PC-03Process measurement · TT
Temperature Sensors, Thermowells & Transmitters
Temperature is one of the most frequently measured process variables. The sensor, the thermowell and the transmitter are selected together — range, response time and mechanical load decide each one.
01Scope
What's included
RTDs, thermocouples, thermowells and temperature transmitters.
- Resistance thermometers (RTD)Pt100 and Pt1000 elements in 2-, 3- or 4-wire connection, as inserts or complete assemblies.
- ThermocouplesBase-metal and noble-metal types (e.g. K, J, N, R, S, B), usually mineral-insulated and sheathed.
- ThermowellsThreaded, flanged, weld-in and hygienic designs — bar-stock or fabricated — sized for the process load.
- Temperature transmittersHead-mounted, rail-mounted or field-housing transmitters with linearization and sensor diagnostics.
- Local and non-contact measurementBimetal and filled-system thermometers for local reading; infrared pyrometers for surfaces and moving material.
02Compare
Sensing principles
Accuracy and stability favour an RTD; high temperature and fast response favour a thermocouple. The thermowell and transmitter complete the measurement.
| Principle | How it works | Suited for | Considerations |
|---|---|---|---|
| RTD (Pt100 / Pt1000)Media: LiquidGasSteam | How it worksPlatinum's resistance rises predictably with temperature — a Pt100 reads 100 Ω at 0 °C. | Suited forProcess control, utilities, pharmaceutical and food applications where accuracy and long-term stability matter, up to roughly 600 °C depending on construction. | Considerations3- or 4-wire connection cancels lead resistance; the tolerance class (IEC 60751) sets interchangeability; more sensitive to vibration than a thermocouple. |
| ThermocoupleMedia: LiquidGasSteamBulk solids | How it worksTwo dissimilar metals joined at the tip generate a small voltage that depends on the temperature difference to the reference junction. | Suited forHigh temperatures — furnaces, kilns, flue gas, superheated steam — and fast response with small sheathed elements. | ConsiderationsLower accuracy and more drift than an RTD; needs cold-junction compensation and matching extension or compensating cable; the type must suit temperature and atmosphere. |
| ThermowellMedia: LiquidGasSteam | How it worksA pressure-retaining sleeve separates the sensor from the process so the sensor can be removed while the process runs. | Suited forPressurized, flowing or corrosive processes where the sensor must stay serviceable. | ConsiderationsAdds response time; a wake-frequency and stress calculation (ASME PTC 19.3 TW) checks it against flow-induced vibration. |
| Temperature transmitter | How it worksConverts the sensor signal close to the measuring point into 4–20 mA / HART or fieldbus, with linearization and sensor diagnostics. | Suited forLong cable runs, electrically noisy plants and anywhere a standard control-system signal is needed. | ConsiderationsChoose head, rail or field mounting; dual-input versions provide sensor redundancy or differential temperature. |
| Infrared (non-contact)Media: Bulk solids | How it worksMeasures the thermal radiation emitted by a surface. | Suited forMoving, very hot or inaccessible surfaces — kiln shells, hot metal, rotating equipment. | ConsiderationsThe target's emissivity and the optical path (dust, steam, windows) affect the reading; it measures the surface only. |
General engineering guidance. Limits and performance depend on the specific instrument and variant — confirm them against the manufacturer's datasheet for the selected model.
03Specify
What to specify
The parameters that decide the selection. State each with its unit; where a value is not known yet, say so — it becomes part of the engineering review.
Temperature range
Decides between RTD and thermocouple and, for thermocouples, the type and sheath material.
Accuracy and tolerance class
Sets the sensor class and whether sensor-transmitter matching or system calibration is needed.
Response time
Fast loops need small sensors and thin, short thermowells — which conflicts with mechanical strength.
Process pressure and flow velocity
Determine the thermowell design and its wake-frequency limit.
Immersion length
Too short and heat conducted along the stem biases the reading; too long and the thermowell may not survive the flow.
Medium and wetted material
Thermowell material must resist corrosion and erosion at process temperature.
Process connection
Threaded, flanged, weld-in or hygienic — matched to the nozzle, pressure rating and cleaning regime.
Output and communication
Direct sensor wiring or a transmitter with 4–20 mA / HART or fieldbus — decided by cable length, noise and the input cards.
Area classification
Hazardous areas need certified transmitters and heads, and intrinsically safe or flameproof installation.
Ambient conditions
Vibration, ambient temperature at the head and washdown decide construction and enclosure rating.
04Integrate
Outputs & communication
Interfaces typically offered in this category, and what each means for the control system. Availability depends on the specific model.
- 4–20 mAAnalog
- Current loop with a live zero: 4 mA is the bottom of the range, so a broken wire (0 mA) is distinguishable from a zero reading. Two-wire (loop-powered) on most transmitters; four-wire with an active output on line-powered instruments such as analyzers — the I/O channel must match. Accepted by practically every PLC and DCS analog input.
- HARTDigital over analog
- A digital signal (Bell 202 FSK, 1200/2200 Hz) superimposed on the 4–20 mA loop — configuration, diagnostics and secondary variables over the existing two wires.
- PROFIBUS PAFieldbus
- Two-wire, bus-powered process fieldbus at 31.25 kbit/s, coupled to PROFIBUS DP — several instruments share one cable, with intrinsically safe variants for hazardous areas.
- FOUNDATION Fieldbus H1Fieldbus
- Two-wire, bus-powered digital fieldbus at 31.25 kbit/s with function blocks in the field devices — used mainly where the DCS architecture is built around it.
- RTD / TC directSensor signal
- Sensor wired straight to a temperature input card, without a transmitter. RTDs need 3- or 4-wire connection to cancel lead resistance; thermocouples need matching extension or compensating cable.
05Install
Installation practice
- Immerse far enough to limit conduction error along the stem — a common rule of thumb is at least ten times the sheath or thermowell diameter — while staying within the thermowell's wake-frequency limit.
- Measure in a well-mixed, flowing section, not in dead legs or directly downstream of a mixing or injection point.
- Mount a transmitter near the sensor for long runs: thermocouple signals are in the millivolt range and pick up noise easily.
- Plan calibration access — removable inserts let the sensor be checked against a reference without opening the process.
06Products
Temperature products
Status · no models listed
Specific models are not listed on the site.
Our engineers can recommend instruments against your process data. The selection checklist on this page is the information that decides it — send what you have.
How a review runs
TT01Share the process data
Medium, range, conditions, connection and the system it reports to.
02Engineering review
The measuring principle and the installation are checked against the application.
03Instrument options
Suitable devices are proposed with the data needed to confirm the selection.
Next steps
Where to go next
- 01SolutionInstrumentationProcess measurement and final control elements — selected for the process conditions, installed, calibrated and loop-checked.
- 02ResourceWhat an Instrumentation & Automation AMC Should CoverScope, preventive schedules, response planning and records for a useful AMC.
- 03Related categoryPressureGauge and absolute pressure transmitters, switches and gauges.
Engineering enquiry
Specifying a temperature measurement point?
Share the medium, the range, the process conditions and the system it has to connect to. Our engineers can help match the principle and the instrument to the application.