Temperature errors cost time, energy, and product quality. If you need steady, repeatable readings, a resistance temperature detector (RTD) is often the right tool. This guide explains what an RTD is, how it works, and how to pick and install one with confidence.
What An RTD Is—In Plain Words
A resistance temperature detector is a sensor that changes resistance with temperature. As the temperature rises, the resistance increases in a stable, predictable way. Your transmitter or controller converts that resistance into a temperature value. Because the change is linear and repeatable, RTDs give clean, stable readings for control and logging.
Why Teams Choose RTDs
- High accuracy and repeatability: Good for holding a tight setpoint.
- Low drift over time: Requires less recalibration compared to many other sensors.
- Wide usable range: Typical -50 °C to 250 °C for standard builds; special designs go higher.
- Simple to integrate: Works with common transmitters and PLC input cards.
The Core Choices: Element, Class, And Wiring
Element material: Most industrial RTDs use platinum because it is stable. You will see Pt100 and Pt1000.
- Pt100 = 100 Ω at 0 °C; common in industry.
- Pt1000 = 1000 Ω at 0 °C; this is particularly helpful when lead lengths are long, as the higher base resistance reduces lead errors.
Accuracy class: Look for Class A or Class B per IEC 60751.
- Class A is tighter—choose it for critical loops.
- Class B is a solid, cost-effective default.
Wiring options:
- 2-wire: simplest, but lead resistance adds error—use only for short runs or where accuracy is not critical.
- 3-wire: cancels most lead error if the three leads match; this is the common plant choice.
- 4-wire: best accuracy; used in laboratories or for long runs where precision is crucial.
Probe, Sheath, And Head
Probe diameter and length set the response and reach. Thinner probes respond faster but may be less rugged.
Sheath material (often 316 stainless steel) must match the process fluid and any cleaning chemicals.
The connection head protects the wiring and may hold a transmitter. For wet or dusty areas, use gasketed heads with the proper IP or NEMA rating.
RTD vs Thermocouple vs Thermistor—Quick Guide
- RTD: best for accuracy and stability from roughly -50 °C to 250 °C (and higher with special builds). Ideal for holding steady product temperature, utility loops, and test stations.
- Thermocouple: better for very high temperatures, fast response, or when you already use them across a furnace or kiln.
- Thermistor: very sensitive at narrow ranges; common in HVAC and electronics, less common in harsh industrial duties.
Sizing And Placement That Prevents Bad Data
Insertion depth: As a general rule of thumb, aim for at least 10 times the probe diameter in moving fluids, so the tip measures the true process temperature. Too shallow, and the reading skews toward wall temperature.
Thermal wells: In pressurized lines or abrasive liquids, use a thermowell. Choose the right bore, immersion length, and material. Streamline (tapered) designs improve strength and response.
Flow matters: Place the tip in a well-mixed region—downstream of a bend or a mixer—rather than in a dead leg.
Transmitters And Signals
Pair the sensor with a head-mount transmitter and convert resistance to a 4–20 mA or fieldbus signal. This reduces noise and lead errors, especially over long runs. Many transmitters support linearization (IEC 60751), sensor error alarms, and simple one-point trims.
Installation: A Short Checklist
- Verify part numbers: element type, class, wiring, length, and sheath material.
- Check continuity and insulation: quick meter check before mounting.
- Use heat-transfer paste in dry wells to improve response.
- Torque fittings to spec; avoid overtightening small-diameter probes.
- Route cables cleanly: keep away from high-power lines and VFD outputs.
- Commission slowly: power the loop, confirm the ambient reading, then bring the process up and watch for stable values.
Calibration And Care
- Traceable checks: For critical loops, do a periodic check at two points near your normal operating range.
- Match replacements: Keep a few spare RTDs with the same class and wiring so swaps do not change readings.
- Watch for drift triggers: harsh chemicals, repeated thermal shocks, or mechanical abuse can shift readings—schedule checks if any of these occur.
Common Mistakes (And Easy Fixes)
- Too little immersion: Extend the probe or change the well to reach the flow.
- Wrong wiring: A 3-wire RTD landed as a 2-wire can read high or low—verify at the head and at the card.
- Lead mix-ups: Use consistent cable colors and ferrules to avoid crossing sense and excitation leads.
- No transmitter in noisy areas: Add a head-mount transmitter to stabilize the signal.
Where RTDs Shine
Food and beverage lines, clean utilities, jacketed reactors, oil circulation skids, and test benches all benefit from the stable reading an RTD provides. In these settings, a resistance temperature detector helps hold quality limits, reduce scrap, and make audits easier because the data stays steady over long runs.
Choosing A Reliable Supplier
A strong vendor will size the element, select the sheath, and recommend the right thermowell and transmitter. You should get drawings, clear response time data, and guidance on mounting. Tempsens offers a wide range of probes, wells, and assemblies backed by application support, so your sensor choice works on day one and keeps working across seasons.
Bottom line: define the temperature range, choose the element class and wiring to match your accuracy need, place the probe in good flow, and pair it with a solid transmitter. With that setup, a resistance temperature detector delivers clean, dependable data that keeps your process on target.
