What Is a Pt100 Sensor? A Complete Guide

What Actually Is a Pt100?

Strip away the jar­gon and a Pt100 is sim­ple: it uses a plat­inum sens­ing ele­ment whose resis­tance changes with tem­per­a­ture. ​“Pt” is plat­inum. ​“100” means it reads 100 ohms at 0°C. That pre­dictable resis­tance change is what makes it so use­ful — your trans­mit­ter or con­trol sys­tem con­verts that resis­tance into a tem­per­a­ture val­ue, and because plat­inum behaves so con­sis­tent­ly, the read­ing is accu­rate and repeatable.

It’s a type of RTD (Resis­tance Tem­per­a­ture Detec­tor), but not all RTDs are Pt100s. You can get nick­el and cop­per RTDs too, though they drift more over time. Plat­inum is chem­i­cal­ly inert and incred­i­bly sta­ble, which is why it dom­i­nates indus­tri­al tem­per­a­ture mea­sure­ment despite cost­ing more than the alternatives.

How It Works (Without the Textbook Waffle)

As tem­per­a­ture ris­es, the atoms in the plat­inum ele­ment vibrate more, mak­ing it hard­er for elec­trons to pass through. Resis­tance goes up. Cool it down and resis­tance drops. The rela­tion­ship is near­ly lin­ear and described by the Cal­len­dar-Van Dusen equa­tion — but you don’t need to wor­ry about that unless you’re doing your own cal­i­bra­tion.

In prac­ti­cal terms: 100 ohms at 0°C, rough­ly 138.5 ohms at 100°C, rough­ly 175.9 ohms at 200°C. Your trans­mit­ter han­dles the maths.

Two-Wire, Three-Wire, or Four-Wire?

This is where we see the most spec­i­fi­ca­tion mis­takes. The wires con­nect­ing your Pt100 to the trans­mit­ter have their own resis­tance, and if you don’t account for it, you’ll get a read­ing that’s con­sis­tent­ly too high.

Two-wire is the cheap­est but least accu­rate. The lead resis­tance gets added to your mea­sure­ment. Fine for short cable runs where you can tol­er­ate ±1°C or worse, but we rarely rec­om­mend it for process applications.

Three-wire is the indus­try stan­dard for most instal­la­tions. A third wire acts as a ref­er­ence to com­pen­sate for lead resis­tance. It’s not per­fect — it assumes both leads have iden­ti­cal resis­tance — but it’s good enough for the vast major­i­ty of indus­tri­al work.

Four-wire uses sep­a­rate cur­rent and volt­age-sens­ing leads to min­imise lead-resis­tance error. It does not estab­lish sen­sor class or guar­an­tee ±0.1°C sys­tem accu­ra­cy: the ele­ment, instru­ment, cal­i­bra­tion and instal­la­tion all contribute.

Choose the wiring to suit the instru­ment and allow­able mea­sure­ment error. Class A can use three- or four-wire mea­sure­ment; assess lead-resis­tance mis­match and the com­plete mea­sure­ment chain.

Accuracy Classes — What They Actually Mean

IEC 60751 tol­er­ance lim­its below are in °C, with |T| the absolute numer­i­cal tem­per­a­ture in °C. Each class applies only with­in its spec­i­fied tem­per­a­ture range for the select­ed construction:

Class B: ±(0.30 + 0.005 × |T|) °C.

Class A: ±(0.15 + 0.002 × |T|) °C.

Class AA: ±(0.10 + 0.0017 × |T|) °C. ¹⁄₃ DIN is not an inter­change­able name for Class AA. Frac­tion­al DIN descrip­tions are sup­pli­er spec­i­fi­ca­tions; con­firm the tol­er­ance and tem­per­a­ture range when ordering.

One thing to note: these tol­er­ances apply at the point of man­u­fac­ture. Over time, mechan­i­cal stress, con­t­a­m­i­na­tion, or ther­mal cycling will cause drift. That’s why recal­i­bra­tion mat­ters — and why the sen­sor con­struc­tion and ther­mow­ell pro­tec­tion are just as impor­tant as the accu­ra­cy class print­ed on the spec sheet.

Temperature Range

The usable range depends on the com­plete probe. H&B’s pub­lished RTD301 datasheet lists low-tem­per­a­ture (-196°C to +100°C), stan­dard (-50°C to +250°C) and high-tem­per­a­ture (-50°C to +650°C) options. These are alter­na­tive con­fig­u­ra­tions, not one uni­ver­sal range. Con­firm the required tol­er­ance over the oper­at­ing range and the lim­its of the sheath, cable, con­nec­tion head and any transmitter.

Most of the Pt100 assem­blies we build at H&B Sen­sors sit in the ‑50 to +400°C range, paired with a ther­mow­ell for pro­tec­tion. That cov­ers the bulk of oil and gas, food, phar­ma­ceu­ti­cal, and water treat­ment applications.

Thin Film vs. Wire Wound

The plat­inum ele­ment has two com­mon forms: thin film, with plat­inum deposit­ed on a sub­strate, and wire wound, with fine plat­inum wire sup­port­ed in an insu­lat­ing body. Their tem­per­a­ture and vibra­tion capa­bil­i­ties depend on con­struc­tion. Wire-wound ele­ments are not auto­mat­i­cal­ly more resis­tant to mechan­i­cal stress.

Select the ele­ment and com­plete assem­bly for the required tem­per­a­ture, tol­er­ance, response and vibra­tion conditions.

When to Choose a Pt100 Over a Thermocouple

A Pt100 is often a suit­able choice where sta­bil­i­ty and accu­ra­cy are pri­or­i­ties, pro­vid­ed the com­plete assem­bly is rat­ed for the process. There is no uni­ver­sal 500°C cut-off: com­pare the actu­al RTD and ther­mo­cou­ple options for tem­per­a­ture range, response, vibra­tion and mea­sure­ment uncertainty.

Where ther­mo­cou­ples still make sense: very high tem­per­a­tures, fast response require­ments, or when you need a dis­pos­able sen­sor for short-term measurement.

Installation Tips from the Workshop Floor

We’ve seen plen­ty of well-spec­i­fied sen­sors let down by poor instal­la­tion. A few things that make a real difference:

Use a ther­mow­ell where the process requires pres­sure con­tain­ment, mechan­i­cal pro­tec­tion or sen­sor removal with­out open­ing the process. Direct-immer­sion, sur­face and clamp arrange­ments can also be appro­pri­ate when designed for the application.

Get the immer­sion depth right. The sens­ing ele­ment needs to be ful­ly immersed in the process flu­id to read accu­rate­ly. As a rule of thumb, the immer­sion length should be at least 10 times the sen­sor diam­e­ter. Too shal­low and you’ll mea­sure the pipe wall tem­per­a­ture, not the fluid.

Keep cable runs short and use shield­ed cable in elec­tri­cal­ly noisy envi­ron­ments. A Pt100 sig­nal is a small resis­tance change — it does­n’t take much inter­fer­ence to throw it off.

Seal the con­nec­tion head prop­er­ly. Mois­ture ingress is one of the most com­mon caus­es of sen­sor fail­ure, par­tic­u­lar­ly in wash­down or out­door environments.

Common Questions We Get

What’s the dif­fer­ence between Pt100 and Pt1000?
Same prin­ci­ple, dif­fer­ent base resis­tance. A Pt1000 reads 1,000 ohms at 0°C instead of 100. The high­er resis­tance means lead wire effects are pro­por­tion­al­ly small­er, which can be use­ful for two-wire instal­la­tions. In prac­tice, Pt100 with three- or four-wire con­nec­tion is still the indus­try standard.

My Pt100 read­ing seems to drift — what’s going on?
Usu­al­ly one of three things: mechan­i­cal stress on the ele­ment (often from vibra­tion), con­t­a­m­i­na­tion inside the ther­mow­ell, or degrad­ed lead wires. Check the ther­mow­ell bore for mois­ture or cor­ro­sion first. If the sen­sor has been run­ning at high tem­per­a­ture for a long time with­out recal­i­bra­tion, that’s anoth­er like­ly culprit.

Can I mix and match sen­sors and ther­mow­ells from dif­fer­ent sup­pli­ers?
Yes — as long as the dimen­sions are com­pat­i­ble, there’s no issue using sen­sors and ther­mow­ells from dif­fer­ent man­u­fac­tur­ers. From an ATEX per­spec­tive, the ther­mow­ell itself isn’t nor­mal­ly part of the sen­sor’s cer­ti­fi­ca­tion. What the approval may spec­i­fy is that the sen­sor needs to be installed in a ther­mow­ell — par­tic­u­lar­ly if it’s a spring-loaded design — but it does­n’t need to be a spe­cif­ic ther­mow­ell from the same supplier.

Three-wire or four-wire — is the upgrade worth the cost?
It depends on the com­plete mea­sure­ment error bud­get. Four-wire mea­sure­ment reduces lead-resis­tance effects, but nei­ther wiring arrange­ment alone guar­an­tees a tol­er­ance or com­pli­ance. Match the sen­sor class, instru­ment, cal­i­bra­tion and instal­la­tion to the requirement.

The Bottom Line

A Pt100 is mature, proven tech­nol­o­gy. It’s been the back­bone of indus­tri­al tem­per­a­ture mea­sure­ment for decades, and that’s not going to change. The key is spec­i­fy­ing the right sen­sor for your con­di­tions — the right accu­ra­cy class, the right wiring con­fig­u­ra­tion, the right ther­mow­ell mate­r­i­al — and installing it prop­er­ly. Get those things right and a Pt100 assem­bly will give you reli­able, accu­rate read­ings for years.

If you’re not sure what to spec­i­fy, give us a call. It’s what we do.