Ask Us Anything: 21 Questions We Get Asked Most About Temperature Measurement and Heating
The short answer: Most temperature measurement problems come down to a small set of recurring questions — which sensor type, how deep to insert it, which junction type, why a reading wanders, and why a heater keeps failing. This page answers the twenty-one questions Thermocouple Products is asked most often, in plain language, in the order they usually come up.
No sign-up, no sales pitch in the middle of a technical answer. If your question is not here, phone (011) 452-6617 — that is what it is for.
Choosing a sensor
1. Which thermocouple type should I use?
Nine times out of ten, type K. It covers −180°C to +1 100°C, it is robust, it is economical, and it is what most instruments are already configured for.
Move away from it only when the application asks you to:
Type | Composition | Range | Choose it when |
K | Chromel / alumel | −180°C to +1 100°C | General purpose — the default |
J | Iron / constantan | −250°C to +750°C | Dry, reducing or vacuum atmospheres |
T | Copper / constantan | −250°C to +370°C | Low and cryogenic temperatures |
E | Chromel / constantan | 0°C to +870°C | Highest EMF output per degree |
R / S / B | Platinum alloys | Up to 1 600°C | Above the base metal range |
There is no advantage in specifying something more exotic than the duty requires. Conversely, a type K asked to work continuously above 1 100°C is not “a type K with a shorter life” — it is outside its specified range.
2. Thermocouple or Pt100 RTD — which one?
Thermocouple where the temperature is high (above roughly 600°C, and certainly above 1 100°C), where response speed matters, or where you need many points economically. Pt100 RTD where accuracy and long-term stability matter more than speed or cost, which in practice means below about 600°C in a clean, stable process.
A Pt100’s resistance is 100 ohms at 0°C by definition, and it is more repeatable over years than a thermocouple. A thermocouple is tougher, faster and cheaper, and it goes to the top of the scale where an RTD cannot follow.
Most plants need both, and the split is usually around 600°C.
3. What is the difference between standard and special limits of error?
Special limits roughly halve the permissible error, and you simply ask for it when ordering. For type K:
Limits | 0–280°C | Above 280°C |
Standard | ±2°C | ±3/4% of reading |
Special | ±1°C | ±3/8% of reading |
The percentage matters more than it looks. At 600°C, ±3/4% is ±4,5°C while ±3/8% is ±2,25°C. If two degrees changes your outcome, special limits are a line on the order, not a special project.
4. Which sheath material do I need?
Start with the maximum temperature, then the process medium.
Material | Maximum temperature |
304 / 316 / 321 stainless | 925°C |
446 stainless | 1 040°C |
310 stainless | 1 095°C |
Inconel 600 | 1 150°C |
For anything corrosive, use the catalogue’s 155-chemical material selection guide rather than guessing — and check the temperature and concentration columns, not just the chemical name. Nitric acid wants 304 stainless at 25°C and 316 stainless or tantalum at 150°C. Chlorine wants Monel when dry and Hastelloy C when moist.
Note also that 304, 316 and 321 all show 925°C and are not interchangeable: 316 resists chlorides better, and 321 resists intergranular corrosion after welding.
5. Mineral insulated or standard construction?
Mineral insulated (MI) is a compacted magnesium oxide construction in a small-diameter sheath — typically 1,5 mm to 8,0 mm — flexible, fast responding, and available in long continuous lengths. It suits tight spaces, fast control loops, vibration and where a slim, robust sensor is needed.
Standard construction uses a larger sheath, often with a connection head and a mounting flange or fitting, ceramic insulators and heavier conductors. It suits process plant, tanks, pipes and heavy industrial installations where the sensor is a permanent fixture rather than a component in a machine.
Both are available in single and duplex, and in all seven calibrations.
6. What does “duplex” mean, and when do I need it?
Duplex means two independent measuring elements in one assembly — a duplex type K is ordered as KK. The same doubling applies across the range: JJ, TT, EE, RR, SS, BB.
You need it when: one element runs the control loop and the other feeds a recorder, alarm or over-temperature cut-out; when a measurement must continue through a sensor change; or where changing a sensor is expensive, so a spare element in place is cheap insurance.
There is also a diagnostic benefit that is rarely mentioned: two elements in the same process should agree. When they start to disagree, one is announcing that it is drifting — while the plant still runs on the other.
Installation and accuracy
7. How deep should a sensor be inserted?
Deep enough for the tip to sit in the representative part of the process — aim for the middle third of the pipe or vessel, in flowing medium rather than the slow boundary layer against the wall.
Insufficient immersion depth is the most common single cause of an inaccurate reading, and it is invisible: the sensor is working perfectly, and measuring the wrong thing. In small pipes the answer is often to angle the sensor or fit it in an elbow rather than to accept a shallow insertion.
8. Grounded, ungrounded or exposed junction?
Code | Junction | Response | Isolation |
G | Grounded to sheath | Fast | None — electrically connected |
U | Ungrounded / insulated | Slower | Isolated from the sheath |
E | Exposed | Fastest | None, and unprotected |
Default to U. An ungrounded junction is isolated from the sheath, which avoids earth loops — a frequent cause of readings that wander or are unstable in electrically noisy plants. Use G where response speed genuinely matters and the process is electrically quiet. Use E only where the medium is benign and you need the fastest possible response.
9. Why does my reading wander or jump around?
In rough order of likelihood:
- Junction type and earth loops — a grounded junction in a noisy plant, or a second earth path through the sheath, shows up as instability
- Loose or corroded terminals — the cheapest thing to check and the most often missed
- Moisture in the connection head — insulation resistance falls and readings become erratic, often reading low
- Mechanical looseness — a sensor rattling in a thermowell or a poorly clamped band heater produces a signal that follows vibration, not temperature
- Genuine sensor failure — less common than the four above, but the first thing people replace
10. Why is my reading consistently low?
A stable, repeatable offset is usually not a sensor fault at all. Common causes:
- Wrong extension cable type — using a cable with different conductor alloys for the thermocouple in question introduces a fixed error at every junction
- Reversed polarity — a compensation cable connected the wrong way round produces a consistent offset that looks like a calibration problem
- Insufficient immersion — the sensor conducts heat away up its own sheath and reads the cooler average
- A sensor reading the wall, not the process
- The instrument’s input type is set for a different calibration
Check the cable, the polarity and the immersion before ordering a replacement sensor. All three cost nothing to inspect.
11. Can I cut a mineral insulated thermocouple to length?
No. An MI thermocouple is packed with compacted magnesium oxide and sealed at the hot end; cutting it destroys both the insulation and the seal.
Specify the length you need when ordering. Lengths are made to order across a wide range, and specifying correctly costs no more than cutting would — it just happens before delivery rather than after.
12. How can I check whether a sensor is accurate, without a laboratory?
Three checks, none needing equipment you do not already have.
Ice point. Fill an insulated flask with crushed ice — not cubes — and add just enough clean water to make a slush. Insert the sensor at least 100 mm, not touching the sides, stir, allow two to three minutes to settle. It should read 0°C.
Comparison. Put a known-good sensor beside it, same depth, same medium, both allowed to settle. If they disagree, at least one is wrong — and now you know to ask which.
Inspection. Immersion depth, insulation condition, terminal tightness and the extension cable type. These produce believable wrong numbers from serviceable sensors, and cost nothing to check.
Heating elements
13. Mica band heater or ceramic band heater?
Temperature decides it. Mica band heaters are typically rated to about 480°C and run at 3 to 7 W/cm². Ceramic band heaters are rated continuously to 760°C, and their ceramic insulation and fibre blanket reduce heat loss by roughly 25% to 30% — which lowers the external surface temperature and the load on factory cooling as well as saving power.
Below 480°C, a mica band is usually the more economical and entirely correct choice. Above it, no amount of care in manufacture makes a mica band suitable. It is the wrong construction for the duty, not a lesser version of the right one.
14. What is watt density, and why does it matter?
Watt density is the heating power divided by the heated surface area — expressed in W/cm² or W/in². It is the number that decides whether an element lives comfortably or works at its limit.
Two heaters can have identical wattage and completely different watt densities, because one has far more surface area to dissipate the heat through. Too high a watt density shortens element life, oxidises the resistance wire faster and accelerates every other failure mode — no matter who manufactured it.
Work it out rather than guessing: if the figure is outside the range for the construction (3 to 7 W/cm² for mica band heaters), the specification is the problem.
15. Why does my band heater keep burning out?
In order of frequency:
- Poor fit — a loose or distorted band leaves air gaps, air insulates, and the element runs hotter internally to push heat across the gap. The process still reaches temperature, so nobody investigates.
- Watt density above the construction’s capability — the wrong product for the duty.
- A worn or badly positioned sensor — the controller drives the heater harder than it should because it is being given the wrong information. The heater is blamed; the sensor is guilty.
- Where the damage is tells you a lot: one localised burnt patch suggests poor contact at that spot or a barrel hot spot, while even ageing over the whole band suggests ordinary end of life.
If the same zone fails twice in the same way, diagnose before ordering a third.
16. Can a cartridge heater be fitted to any hole?
Not quite. A cartridge heater is a tube-shaped element inserted into a drilled metal hole, relying on contact for heat transfer — so the bore fit matters. Too loose and heat transfer suffers (the element runs hotter than it should); too tight and removal becomes a machining job rather than a maintenance task.
The hole also has to be deep enough for the element to sit fully home, clean and properly drilled, and the leads need somewhere to go that is not hotter than they can survive. Cartridge heater failures are frequently lead failures.
17. Over-the-side or ceramic withdrawable immersion heater?
Both heat a tank. They answer different constraints.
Over-the-side (OTS) heaters hang over the tank rim with the terminal housing outside, a vertical cold riser and the active elements submerged. No tank modification is required, and they can be moved between vessels — useful for open tanks, temporary duty and mobile heating. Sheaths come in stainless steel, titanium, Incoloy or fluoropolymer to suit the liquid.
Ceramic withdrawable elements slide into a sealed metallic pocket or thermowell fitted permanently into the tank. The element never touches the process, and when it fails it is withdrawn and replaced without draining the tank or cooling the contents — which matters where the tank is large, the product is valuable, or the contents solidify when cold (wax, bitumen and similar).
If a tank can only be repaired by emptying it, that is worth knowing before the next failure rather than during it.
18. What is Incoloy, and when do I need it?
Incoloy is a nickel-iron-chromium superalloy used as an element sheath. It runs continuously to about 870°C without losing mechanical strength, and resists pitting, oxidation, mineral scale and chloride stress-corrosion cracking.
It is the material for water heating where limescale is a problem, foodservice equipment, chemical immersion and process air. It is not the answer to every corrosive duty — for aggressive chemicals, check the process medium against the catalogue’s chemical selection guide and specify accordingly.
Ordering and replacing
19. How do I read your part number?
A Thermocouple Products part number is built from one code per specification group, joined by dashes, following the sensor from tip to termination:
60 – K – 316 – U – 150 – A – FSS
- 60 → 6,0 mm sheath diameter
- K → type K element
- 316 → 316 stainless steel sheath
- U → ungrounded junction
- 150 → 150 mm immersion depth
- A → termination
- FSS → fixed stainless steel fitting
Seven decisions, one unambiguous code. Every option is listed in the catalogue, which also carries the equivalent tables for RTDs, inserts and protection sheaths.
20. Can you match an old part, or work from a photograph?
Often, yes — with the right photograph. A tape measure or ruler in frame, the damage visible, and the voltage and wattage get a long way.
But the best option is the old part itself, and the second best is a drawing. What a photograph cannot show is what the part has to do, the process temperature, or how it mounts — and those three usually decide the specification.
21. What do you need from me to quote?
Five things, and a photograph or the old part answers most of them:
- What it heats or measures, and the medium
- The operating temperature, and the maximum it will ever see
- Voltage and wattage for an element, or the calibration and length for a sensor
- Dimensions — diameter, length, immersion depth, thread or fitting
- How it mounts, and how it terminates
Send what you have. Three answers and a photograph is a perfectly normal way to start an enquiry here — most orders arrive that way.
What we make
Everything above refers to products manufactured in Edenvale, Gauteng, by Thermocouple Products (Pty) Ltd — trading since 1992, with over 89 years of combined experience across the team.
Temperature measurement: thermocouples in types J, K, T, E, R, S and B, single and duplex; mineral insulated and standard constructions; Pt100 RTDs; thermocouple inserts; protection sheaths; thermowells; extension cable and connectors.
Heat generation: mica and ceramic band heaters, cartridge heaters, Incoloy elements, supercoil heaters, ceramic withdrawable elements, over-the-side immersion heaters, and NFG high-temperature cable.
The range: −250°C to +1 600°C, from a 1,5 mm mineral insulated sensor to a heavy assembly in a 32 mm protection sheath.
That combination — heaters and sensors from one manufacturer — is unusual, and it is the reason a single phone call can resolve a problem that would otherwise be divided between a heater supplier and a sensor supplier who each believe the other is at fault.
Ask us something difficult. It is genuinely the best part of the job.
CTA: Request a Quote → /contact · Call (011) 452-6617 · Email tcprod@mweb.co.za
Related reading:
- The Complete Buyer’s Guide to Thermocouples, RTDs and Heating Elements
- How to Read the Thermocouple Products Catalogue
- What a Failed Heater or Sensor Is Trying to Tell You
FAQ schema for this page
Which thermocouple type should I use? Type K is the correct choice for most industrial work, covering −180°C to +1 100°C. Choose type J for dry, reducing or vacuum atmospheres up to 750°C, type T for low and cryogenic temperatures from −250°C to +370°C, type E where the highest output per degree is wanted up to 870°C, and noble metal types R, S or B above 1 100°C up to 1 600°C.
Can I cut a mineral insulated thermocouple to length? No. A mineral insulated thermocouple is packed with compacted magnesium oxide and sealed at the hot end, so cutting it destroys the insulation and the seal. The length must be specified when ordering.
Why does my thermocouple reading wander? The most common causes are a grounded junction creating earth loops in an electrically noisy plant, loose or corroded terminals, moisture in the connection head lowering insulation resistance, mechanical looseness causing the signal to follow vibration, and genuine sensor drift. Checking the junction type, terminals, cable type and head condition resolves most cases before a sensor is replaced.
What is the difference between a mica band heater and a ceramic band heater? Mica band heaters are typically rated to about 480°C at 3 to 7 W/cm² and suit plastics barrels, nozzles, pipes and drums. Ceramic band heaters operate continuously to 760°C, and their ceramic insulation and fibre blanket reduce heat loss by roughly 25% to 30%. The required temperature decides which construction is correct.
Why does my band heater keep burning out in the same place? Repeat failure in one zone is usually a fit or duty problem rather than a defective heater: a loose band leaves air gaps and forces the element to run hotter internally, a watt density above the construction’s range, or a worn or misplaced thermocouple that makes the controller overdrive the heater. The location of the damage is informative — a single localised burnt patch indicates poor contact or a hot spot, while even ageing indicates normal end of life.