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.

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