How do I choose the right Thermocouple?
“what kind of thermocouple?” Is a question that needs to be answered but requires a few more questions to be answer first to get the right thermocouple for your application. Temperature measurement is important in manufacturing and processing operations in many industries. Accurately monitoring the temperature helps ensure safe, efficient, and optimal outcomes. The two most common industrial temperature measurement sensors are resistance temperature detectors (RTDs) and a thermocouple offers relatively low cost, fast response time, and the ability to measure media up to 2,300°C (RTDs are suitable for use for up to only 850°C). However, thermocouples are less accurate than RTDs (their accuracy degrades over time), and their output is nonlinear. RTDs offer greater accuracy, repeatability, and stability, but they have slower response times and are more expensive than thermocouples.
Your first steps to select the correct thermocouple for you is to choose the application where the thermocouple will be used, research the temperature ranges the thermocouple will be exposed to, consider any chemical resistance needed for the thermocouple or sheath material, assess the need of abrasion and vibration resistance. Also, any installation requirements such as the need to be compatible with existing equipment eg: existing fittings/holes may determine probe diameter.
The process is then specific to your application and requirements you need to consider the sensor type, diameter of the probe, the length of the probe, if the sensor requires a sheath, the junction type, if you require fittings, floats or a thermowell. Also, if you require cable and what type of cable. If you requires accessories.
The process is then specific to your application and requirements you need to consider the sensor type, diameter of the probe, the length of the probe, if the sensor requires a sheath, do you require a terminal head and/or transmitter. Which junction type, if you require fittings, floats or a thermowell. Also, if you require cable and what type of cable. If you requires accessories.
The following details are a brief introduction to the types and uses of thermocouples.
There are numerous styles of sensors to choose from below we have attached a few Data sheets to show some of the variety.
You will have to know what kind of junction you require for your application, there are three versions.
1-A Grounded Junction Thermocouples: at the tip of a grounded junction probe, the thermocouple wires are physically attached to the inside of the probe wall. This results in good heat transfer from the outside, through the probe wall to the thermocouple junction. This means that grounded thermocouples will have faster response times than ungrounded thermocouples. The grounded junction is recommended for the measurement of static or flowing corrosive gas and liquid temperatures and for high-pressure applications. The junction of a grounded thermocouple is welded to the protective sheath giving faster response than the ungrounded junction type. However, grounded thermocouples are highly susceptible to noise-induced by ground loops, resulting in less accurate readings.
2- Ungrounded Junction Thermocouples: In an ungrounded probe, the thermocouple junction is detached from the probe wall. Response time is slower than the grounded style. On the other hand, the junction is electrically isolated from the sheath, which prevents electrical noise from interfering with the signal. This yields much greater temperature measurement accuracy, especially with very low-level signals. An ungrounded junction is recommended for measurements in corrosive environments where it is desirable to have the thermocouple electronically isolated from and shielded by the sheath. The welded wire thermocouple is physically insulated from the thermocouple sheath by MgO powder (soft).
3- Exposed Junction Thermocouples: The thermocouple in the exposed junction style protrudes out of the tip of the sheath and is exposed to the surrounding environment. This type offers the best response time but is limited in use to noncorrosive and non-pressurised applications. An exposed junction is recommended for the measurement of static or flowing non-corrosive gas temperatures where fast response time is required. The junction extends beyond the protective metallic sheath to give accurate fast response. The sheath insulation is sealed where the junction extends to prevent penetration of moisture or gas which could cause errors. The choice of the hot junction will be dependent upon the response of the thermocouple, contamination hazards, physical restraints, and electrical earthing considerations.
Pyrosales has extensive knowledge and experience in temperature management and process control. We provide advice, design, repair, and fabrication to our clients on a wide range of industries, uses, and capabilities of temperature management.