bri­cat­ing and hy­draulic oils should not leave an op­ti­mum tem­per­a­ture range in order to main­tain their favourable char­ac­ter­is­tics. The con­tin­u­ous mea­sure­ment of the oil tem­per­a­ture can de­tect a de­vi­a­tion from the per­mis­si­ble range and coun­ter­mea­sures for cool­ing/heat­ing can be ini­ti­at­ed.

Con­ver­sion of chem­i­cal en­er­gy re­quires good ther­mal man­age­ment

In many ma­chines, chem­i­cal en­er­gy is con­vert­ed into me­chan­i­cal or elec­tri­cal en­er­gy, e.g. in com­bus­tion en­gines, gas tur­bines or elec­tric cars (there in the bat­ter­ies or fuel cells). In all cases, heat is gen­er­at­ed that has to be re­moved for the con­tin­u­ous op­er­a­tion of the ma­chine. This re­quires well planned ther­mal man­age­ment, which can be ef­fec­tive­ly val­i­dat­ed and mon­i­tored by tem­per­a­ture mea­sure­ments.

Elec­tri­cal ma­chines also ex­pe­ri­ence sig­nif­i­cant heat input

The same ap­plies to elec­tri­cal com­po­nents, such as elec­tric mo­tors or gen­er­a­tors, ex­cept that the rea­son for the heat input here - in ad­di­tion to me­chan­i­cal fric­tion - are cur­rent flow or mag­net­ic hys­tere­sis loss­es of com­po­nents.

Ther­mome­ter Er­rors

Chal­lenges in tem­per­a­ture Mea­sure­ments

The most com­mon mea­sure­ment prin­ci­ple for de­ter­min­ing tem­per­a­tures is based on bring­ing a sens­ing el­e­ment into ther­mal con­tact with the mea­sured ob­ject. In doing so, the sens­ing el­e­ment adapts to the tem­per­a­ture of the mea­sured ob­ject and gen­er­ates an elec­tri­cal mea­sure­ment vari­able from which we can con­clude the tem­per­a­ture of the sens­ing el­e­ment - and thus of the mea­sured ob­ject. Ba­si­cal­ly, two types of er­rors al­ways occur, which are of vary­ing im­por­tance de­pend­ing on the ap­pli­ca­tion and the mea­sur­ing de­vice used:

Type I Error

Since the sens­ing el­e­ment adopts the tem­per­a­ture of the mea­sured ob­ject only after a cer­tain pe­ri­od of time, the mea­sured tem­per­a­ture al­ways lags some­what be­hind the tem­per­a­ture to be mea­sured. This time lag, char­ac­ter­ized by a time con­stant, be­comes short­er the lower the trans­duc­er's heat ca­pac­i­ty and the bet­ter the ther­mal con­tact with the mea­sured ob­ject is.

Type II Error

If the trans­duc­er is not ex­clu­sive­ly in ther­mal con­tact with the mea­sured ob­ject, it can  adopt a tem­per­a­ture that dif­fers from the one of the mea­sured ob­ject. For ex­am­ple, in high-tem­per­a­ture ap­pli­ca­tions heat trans­fer by ra­di­a­tion plays an im­por­tant role. When de­ter­min­ing the hot gas tem­per­a­ture in a com­bus­tion cham­ber, it can hap­pen that the trans­duc­er is not only in ther­mal con­tact with the gas, but also - via ra­di­a­tion - with the cool­er com­bus­tion cham­ber walls. As a re­sult, the tem­per­a­ture mea­sured is sys­tem­at­i­cal­ly too low. This ef­fect can be mit­i­gate, for ex­am­ple, by mir­ror­ing the probe or by at­tach­ing a ra­di­a­tion shield.

The Selec­tion of Sen­sors

In in­dus­try, one en­coun­ters main­ly three types of elec­tri­cal tem­per­a­ture mea­sure­ment sen­sors: re­sis­tance ther­mome­ters, ther­mis­tors and ther­mo­cou­ples. All of them have in­di­vid­u­al ad­van­tages and dis­ad­van­tages and are there­fore suit­able for dif­fer­ent ap­pli­ca­tions.

Re­sis­tance Ther­mome­ters/Re­sis­tance Tem­per­a­ture De­tec­tors - RTD

This type of sen­sor ex­ploits the fact that the re­sis­tance of metal­lic con­duc­tors is tem­per­a­ture-de­pen­dent. Tem­per­a­ture mea­sure­ment is thus traced back to a re­sis­tance mea­sure­ment. A metal con­duc­tor, which has a de­fined re­sis­tance for a ref­er­ence tem­per­a­ture, serves as the sens­ing el­e­ment. The so-called Pt-100 re­sis­tance ther­mome­ter, which has a plat­inum (chem­i­cal Pt) sen­sor with a re­sis­tance of 100 Ohms at 0°C, is a wide­ly used ex­am­ple. Due to the rel­a­tive­ly low sens­ing el­e­ment re­sis­tance of RTDs, the re­sis­tance of the wires con­nect­ing the sens­ing el­e­ment to the mea­sure­ment de­vice can have a sig­nif­i­cant in­flu­ence on the mea­sured value. In order to com­pen­sate this in­flu­ence, in­stead of a sim­ple 2-wire mea­sure­ment, often the more com­plex 3- or 4-wire method is used.

Ad­van­tages:

  • Long-term sta­ble
  • Large mea­sure­ment range
  • Rel­a­tive­ly lin­ear tem­per­a­ture be­hav­ior
  • Pre­cise (es­pe­cial­ly at high tem­per­a­tures)
  • Solid against elec­tro­mag­net­ic noise
  • Long ser­vice life

Disad­van­tages:

  • Ex­pen­sive
  • Slow re­ac­tion times/large time con­stant
  • Com­par­a­tive­ly large size
  • Low trans­duc­er re­sis­tance, which means that long con­nec­tion ca­bles af­fect the mea­sure­ment re­sult

Typ­i­cal ap­pli­ca­tions:

  • Pre­cise mea­sure­ments at high tem­per­a­tures (process mon­i­tor­ing  and con­trol)
  • Mea­sure­ments under elec­tro­mag­net­ic in­flu­ences (gen­er­a­tors, high volt­age sys­tems)

 

 

Ther­mis­tors: NTC and PTC

Like RTDs, this type of sen­sor is based on re­sis­tance mea­sure­ment to de­ter­mine tem­per­a­ture. Con­trary to RTDs, how­ev­er, ther­mis­tors are based on semi­con­duc­tor met­als. A dis­tinc­tion is made be­tween (1) PTC - Pos­i­tive Tem­per­a­ture Co­ef­fi­cient - ther­mis­tors, whose re­sis­tance in­creas­es with tem­per­a­ture, and (2) NTC - Nega­tive Tem­per­a­ture Co­ef­fi­cient - ther­mis­tors, whose re­sis­tance de­creas­es with tem­per­a­ture. Due to their high­er re­sis­tance and their sig­nif­i­cant­ly larg­er dy­nam­ic range, main­ly NTC ther­mis­tors are used for tem­per­a­ture mea­sure­ment. Sil­i­con-based lin­ear PTC ther­mis­tors are an ex­cep­tion, but they are rel­a­tive­ly new and there­fore not yet very com­mon.

Ad­van­tages:

  • High pre­ci­sion
  • Short re­sponse times/small time con­stant
  • High re­sis­tance which al­lows the use of longer ca­bles
  • Re­sis­tant to shock and vi­bra­tion
  • Fa­vor­able price

Disad­van­tages:

  •  Only for low tem­per­a­tures with small tem­per­a­ture ranges
  • Strong­ly non-lin­ear tem­per­a­ture be­hav­ior

Typ­i­cal ap­pli­ca­tions:

  • Eco­nom­i­cal and pre­cise tem­per­a­ture mea­sure­ment at low tem­per­a­tures (<130°C)
  • Large quan­ti­ties pos­si­ble at small re­quired space (house­hold ap­pli­ances, med­i­cal tech­nol­o­gy, au­to­ma­tion)

Ther­mo­cou­ples

This type of sen­sor con­sists of two metal­lic con­duc­tors of dif­fer­ent ma­te­ri­al, which are con­nect­ed to each other at the mea­sure­ment point. At a ref­er­ence junc­tion, a volt­age can then be mea­sured be­tween the two con­duc­tors, which is pro­por­tion­al to the tem­per­a­ture dif­fer­ence be­tween the mea­sure­ment point and the ref­er­ence junc­tion. Know­ing the ref­er­ence junc­tion tem­per­a­ture, the ab­so­lute tem­per­a­ture at the mea­sure­ment point can be de­ter­mined from this dif­fer­en­tial tem­per­a­ture. The re­quired ref­er­ence junc­tion tem­per­a­ture is usu­al­ly mea­sured with the aid of an RTD. Depend­ing on the ma­te­ri­al com­bi­na­tions, sen­si­tiv­i­ties and mea­sur­ing ranges of the re­spec­tive ther­mo­cou­ples vary. The most im­por­tant com­bi­na­tions are stan­dard­ized ac­cord­ing to DIN EN 60584-1 and are marked with a cap­i­tal let­ter, e.g. "K" for chromel (NiCr) and nick­el (Ni) or "J" for iron (Fe) and cupron­ick­el (CuNi). More­over, for easy iden­ti­fi­ca­tion of the ther­mo­cou­ple type, con­nect­ing ca­bles have dif­fer­ent color codes.

Ad­van­tages:

  • Cover a large tem­per­a­ture range 
  • Suitable for very high tem­per­a­tures 
  • Short re­sponse times/small time con­stant
  • Re­sis­tant to shock and vi­bra­tion
  • Fa­vor­able price

Disad­van­tages:

  • Com­par­a­tive­ly in­ac­cu­rate
  • Sig­nal qual­i­ty de­creas­es over time
  • Prone to elec­tro­mag­net­ic noise due to small mea­sure­ment sig­nals
  • Com­plex sig­nal pro­cess­ing nec­es­sary
  • Re­quires cold junc­tion com­pen­sa­tion

Typ­i­cal ap­pli­ca­tions:

  • Mea­sure­ment of very large tem­per­a­ture ranges at high tem­per­a­tures
  • Func­tion­al even under harsh con­di­tions (an­neal­ing and fir­ing fur­naces, process mon­i­tor­ing)

Mea­sure­ment Sys­tem with In­te­grat­ed Tem­per­a­ture Input Mo­d­ule

No mat­ter which choice you take - one so­lu­tion for all cases

The IfTA tem­per­a­ture mea­sure­ment card TI4 of­fers a care­free pack­age for tem­per­a­ture mea­sure­ment: To each of Its 4 elec­tri­cal­ly iso­lat­ed in­puts RTDs, ther­mis­tors or all com­mon ther­mo­cou­ples can be flex­i­bly con­nect­ed. It sup­­ports all com­­mon meas­ure­­ment prin­­ci­ples, for ex­am­ple: 2-, 3- and 4-wire meas­ure­­ment for RTDs, high-pre­­ci­­sion cold junc­­tion com­pen­s­a­­tion using Pt-100 res­ist­ance ther­­mo­met­ers as well as sup­­port for dif­fer­­en­­tial meas­ure­­ments for ther­­mo­­cou­ples, or ac­cur­ate pro­­cess­ing of non-lin­ear be­hav­ior for ther­mis­tors. In our fa­mil­iar graph­i­cal user in­ter­face all mea­sure­ment set­tings can be ad­just­ed con­ve­nient­ly and saved in con­fig­u­ra­tion files for later use or ex­change with col­leagues.

Rec­om­mend­ed Prod­ucts

TI4 Tem­per­a­ture Card

Tem­per­at­ure meas­ure­­ment by ther­mo ele­­ments or res­ist­ance ther­­mo­met­ers.

Ar­gusOMDS

Pro­tec­tion sys­tem with di­ag­nos­tics and mon­i­tor­ing func­tion­al­i­ty.

Dy­naMaster

Di­ag­nos­tic tool for high-speed anal­y­sis & in­tel­li­gent vi­su­al­iza­tion.

TrendView­er

Fast & in­tu­itive on­line/off­line anal­y­sis soft­ware for ef­fi­cient vi­su­al­iza­tion.