FAQ
Sensors, Fittings & Accessories
Can Teflon and Kapton type T thermocouple probes be interchanged?
They should not. Although both probes use copper/constantan wire, each contains different insulation, which provides unique properties. Teflon probes with properly sealed tips are ideal for temperature measurement in steam autoclaves. Teflon insulation protects the sensor from moisture pulled through the wire, and therefore minimizes oxidation.
Teflon wire is rated for continuous use up to 200°C, with a peak rating of 260°C. Using it at higher temperatures will melt the insulation and create toxic gas. (These probes eventually need replacement due to increased permeability, resulting from wear and tear.)
Kapton probes are used for dry heat applications ranging from 150 to 375°C. The higher the temperature, the faster the Kapton will degrade. For example, the life of a Kapton sensor operating at 300°C vs. 375°C is reduced from several months to several days.
The difference in Kapton insulation from others such as fiberglass and ceramic is that Kapton will not leave particulates in your sterilizer or tunnel.
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If the two wires of a thermocouple are twisted together before welding, does it affect measurement accuracy?
It depends on the application. Twisting wires together increases the contact between them over the length of the twisted portion. Your instrument measures the temperature at the first point of contact, i.e., the furthest point from the tip.
Using a twisted thermocouple to measure air temperature in a steam sterilizer would not significantly affect accuracy because the difference in air temperature between the tip and the first point of contact is negligible. An example where twisted conductors could produce incorrect data is when the thermocouple is used to measure the temperature of liquid in a vial. Inserting this thermocouple causes the instrument to measure the air temperature inside the vial instead of the temperature of the liquid.
You can avoid this problem by reducing the junction to a single point. Kaye uses an argon welder to create a thermocouple junction, resulting in a small bead that joins the wires at the tip. In a Kaye Teflon probe, the wires are stripped no more than necessary to create a weld. The nonconductive coating that is left on each wire separates the unwelded bare length of wire. In a Kapton probe the tip is very small and is protected by a stainless steel tube filled with ceramic epoxy.
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Varied thermocouple readings during qualification study.
The cold junction compensation provided by a passive UTR (contained in any thermocouple measuring system) will always be sensitive to uneven heating.
Avoid this condition by positioning the Digi in a location with stable and even temperature, not exposed to local heat sources, e.g. close to a sterilizer or near drafts. Temperature variations or exposure to external heat sources during calibration or qualification studies may cause temperature measurement errors.
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