Practical Procedure · Calibration

RTD Calibration and Verification — Dry-Block vs. Ice-Point Method

Applies to: RTD temperature sensors (Pt100/Pt1000, 2/3/4-wire) | Equipment needed: Dry-block calibrator or ice bath, precision reference thermometer, multimeter/ohmmeter

Pt100 RTD: Resistance Rises With Temperature

Resistance (Ω)

194Ω 138Ω 100Ω

0°C 100°C 250°C

Temperature (°C)

A Pt100 RTD starts at 100Ω at 0°C and increases by roughly 0.385Ω per °C — nearly linear across its working range. That predictable, repeatable relationship is exactly why RTDs are trusted for precision temperature measurement over thermocouples in the range where both could work.

Before You Start

Note the RTD type (Pt100 or Pt1000) and wiring configuration (2-wire, 3-wire, or 4-wire) — this affects both how you measure and how much lead resistance error matters.
Isolate the RTD from its transmitter/input card if you're testing the sensor alone, or plan to check the full loop if you're verifying end-to-end accuracy.
Inspect the sensor and cable for physical damage or corrosion at the termination before assuming any reading error is in the sensor element itself.

Method 1 — Ice-Point Check

A quick, low-cost single-point verification:

Prepare a bath of crushed ice and water, well-mixed, to establish a stable 0°C reference.
Immerse the RTD sensing tip fully in the ice bath, away from the container walls.
Allow time to stabilize, then measure resistance. A Pt100 RTD should read close to 100.00 Ω at 0°C.
This method only verifies one point (0°C) — it confirms the sensor hasn't drifted at that reference, but says nothing about accuracy at your actual operating temperature.

Method 2 — Dry-Block Calibration

The more complete method, used for multi-point calibration across the actual operating range:

Insert the RTD into the dry-block calibrator's test well alongside the block's own reference probe.
Set the dry-block to stabilize at your first test point — typically matching your process operating range (e.g. if the RTD monitors a 0–200°C range, test at 0%, 25%, 50%, 75%, 100% of that span).
Allow adequate soak time at each point — RTDs and dry-blocks both need time to reach thermal equilibrium; rushing this step is one of the most common sources of calibration error.
Compare the RTD's resistance reading (converted to temperature) against the dry-block's reference temperature at each point.
Repeat across all test points and record both as-found and as-left values.

Checking Lead Resistance (2-Wire vs. 3-Wire vs. 4-Wire)

Lead wire resistance can introduce measurement error, especially on long cable runs:

2-wire — most affected by lead resistance since it's included directly in the measurement. Only acceptable for short cable runs or low-accuracy applications.
3-wire — compensates for lead resistance assuming both leads have equal resistance. The standard choice for most industrial applications.
4-wire — fully eliminates lead resistance error through separate sense and current-carrying leads. Used where highest accuracy is required.

If you're seeing a consistent offset error that doesn't match the dry-block calibration results, check lead resistance and wiring configuration before condemning the sensor.

Signs of a Drifting RTD to Watch For

Resistance reading at 0°C (ice-point check) has shifted from the sensor's known baseline.
Error increases at higher temperatures but is fine near 0°C — often points to sensor element aging rather than a wiring problem.
Sudden step-changes in reading rather than gradual drift — more likely a connection/termination issue than sensor drift.
Reading fluctuates rapidly with no real process change — check for a loose termination or damaged lead insulation before condemning the element.

Common Mistakes to Avoid

Not allowing enough soak time at each dry-block test point, leading to false error readings.
Testing only at 0°C (ice point) and assuming that verifies accuracy across the whole operating range.
Ignoring lead resistance on 2-wire installations with long cable runs.
Not checking termination/connection quality before assuming the sensor element itself has drifted.

This is a general field procedure — specifics can vary by RTD type, plant calibration standards, and equipment available. Adjust to your own SOP where needed.

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