Thermocouple Junction Configurations

Thermocouples are simple devices made up of several key components: thermocouple wire, electrical
insulation, and the a welded wire sensing junction. Many thermocouple designs also include a stainless steel sheath that protects the thermocouple from vibration, shock, and corrosion.

A thermocouple has three variations of sensing tip (or junction):

1. Exposed junction, where the exposed wire tips and welded bead have no covering or protection.
2. Grounded junction, where the welded bead is in physical contact with the thermocouple's sheath.
3. Ungrounded junction, where the tip is inside the thermocouple sheath, but is electrical (and somewhat thermally) insulated from the sheath (no sheath contact).

Exposed junction thermocouples respond to temperature change quickly and are less costly, but their signals are susceptible erratic reading caused by induced or conducted electrical noise. Because there is no sheath, they are also prone to mechanical damage and ambient contamination.

Grounded junction thermocouples provide fast response and are mechanically more robust, with a metallic sheath that protects the thermocouple both mechanically and from contaminants. But because their sensing tip is in contact with the external sheath, their signal still can be affected by externally induced or conducted electrical noise.

Ungrounded thermocouples, like grounded, are protected mechanically and from ambient contaminants by their sheath. However, their sensing junctions are kept separate from their metallic sheath, isolating the junction from external electrical  interference. This separation does come at a small cost in temperature sensing responsiveness though.

For safety, precision, and optimum performance, always talk to an applications specialist when applying temperature sensors. A short phone call can prevent major headaches and lost time in  troubleshooting a misapplied thermocouple.

Thermocouples and RTDs Used in Power Plants

The majority of temperature measuring in a electrical generating plant are done with RTDs (resistance temperature detectors) and thermocouples (T/Cs).

RTD's are devices that produce a measurable resistance change with temperature change, while thermocouples produce a mV signal change in response to temperature change.

RTD's are constructed of a a thin conductor (nickel, platinum, copper) wrapped around a glass or ceramic bobbin, inserted into a protective sheath, and backfilled with an electrically inert, but thermally conductive, material.

Power plants historically use 100-ohm platinum, 100-ohm nickel, 120-ohm nickel, and 10-ohm copper RTDs. While providing excellent accuracy and long term stability, RTDs are prone to mechanical shock and vibration found in a generating facility. They are more expensive than thermocouples and application temperatures are generally limited to around up to 1110°F. One very attractive feature for RTDs are their inherent electrical noise immunity, a significant advantage over thermocouples. Finally, common, inexpensive instrument wire is used for connecting the RTD to the measuring instrumentation.

A thermocouple consists of two wires, made of dissimilar alloys, joined together at each end. One junction is designated the hot junction, the other junction is designated as the cold (or reference junction). When the hot junction experiences a change in temperature, a voltage is generated that is proportional to the difference in temperature between the hot and cold junctions. 

T/Cs are be made of different combinations of alloys and "calibrations" for use at various temperature ranges. The most common thermocouples for the power generation industry applications under 1800 °F are type are J, K and N ; for applications over 1800 °F,  types R and S are common. Aside from the obvious higher temperature capability, thermocouples provide faster response and greater shock and vibration endurance. However, thermocouples, due to the minuter signals the produce, are more susceptible to conducted and radiated electrical noise.  Another concern with thermocouples are their degradation over time when used at elevated temperatures and are therefore less stable than RTDs. One final issue is the need to run costly thermocouple extension wire of the same type as the thermocouple between sensor and measuring instrument.

When in doubt about which sensor is best to apply in a power plant application, contact an application expert who will help you choose the ideal sensor for your requirements.

https://duro-sense.com
Ph: 310-533-6877

What are Plastics Thermocouples?

Right Angle, Bayonet Style, Plastics Thermocouples

The term "plastics thermocouple" refers to a style of thermocouple designed and used by the plastics, packaging and rubber industries.  They are installed on injection molding, thermoforming, vacuum forming, and extruding equipment to accurately sense the temperature of the plastic molds and nozzles. While there are a variety of configurations of plastics thermocouples - such as bayonet, washer style, shim style, nozzle, and right angle - their basic components remains the same.

In most cases, plastics thermocouples are ANSI type J or type K calibration. Type J or K lead wire is available in a variety of insulation materials and protection options, such as high temperature fiberglass, PVC, stainless steel braided fiberglass, or stainless steel flexible armor cable. Electrical connections are most commonly bare leads, male thermocouple jacks, female thermocouple plugs, or spade lugs.

Bayonet designs are straight or right angle, and use industry standard bayonet fittings that easily retrofit most injection molding and plastics processing equipment. These fittings allow for adjustable depth and are spring loaded for maintaining goos contact with the media. Washer and shim style thermocouples weld or crimp the thermocouple sensing junction right to the washer or shim.

Bayonet thermocouples use a tube and wire design utilizing stranded thermocouple cable through out the probe. The metallic probe is made of 301, 304 or 316 series stainless steel. The thermocouple can made with a grounded, or ungrounded junction. A grounded junction is welded to the tip of the probe and, while it has very fast response, it can conduct electrical noise back to the instrumentation. An ungrounded junction is isolated from the metallic probe, and prevents electrical noise transmission. However, ungrounded junctions are slightly slower to respond to temperature changes.

For more information on plastics thermocouples, contact Duro-Sense by visiting https://duro-sense or by calling 310-533-6877.