RTD / Thermocouple Conversion Tool

This comprehensive tool converts resistance (for RTDs) or millivolt signals (for Thermocouples) to temperature, and vice versa. Designed for high precision and reliability, it supports various sensor types and is essential for calibration, troubleshooting, and process control in all industrial sectors worldwide.

Sensor Configuration

RTD Parameters (IEC 60751)

Thermocouple Parameters (NIST ITS-90)

Professional Insights: RTD vs. Thermocouple

What is an RTD (Resistance Temperature Detector)?

An RTD is a high-precision temperature sensor. Its principle is simple: as the temperature of a metal (like platinum) increases, its electrical resistance increases in a very stable and predictable way. A "Pt100" is the most common type, meaning it has a resistance of 100.0 Ohms at 0°C. A "Pt1000" has 1000.0 Ohms at 0°C.

The relationship between resistance ($R$) and temperature ($T$) is defined by the Callendar-Van Dusen equation (part of the IEC 60751 standard):

$R_T = R_0 \cdot (1 + A \cdot T + B \cdot T^2)$     (for $T \ge 0^\circ C$)
$R_T = R_0 \cdot [1 + A \cdot T + B \cdot T^2 + C \cdot (T-100) \cdot T^3]$     (for $T < 0^\circ C$)

Where $R_0$ is the resistance at 0°C, and $A$, $B$, and $C$ are standard coefficients for platinum.

  • Pros: Very high accuracy, excellent stability and repeatability, good linearity over a wide range.
  • Cons: More expensive, slower response time, susceptible to self-heating (from the measurement current), more fragile.

What is a Thermocouple?

A thermocouple is a rugged, versatile temperature sensor. It works on the Seebeck effect: when two different types of metal wires (e.g., Nickel-Chromium and Nickel-Alumel for a Type K) are joined at one end, they create a tiny, temperature-dependent voltage (in millivolts). As the temperature at the "hot junction" (the measurement tip) changes, this voltage changes in a predictable, non-linear way defined by NIST tables.

  • Pros: Extremely wide temperature range (e.g., -200°C to over 2000°C), very fast response, durable, self-powered (generates its own voltage), and inexpensive.
  • Cons: Less accurate than an RTD, non-linear, and requires **Cold Junction Compensation**.

The "Gotcha!": Cold Junction Compensation (CJC)

This is the most critical and most misunderstood concept in thermocouple measurement. A thermocouple **does not measure absolute temperature**. It only measures the **temperature difference** between its hot junction (the tip) and its cold junction (the terminals on the transmitter or PLC card).

If the hot junction is 100°C and the terminal block (cold junction) is 25°C, the thermocouple will only output the millivolts for 75°C (100 - 25).

To get the correct reading, the transmitter *must* have a second, built-in thermometer (a thermistor) to measure the temperature of the terminal block. This is the **Cold Junction Compensation (CJC)**.

$T_{\text{Actual}} = T_{\text{Measured (from mV)}} + T_{\text{Cold Junction}}$
This calculator requires you to enter the cold junction temperature (often 0°C for tables, or a real-world 20-25°C) to perform this essential compensation. A 0.0mV reading does not mean 0°C; it means the hot end is the exact same temperature as the cold end.

Why 3-Wire and 4-Wire RTDs are Better than 2-Wire

The problem with RTDs is that the connecting copper wires also have resistance. This "lead resistance" adds to the RTD's resistance, making the transmitter think the temperature is higher than it really is. A 100-foot run of wire could add several Ohms, translating to many degrees of error.

  • 2-Wire RTD: The cheapest method. The transmitter measures the total resistance of the RTD *plus* both lead wires. It's impossible to separate the true RTD resistance from the wire resistance. This method is inaccurate and should not be used in industry.
    $R_{\text{Total}} = R_{\text{RTD}} + R_{\text{Wire1}} + R_{\text{Wire2}}$
  • 3-Wire RTD: The most common industrial standard. It uses a clever bridge circuit. It measures the resistance of the RTD + 2 wires, then measures the resistance of *just* 2 wires on a "compensation loop." By subtracting the wire resistance, it calculates the *true* resistance of the RTD element. This works perfectly as long as all three wires are the same length, gauge, and material.
  • 4-Wire RTD: The most accurate (laboratory grade). It uses two wires to send a tiny, precise measurement current (e.g., 1mA) *through* the RTD. It then uses two *completely separate* wires to measure the voltage drop *directly across* the RTD element. Since no current flows in the measurement wires, their resistance doesn't matter. Ohm's Law ($V=IR$) gives the true resistance of the RTD with zero error from the lead wires.

When to Choose Which Sensor

Use an RTD (e.g., Pt100) when you need:

  • High Accuracy: For critical process control, custody transfer, or scientific measurement where 0.1°C matters.
  • High Stability: For applications that must not "drift" over time.
  • Good Linearity: When you need a signal that is nearly linear with temperature.
  • Typical Range: -200°C to 600°C.

Use a Thermocouple (e.g., Type K) when you need:

  • High Temperature: For furnaces, ovens, engines, or anything over 600°C.
  • Fast Response: For processes that change temperature very quickly.
  • Durability: For high-vibration or high-impact environments.
  • Low Cost: When you need to measure many points cheaply.
  • Point Sensing: When you need to measure the temperature of a tiny, specific point.