Technical Information

Thermocouple Selection Guide

Thermocouple Probes are composed of two dissimilar metals, joined to produce a voltage when the applied (measured) temperature differs from the reference temperature.

Thermocouple Thermometers measure, amplify, linearize, and display the proportional voltage signal generated by the thermocouple probe.

Extend Your Thermocouples up to 2000 feet without signal loss. Extension wire must be the same type as the thermocouple.

System Error becomes important when you select a probe and meter to make a complete temperature measurement system. For example: a meter has an accuracy of ±0.7°F; from the probe-error limits table below, type T probes with metal sheaths, straight cables, and stripped ends have an error limit of ±1.8°F at 400°F. Therefore, the probe-meter system accuracy will be (±0.7) + (±1.8) = ±2.5°F at 400°F.

NIST Traceability is required for many applications. In order to make an item traceable to NIST standards, the item and the standard are exposed to the same conditions, the readings are noted, and the difference between the readings is recorded on a NIST certificate. When taking future readings with the item, the value on the certificate must be added or subtracted from the measured value.

Sheaths with small diameters have faster response times; sheaths with larger diameters have longer life and are better for measuring higher temperatures.

Exposed junction

Exposed Junction has the fastest response time. Ideal for measuring rapid temperature changes. Clear coating on most models provides a humidity barrier for the thermocouple. Do not use with corrosive fluids or atmospheres. See table at right for recommended atmosphere type for exposed-junction probes.

Ungrounded junction

Ungrounded Junction has a welded junction insulated from the protective sheath and is electrically isolated. Longer response time; use for conductive solutions or where isolation of the measuring circuitry is required.

Grounded junction

Grounded Junction has a junction welded to tip of sheath. Wires are completely sealed from contaminants. Good response time.

Tolerances apply only to new thermocouples from -200°C to the recommended upper temperature limit of the probe. Tolerances change with use and it is up to the user to establish acceptable limits of error for used thermocouples. Calculated from ASTM tolerances.