Thermocouple Cable Insulation
The practical operating temperature of the thermocouple probes is limited by
the cable insulation material’s temperature characteristics.
Insulation Upper Temperature Limit
Glass fiber, thin 500°C/932°F
Glass fiber, thick 500°C/932°F continuous, 700°C/1,292°F peak
Mineral insulation (MI) 1,250°C/2,282°F
PTFE 265°C/509°F
Glass-fiber-insulated probes are available in both thin and thick forms.
Impregnated with a silicone resin binder, thin-glass-fiber-insulated probes are
more robust than thick-glass-fiber-insulated probes; they are suitable for use at
temperatures up to 500°C/932°F. Thick-glass-fiber-insulated probes are more
flexible than thin, being less impregnated with silicone resin; they are more
suitable for oven use, and are capable of continuous operation at temperatures
up to 500°C/932°F, and short-term operation at up to 700°C/1,292°F. They
can be used when probe cables are close to heating elements.
Mineral-insulated (MI) probes have an enclosed junction providing increased
immunity to electrical interference. Less flexible than PTFE or glass fiber, but
suitable for use up to 1,250°C/2,282°F, they can be used when probe cables
may be close to heating elements.
PTFE (polytetrafluoroethylene)-insulated probes are suitable for general-
purpose use at temperatures up to 265°C/509°F. PTFE is a robust, flexible,
non-stick material, with a low thermal mass and therefore a quick response
time. This is the standard insulation for oven use, though it cannot be used
when probe cables may be close to infra-red heating elements.
WARNING
PTFE does not support combustion, but decomposes above 265°C/509°F
producing small amounts of toxic fumes.
The important products from PTFE thermal decomposition are as follows.
36 Thermocouple Probes OVEN TRACKER XL2
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