How they work
RTD (resistance temperature detector)
A fine platinum wire or film whose resistance rises with temperature. A Pt100 is 100 Ω at 0 °C and about 138.5 Ω at 100 °C (α = 0.00385). The transmitter passes a small current and measures the resistance.
Thermocouple (TC)
Two different metals joined at the hot end. The junction creates a small voltage (millivolts) that depends on the difference between the hot junction and the cold end where the wires meet copper. Type K gives about 41 µV/°C.
Convert either one in the app: RTD Converter · Thermocouple Converter.
Which one to choose
| RTD (Pt100) | Thermocouple | |
|---|---|---|
| Typical range | −200 to about 600 °C | −200 to 1700+ °C, depending on type |
| Accuracy | Better: Class A ±(0.15 + 0.002·|t|) °C | Lower: type K class 1 about ±1.5 °C or 0.4 % |
| Stability / drift | Very stable | Drifts more, especially at high temperature |
| Response | Slower (larger element) | Faster (tiny junction) |
| Vibration | Wire-wound can be fragile; thin film is better | Rugged |
| Wiring | Ordinary copper, 3- or 4-wire | Matching extension/compensating cable |
| Cost | Higher sensor cost | Lower sensor cost |
Rule of thumb: use an RTD for accuracy below about 500–600 °C (process control, custody transfer, bearings). Use a thermocouple for high temperatures (furnaces, heaters, exhaust, flue gas) or where fast response and ruggedness matter more than accuracy.
RTD wiring: 2, 3 and 4 wire
Best practice: mount a head-mounted transmitter on the sensor and send 4–20 mA back to the control room. The short RTD leads then don't matter, and the signal is immune to lead resistance and noise.
Thermocouple types and cable
| Type | Metals | Approx. range | Typical use |
|---|---|---|---|
| K | Chromel / Alumel | −200 to 1250 °C | General purpose, most common |
| J | Iron / Constantan | 0 to 750 °C | Older plants, reducing atmospheres |
| T | Copper / Constantan | −200 to 350 °C | Low temperature, cryogenic |
| N | Nicrosil / Nisil | −200 to 1250 °C | More stable alternative to K |
| E | Chromel / Constantan | −200 to 900 °C | Highest output (mV/°C) |
| R, S, B | Platinum / Rhodium | up to 1600–1700 °C | Furnaces, glass, very high temperature |
- Between the TC head and the transmitter, use matching extension or compensating cable of the same type. Plain copper creates a new junction and a large error.
- Watch the polarity: reversing the wires makes the reading go the wrong way as temperature rises.
- Colour codes differ between standards (IEC 60584 vs ANSI). Check which one your project uses before trusting colours.
Cold-junction compensation
A thermocouple measures the difference between its hot junction and the point where it connects to the copper terminals (the cold junction). The transmitter or input card measures the terminal temperature and adds it back:
mV(hot) = mV(measured) + mV(cold-junction temperature) → convert to °C
That is why you cannot just read the mV with a multimeter and look it up in a table without correcting for terminal temperature. A calibrator in "TC simulate" mode handles this automatically.
Thermowells and response time
- A thermowell protects the sensor and lets it be removed without opening the process. It also slows the response (often 30 s to minutes).
- Spring-loaded sensors pressed to the bottom of the well, and a well bore matched to the sensor, speed up response.
- Long wells in fast flow can vibrate and break: they need a wake-frequency calculation (ASME PTC 19.3 TW).
- Insertion length: typically the tip in the middle third of the pipe, and at least about 10 sensor diameters immersion to avoid stem conduction errors.
Troubleshooting
| Symptom | Likely cause |
|---|---|
| RTD reads very high / over-range | Open circuit (broken element or loose terminal) |
| RTD reads very low / under-range | Short circuit, moisture in the head |
| RTD reads a few °C high, steady | 2-wire connection or unbalanced 3-wire leads |
| TC reads near ambient while process is hot | Copper cable used instead of extension cable, or a shorted junction near the head |
| TC reading falls as process heats | Reversed polarity |
| TC reading wrong by a constant offset | Wrong TC type configured, or cold-junction compensation off |
| Noisy, jumping reading | Grounded junction plus earth loop, cable near VFD/power cables |
| Slow to respond | Sensor not touching the thermowell bottom, oversized well |
Quick check: disconnect the RTD and measure it with a multimeter (≈ 100 Ω at 0 °C, ≈ 108 Ω at 20 °C). For a TC, check continuity (a few ohms) and insulation to earth.
Q&A: test yourself
Related: Temperature Measurement · Pressure Calibration · Loop Checking