The Sheath is the Sensor's Lifeline
The sheath protects the sensing element from mechanical damage, chemical attack, and high-pressure process media. Choosing the right alloy or ceramic for the service is usually the single most important decision in specifying a temperature sensor — wrong choice and the element fails within weeks; right choice and it serves for years.
Five characteristics matter for any sheath material: maximum continuous service temperature, corrosion resistance to the specific process fluid, mechanical strength at temperature, machinability or weldability, and cost. The materials below are arranged roughly in order of increasing capability and cost — start at the top and work down until you find the one that survives your service.
Sheath Construction Conventions
- Standard outside diameters: 1/8", 3/16", 1/4", 3/8", 6 mm, 8 mm
- Wall thickness: 0.020″ to 0.065″ depending on OD and pressure
- Closed end: hemispherical TIG-welded tip is standard
- Junction: grounded, ungrounded, or exposed (TC only)
- Surface finish: 32 Ra (sanitary), 64 Ra (general), or as-drawn
Selection Criteria
1. Temperature — process maximum, with margin
2. Corrosion — match alloy to media
3. Pressure — wall thickness, weld integrity
4. Abrasion — flow velocity, solids loading
5. Vibration / fatigue — natural frequency, support
6. Cost — material price, machining premium
Common Failure Modes
• Oxidation scaling at elevated temperature
• Chloride stress-corrosion cracking (304/316)
• Sulfidation in petrochemical service
• Hydrogen embrittlement (high-pressure H₂)
• Flow-induced vibration fatigue
• Carburization in hydrocarbon atmospheres
Sheath Material Comparison
Maximum temperatures shown are recommended continuous-service limits in clean oxidizing atmospheres. Aggressive media usually require derating; consult applications engineering for confirmation.
| Material | Max Temp (°C) | Key Properties | Common Applications |
|---|---|---|---|
| 304 Stainless Steel | 870 | Austenitic Cr-Ni; good general corrosion resistance; easily welded; lowest cost stainless | General industrial, food, pharmaceutical, HVAC, water |
| 316 Stainless Steel | 870 | Mo addition (~2%) for chloride pitting and crevice corrosion resistance; marine grade | Marine, chemical, sanitary (3-A), brewing, seawater |
| 316L Stainless Steel | 870 | Low-carbon 316 for post-weld corrosion resistance; standard sanitary specification | Pharma, biotech, sterile fill, food and beverage |
| 321 Stainless Steel | 870 | Titanium-stabilized; resists chromium-carbide precipitation during high-temp cycling | Heat-treating fixtures, exhaust, aerospace ducts |
| 310 Stainless Steel | 1100 | 25% Cr, 20% Ni — superior oxidation resistance up to 1150 °C | Furnace tubes, kilns, carburizing atmospheres |
| Inconel 600 | 1175 | Ni-Cr-Fe (76/15/8); outstanding oxidation resistance; standard for high-temp service | Heat treating, salt baths, furnace tubes, high-temp gas |
| Inconel 625 | 980 | Ni-Cr-Mo-Nb; excellent corrosion strength & pitting resistance; high tensile | Aerospace, marine, chemical process, seawater piping |
| Inconel 601 | 1180 | Ni-Cr-Fe with aluminum addition for tenacious oxide; high-cycle service | Radiant tubes, muffle furnaces, ethylene cracker |
| Hastelloy C-276 | 1100 | Ni-Mo-Cr-W; severe corrosion (HCl, FeCl₃, Cl₂ wet/dry); excellent localized resistance | Chemical processing, scrubbers, pulp & paper bleach |
| Hastelloy X | 1200 | Ni-Cr-Fe-Mo; high-temp strength & oxidation; aerospace combustor | Gas turbine combustor, jet engine, industrial furnace |
| Monel 400 | 540 | Ni-Cu (66/33); HF and seawater resistance; reducing acid service | HF alkylation, marine, brine, hydrocarbon refining |
| Tantalum | 250 (chemical) / 1500 (vacuum) | Outstanding resistance to HCl, H₂SO₄, HNO₃, HBr at all concentrations; embrittles in H above 300 °C | Acid digestion, pickling, chlor-alkali, pharma reactor |
| Titanium Grade 2 | 540 | Chloride seawater immunity; lightweight; reducing acid limit | Chlorination, desalination, hypochlorite bleach |
| Platinum (Pt-10%Rh) | 1450 | Noble metal; required for Type R/S/B; non-reactive in oxidizing atmospheres | Glass furnace, heat treating, precious metal refining |
| Alumina Ceramic (99.7%) | 1700 | Refractory; electrically insulating; brittle; cannot tolerate thermal shock | Furnace protection tubes, MgO-insulated TC inner |
| Mullite (3Al₂O₃·2SiO₂) | 1500 | Lower thermal expansion than alumina; better thermal shock resistance | Outer protection tubes in furnace service |
| Silicon Carbide | 1650 | High thermal conductivity; excellent thermal shock; abrasion resistance | Aluminum smelter, glass furnace, foundry |
| Molybdenum | 1650 (vacuum / H₂) | Refractory metal; oxidizes catastrophically in air above 600 °C — vacuum or reducing only | Vacuum furnaces, hydrogen-atmosphere brazing |
Material Compatibility by Media
Quick chemical compatibility reference. Ratings are A = excellent, B = acceptable, C = limited / monitor closely, D = not recommended. These are starting points only — concentration, temperature, and aeration always matter.
| Media | 304 SS | 316 SS | Inconel 600 | Hastelloy C-276 | Tantalum |
|---|---|---|---|---|---|
| Hydrochloric acid (HCl) | D | D | D | B | A |
| Sulfuric acid (H₂SO₄) < 50% | C | C | C | A | A |
| Nitric acid (HNO₃) < 65% | A | A | B | B | A |
| Phosphoric acid (H₃PO₄) | B | A | A | A | A |
| Acetic acid (CH₃COOH) | B | A | A | A | A |
| Sodium hydroxide (NaOH) | A | A | A | A | D |
| Sodium chloride (NaCl) brine | C | B | B | A | A |
| Hot chloride process water | D | C | B | A | A |
| Seawater & marine | C | B | B | A | A |
| Steam & pure water | A | A | A | A | A |
| Hydrogen (high pressure) | B | B | B | B | D |
| Wet chlorine (Cl₂) | D | D | C | A | A |
| Sulfur dioxide (SO₂) hot | C | B | B | A | B |
| Hydrofluoric acid (HF) | D | D | C | B | D |
Above 870 °C — Where Stainless Stops Working
The conventional 300-series stainless steels are limited to roughly 870 °C in oxidizing service. Above that, the chromium-rich passivation layer thickens, scales, and eventually spalls. For continuous service above 900 °C, the choices narrow:
- 310 / 314 stainless — 25% Cr gives an extra ~200 °C of headroom; economical for muffles and recuperators
- Inconel 600 — Ni-base; thick adherent NiCr₂O₄ oxide; standard for heat treating and salt baths
- Inconel 601 / 800H — alloyed with Al for even more stable oxide; furnace radiant tubes
- Hastelloy X — high-temp strength to 1200 °C; turbine combustor liners
- Platinum (Pt-Rh) — for noble-metal thermocouples; required above 1400 °C in oxidizing
- Alumina or mullite ceramic — non-metallic protection tube outside a Pt or graphite inner
- Silicon carbide — refractory non-oxide; resists glass-melt and aluminum-melt slag
The Carburization Trap
In hydrocarbon atmospheres above 800 °C, carbon diffuses into stainless steel and Ni-Cr alloys, embrittling them and shifting the calibration of any thermocouple inside. Aluminized or aluminided coatings (Sermaloy-type) on Inconel substrates dramatically improve carburization resistance — essential in ethylene cracker tubes and reformer service.
Salt Bath Service
Molten chloride/nitrate salt baths at 400–950 °C are very aggressive — chloride attack on stainless is immediate. Inconel 600 is the standard sheath. Replace at first sign of pitting; spent sheaths cannot be reused.
Vacuum Furnace
In vacuum or pure hydrogen above 1100 °C, even Inconel evaporates Cr and Ni. Use molybdenum, tungsten, or tantalum sheaths — but understand that all three oxidize catastrophically if exposed to air at temperature.
Glass Melt
Molten soda-lime glass at 1300–1500 °C dissolves platinum slowly and dissolves nickel quickly. Use solid alumina or sapphire protection tubes around Pt-Rh thermocouples. Replacement is scheduled by mass loss, not by time.
Relative Material Cost
Use 304 stainless as the cost baseline (1×). Multipliers below are approximate raw-material cost ratios for a 6 mm OD × 300 mm long tube — actual cost depends on quantity, length, and current commodity pricing.
| Material | Relative Cost | Performance Tier |
|---|---|---|
| 304 SS | 1.0× | General — start here |
| 316 / 316L SS | 1.3× | Marine, sanitary, pharma |
| 321 SS | 1.4× | Cycled high-temp, exhaust |
| 310 SS | 1.8× | Up to 1100 °C oxidizing |
| Inconel 600 | 4–6× | Furnace, salt bath, heat treat |
| Inconel 625 | 6–8× | Aerospace, severe marine |
| Hastelloy C-276 | 8–12× | Severe chemical, halogens |
| Tantalum | 30–50× | Strong acids, pharma reactor |
| Platinum-10% Rhodium | 500–800× | Glass melt, precious metals |
| Alumina ceramic | 3–5× | Refractory, electrical isolation |
Rule of Thumb
Specify the cheapest sheath that will survive your service for the planned maintenance interval. Over-spec'ing wastes capital; under-spec'ing produces unplanned outages. When in doubt, send us a copy of the process stream composition and we will recommend a sheath based on internal corrosion test data.
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