The Complete Buyer’s Guide to Thermocouples, RTDs and Heating Elements
The Complete Buyer’s Guide to Thermocouples, RTDs and Heating Elements
The short answer: Choosing a temperature sensor or heating element comes down to a small number of decisions made in the right order — what temperature, in what medium, with what accuracy, fitted how, and replaceable when. Get those right and the specification follows almost mechanically. Get them out of order, and no amount of product knowledge rescues the result.
This page is the index to everything Thermocouple Products has published on those decisions. Each section summarises the essentials and links to the full guide. Read it top to bottom once, then bookmark it and return to the section you need.
Thermocouple Products (Pty) Ltd has manufactured thermocouples, Pt100 RTDs, thermowells and industrial heating elements in Edenvale, Gauteng since 1992 — 34 years of trading, with over 89 years of combined experience across the team. Between them, the guides below cover a range from −250°C to +1 600°C, and from a 1,5 mm mineral insulated sensor to a heavy assembly in a 32 mm protection sheath.
Starting point: thermocouple or RTD?
Two sensor technologies cover almost all industrial temperature measurement, and the choice between them is decided by range, accuracy and cost rather than by preference.
Thermocouples — two dissimilar conductors joined at a measuring junction, generating a voltage proportional to the temperature difference between that junction and a reference. Robust, inexpensive, fast responding, wide ranging and usable to the very top of the industrial scale. Types J, K, T and E cover the base metal range; R, S and B the noble metal range to 1 600°C.
Pt100 RTDs — a platinum resistance element whose resistance rises with temperature in a defined, repeatable way: 100 ohms at 0°C by definition. More stable and more repeatable than a thermocouple, excellent at low and moderate temperatures, and the usual choice where accuracy and long-term stability matter more than speed or first cost.
In practice, a plant needs both: RTDs where the measurement must be trusted over years, thermocouples where the temperature is high, the environment harsh or the response time critical.
Decision | Points towards a thermocouple | Points towards a Pt100 RTD |
Temperature | Above about 600°C, and certainly above 1 100°C | Below about 600°C |
Accuracy and stability | Adequate with special limits | Better, and more repeatable |
Response speed | Needed | Not critical |
Environment | Harsh, vibrating, high temperature | Clean and stable |
Cost per point | Lower | Higher |
Full guides: Thermocouples · Pt100 RTDs · Mineral Insulated Thermocouples
Temperature ranges and element types
The first hard constraint is temperature. These are the ranges for the thermocouple calibrations manufactured by Thermocouple Products:
Type | Composition | Temperature range | Notes |
J | Iron / constantan | −250°C to +750°C | Dry, reducing or vacuum atmospheres |
K | Chromel / alumel | −180°C to +1 100°C | The general-purpose workhorse |
T | Copper / constantan | −250°C to +370°C | Excellent low and cryogenic |
E | Chromel / constantan | 0°C to +870°C | Highest EMF output per degree |
R | Pt 13% Rh / Pt | 0°C to 1 600°C | Noble metal |
S | Pt 10% Rh / Pt | 0°C to 1 600°C | Noble metal |
B | Pt 30% Rh / Pt 6% Rh | Up to 1 600°C | Noble metal, highest range |
Duplex versions use the doubled letter — JJ, KK, TT, EE, RR, SS, BB — and carry two independent elements in one assembly.
Type K is the correct answer for the majority of industrial work, and there is no advantage in specifying anything more exotic until the application demands it. Above 1 100°C, base metal types are outside their specified range, and the noble metal types take over.
Sheath material: the decision that decides service life
The element measures temperature. The sheath decides whether the element survives long enough to be useful. For any process that is corrosive, hot or both, this is the most consequential decision in the specification.
The maximum temperatures for the standard sheath materials:
Material | Maximum temperature |
304 / 316 / 321 stainless steel | 925°C |
446 stainless steel | 1 040°C |
310 stainless steel | 1 095°C |
Inconel 600 | 1 150°C |
Note that three stainless grades share the 925°C figure and are still not interchangeable: 316 resists chlorides better, and 321 resists intergranular corrosion after welding. Temperature is one axis; chemistry is the other.
For corrosive service, the catalogue carries a material selection guide covering 155 chemicals, each with an operating temperature, a concentration and a recommended material. Two lessons it teaches repeatedly:
- Temperature changes the answer. Nitric acid at 25°C calls for 304 stainless; at 150°C, 316 stainless or tantalum.
- Wet or dry changes everything. Chlorine dry at 150°C points to Monel; moist at 150°C, to Hastelloy C.
Specifying from the chemical name alone will eventually produce a failure.
Specifying: how an ordering code is built
A Thermocouple Products part number is built by choosing one option from each of seven groups, joined by dashes, following the sensor from tip to termination:
Sheath diameter · Element type · Sheath material · Junction type · Immersion depth · Termination · Mounting
60 – K – 316 – U – 150 – A – FSS
6,0 mm sheath · type K element · 316 stainless steel sheath · ungrounded junction · 150 mm immersion · plug termination · fixed stainless steel fitting.
Three of those decisions deserve particular attention:
- Diameter — thinner responds faster and fits tighter; thicker lasts longer. 1,5 mm is quick and delicate, 6,0 mm is the general-purpose choice, 8,0 mm is built for abuse.
- Junction — G grounded is fastest and not isolated; U ungrounded is isolated from the sheath and avoids earth loops, which makes it the sensible default; E exposed is fastest of all and unprotected.
- Immersion depth — aim for the tip in the middle third of the pipe or vessel, in flowing process rather than the boundary layer at the wall. Insufficient immersion is the single most common cause of an inaccurate reading.
An RTD follows the same logic with fewer groups. 32MP – 150 – SS6 – N is a duplex Pt100, 3 wire, mineral insulated, 150 mm long, in a 6 mm 316 stainless sheath, with a nylon connection head.
Accuracy: limits of error and drift
Thermocouples are supplied to standard limits of error by default, and special limits are available on request. Special limits roughly halve the permissible error.
For type K: standard limits are ±2°C from 0°C to 280°C and ±3/4% above that; special limits are ±1°C and ±3/8%.
The percentage figure is where specifications are misunderstood. ±3/4% is three quarters of one percent of the reading, so the permitted error grows with temperature:
Reading | ±3/4% equals |
400°C | ±3,0°C |
600°C | ±4,5°C |
900°C | ±6,75°C |
1 200°C | ±9,0°C |
At 1 200°C, a type K may be nine degrees out and still be entirely within specification.
The other half of accuracy is drift — the slow change in a sensor’s behaviour over time from high temperature exposure, thermal cycling, oxidation, contamination or moisture. A drifting sensor produces a steady, believable, incorrect reading, which is why it survives so long. Three checks that need no laboratory: an ice point check in a properly made slush of crushed ice at 0°C; comparison against a known-good sensor at the same depth in the same medium; and inspection of the installation — immersion depth, insulation condition and whether the extension cable is the correct type.
Heating elements: match the construction to the duty
On the heating side the same principle applies — the specification follows the duty, and each construction exists because a different duty defeated the others.
Element | How it works | Limits | Typical duty |
Mica band heater | Clamps around cylinders; NiCr wire, mica insulation | To about 480°C, 3–7 W/cm² | Plastics barrels, nozzles, pipes, drums |
Ceramic band heater | Ceramic insulation, fibre blanket | To 760°C, 25–30% less heat loss | Injection and extrusion barrels, blow moulding, autoclaves |
Cartridge heater | Swaged tube inserted into a drilled hole | High watt density, localised | Manifolds, hot runners, nozzles, platens, dies |
Incoloy elements | Nickel-iron-chromium sheath | To 870°C, resists scale and chloride SCC | Water heating, foodservice, chemical immersion |
Supercoil heater | Compact flat coil, often with a built-in J or K thermocouple | High watt density in small space | Hot runner nozzles, sprue bushes, packaging jaws |
Ceramic withdrawable | Wire on ceramic bobbins, slides into a sealed pocket | Withdrawable without draining the tank | Large tanks, wax and bitumen, costly contents |
Over-the-side immersion | Hangs over the tank rim, no tank modification | Sheath material to suit the liquid | Open tanks, temporary or mobile heating |
NFG cable | Nickel conductor, fibreglass braid, silicone varnish | Continuous 250°C to 450°C | Ovens, kilns, furnaces, element connections |
Two points from that table are worth taking away. Mica band versus ceramic band is a temperature decision — 480°C against 760°C — and no amount of care in manufacture makes a mica band suitable for a ceramic duty. And the withdrawable arrangement is an access decision, not a heating one: it exists so that a failed element can be changed without emptying a tank.
Full guides: Band Heaters, Cartridge Heaters and Supercoils: Which One Does Your Machine Actually Need? · How to Order a Custom Heating Element or Temperature Sensor
Continuous plant: thermowells, inserts and duplex sensors
Where the process cannot be opened during operation — power generation, petrochemical, chemical and any continuous line — the sensor question becomes an access question.
A thermowell is a pressure-retaining fitting installed permanently in the pipe or vessel. It holds the pressure, absorbs corrosion erosion and mechanical abuse, and allows the sensor to be withdrawn and replaced while the plant keeps running. It also standardises the connection and immersion, so the insert inside becomes a repeatable, reorderable spare.
Four enemies act on a well: pressure, corrosion (use the chemical selection guide), flow — a long slender well in fast media can vibrate, which is an engineering check rather than a guess — and the air gap inside the assembly, where a sensor sitting short of the well’s bottom measures the gap as enthusiastically as the process.
Duplex sensors carry two elements in one assembly — KK, SS and so on. One controls, one records or alarms; and because two elements in the same process should agree, a growing difference between them reveals that one is drifting, while the plant is still running on the other.
Full guide: Thermowells Explained: Measuring Temperature in Pressurised Pipework Without Shutting Down
Industry-specific guides
Where the application is the best place to start, three guides are written around a single sector:
- Plastics — The Complete Guide to Heating and Measuring an Injection Moulding Machine: every zone, from barrel band heaters to hot runner supercoils, and the sensors that control them
- Food, beverage and pharmaceutical — Temperature Measurement and Heating in Food, Beverage and Pharmaceutical Production: hygienic sensors, Incoloy and immersion heating, and what the process demands
- Cable and connection — Thermocouple Extension Cable: The Hidden Cause of Measurement Error: why the right sensor can still produce the wrong number
When something fails: reading the evidence
A failed element or sensor is information before it is scrap. If the same part fails twice in the same way, a like-for-like replacement simply restarts the same failure.
Repeat burnout in one zone usually means poor contact between band and barrel, a watt density above the construction’s capability, or a worn sensor making the controller overdrive the heater.
A tank element that cannot be changed is a vessel problem, not an element problem — and a withdrawable or over-the-side arrangement may remove the constraint entirely.
A sensor that looks healthy and reads wrong is the most expensive failure of all, and the checks in section 5 are what find it.
The five things that let a manufacturer help you: a photograph of the failed part showing the damage; the dimensions; the voltage and wattage; what it heats or measures, including medium and temperature; and its history — how long it lasted and whether it has failed this way before.
Full guide: What a Failed Heater or Sensor Is Trying to Tell You
Industry-specific guides
Where the application is the best place to start, three guides are written around a single sector:
- Plastics — The Complete Guide to Heating and Measuring an Injection Moulding Machine: every zone, from barrel band heaters to hot runner supercoils, and the sensors that control them
- Food, beverage and pharmaceutical — Temperature Measurement and Heating in Food, Beverage and Pharmaceutical Production: hygienic sensors, Incoloy and immersion heating, and what the process demands
- Cable and connection — Thermocouple Extension Cable: The Hidden Cause of Measurement Error: why the right sensor can still produce the wrong number
The article library
# | Guide | Subject |
1 | Why Is My Thermocouple Reading Wrong? | Diagnosing a suspect measurement |
2 | How to Choose a Sheath Material for Corrosive Service | The 155-chemical approach |
3 | Which Heating Element Does Your Machine Need? | Band, cartridge and supercoil |
4 | Thermocouple Extension Cable | The hidden source of error |
5 | Heating and Measuring an Injection Moulding Machine | The complete plastics guide |
6 | How to Order a Custom Heating Element or Sensor | Specifying a one-off part |
7 | Temperature Measurement in Food, Beverage and Pharma | Hygienic and immersion applications |
8 | Temperature Sensor Accuracy | Drift, limits of error, when to replace |
9 | How to Read the Catalogue | Ordering codes, table by table |
10 | What a Failed Heater or Sensor Is Telling You | Reading failures before reordering |
11 | Noble Metal Thermocouples | Measuring above 1 100°C |
12 | Thermowells Explained | Sensors in plants that cannot stop |
Plus: the 29-page catalogue, containing the complete ordering tables, the 155-chemical material selection guide and the temperature conversion data.
Where to start, depending on who you are
If you are specifying something new: temperature range first (section 2), then sheath material (section 3), then the ordering code (section 4).
If you are replacing a part: check whether the original failed within expectation. If it did, replace like-for-like. If it failed early or twice, read section 8 before ordering.
If you suspect a reading is wrong: section 5, starting with an ice bath.
If the process cannot be opened: section 7.
If you are not sure: phone us. Describe the application — what you are measuring or heating, the temperature, the medium, and how it mounts. That is how most orders arrive here, and it works perfectly well.
CTA: Request a Quote → /contact · Call (011) 452-6617 · Email tcprod@mweb.co.za
FAQ schema for this page
Should I use a thermocouple or a Pt100 RTD? Use a thermocouple where the temperature is high — above roughly 600°C, and certainly above 1 100°C — where response speed matters, or where the environment is harsh and the cost per point is significant. Use a Pt100 RTD where accuracy and long-term stability matter more than speed or cost, which is typically below about 600°C in a clean, stable process. A Pt100 has a nominal resistance of 100 ohms at 0°C and is more repeatable than a thermocouple.
What is the most important decision when specifying a thermocouple? The maximum temperature decides the element type, and the process medium decides the sheath material. The sheath decision is the more consequential of the two for service life: standard sheaths reach their limits at 925°C for 304, 316 and 321 stainless steels, 1 040°C for 446, 1 095°C for 310 and 1 150°C for Inconel 600. For corrosive media, the catalogue’s 155-chemical selection guide should be used rather than the chemical name alone, because temperature and concentration change the recommendation.
What does ±3/4% accuracy mean on a thermocouple? It means the permissible error is three quarters of one percent of the reading, so the error increases with temperature — ±3,0°C at 400°C, ±4,5°C at 600°C, ±6,75°C at 900°C and ±9,0°C at 1 200°C. Special limits of error, available on request, roughly halve these figures, for example ±1°C to 280°C and ±3/8% above that for type K.
How do I know if a temperature sensor is drifting? Perform an ice point check using crushed ice and just enough clean water to form a slush at 0°C, inserting the sensor at least 100 mm and allowing two to three minutes to settle. Compare the sensor against a known-good reference at the same depth in the same medium. Also inspect the installation, since insufficient immersion depth, damaged insulation and the wrong extension cable all produce believable but incorrect readings from a serviceable sensor.
Why does temperature sensor or heating element choice depend on the application rather than the product? Because each construction exists to survive a specific duty. A mica band heater is rated to about 480°C at 3 to 7 W/cm², while a ceramic band heater reaches 760°C with 25–30% lower heat loss; an Incoloy element runs to 870°C and resists scale and chloride stress-corrosion cracking; a ceramic withdrawable element exists so a failed heater can be replaced without draining the tank. Matching the construction to the duty is what determines service life.