How to

# RTD Tolerance Calculation

• Resistance Temperature Detectors (RTDs) are widely used in industrial and scientific applications for accurate temperature measurement.
• Understanding and calculating the tolerance of RTDs is crucial for ensuring precise and reliable readings.
• This guide will explain the fundamentals of RTD tolerance, the standards governing it, and how to calculate it effectively.
• RTD tolerance refers to the acceptable range of deviation in the resistance value of the RTD sensor at a given temperature.
• This deviation impacts the accuracy of the temperature readings. The tolerance is typically defined by international standards such as IEC 60751 and ASTM E1137, which classify RTDs into different accuracy classes.

The European standard, often referred to as the DIN or IEC standard, plays a crucial role in defining the specifications for Pt100 resistance temperature detectors (RTDs). Let’s break down the details:

1. Standard Name: DIN/IEC 60751 (or simply IEC751)
2. Nominal Resistance at 0°C: The RTD is required to have an electrical resistance of 100.00 ohm at 0°C. This value serves as the reference point for temperature measurement.
3. Temperature Coefficient of Resistance (Alpha): The alpha coefficient, denoted by α, represents the change in resistance with respect to temperature. For Pt100 RTDs, the standard alpha value is 0.00385 ohm/ohm/°C between the temperature range of 0°C to 100°C.

The below  table summarizes the electrical resistance standards set by various organizations. The “Alpha” column represents the temperature coefficient of resistance, and the “Nominal Resistance at 0°C” column provides the resistance value at 0 degrees Celsius. Professionals working with precision instruments or components should consider these standards during design, calibration, or measurement processes.

Pt100 RTDs following the DIN/IEC 60751 standard exhibit a consistent resistance of 100.00 ohm at 0°C and a predictable change in resistance as the temperature varies within the specified range. These RTDs are widely used in industrial applications, process control, and instrumentation due to their accuracy and reliability.

Standards and Accuracy Classes The most commonly referenced standard for RTDs is IEC 60751. According to this standard, RTDs are classified into several accuracy classes, including Class AA, A, B, and C.

Each class specifies the allowable deviation in resistance at 0°C, which is crucial for high-precision applications.

Class AA: ±(0.1 + 0.0017|t|)°C

Class A: ±(0.15 + 0.002|t|)°C

Class B: ±(0.3 + 0.005|t|)°C

Class C: ±(0.6 + 0.01|t|)°C

Here, |t| represents the absolute temperature in degrees Celsius.

The resistance vs. temperature properties of the RTD sensor are determined by the combination of temperature coefficient and resistance tolerance. The more variance between sensors (interchangeability) and the more the sensor deviates from a generalized curve, the larger the element tolerance. This is crucial for users who need to reduce interchangeability errors while changing or replacing sensors.

• Tolerance (°C)=±(0.15+0.002⋅∣t∣)
• Tolerance (°C)=±(0.30+0.005⋅∣t∣)
• Tolerance (°C)=±(0.13+0.0017⋅∣t∣)
• Tolerance (°C)=±(0.25+0.0042⋅∣t∣)

These formulas account for errors arising from deviations in nominal resistance (R0) and from errors in the slope (alpha coefficient). It’s important to choose the appropriate accuracy class based on your specific application requirements.

Example 1: Calculate the Accuracy of a 0.1 ASTM Class A Probe at 100 °C

Example temperature t=100.0 °C

For ASTM 1137 Class A:

Tolerance (°C)=±(0.13+0.0017⋅∣t∣)

For t=100.0 °C

Tolerance=±(0.13+0.0017⋅100)

Tolerance=±(0.13+0.17)

Tolerance=±0.30°C

Multiply the calculated tolerance by the fractional factor:

Fractional Tolerance=0.10 X 30

Fractional Tolerance=±0.03 °C

Reference Temperature Tref=100.0 °C

Indicated Temperature Tind=100.5 °C

Error = Tind−Tref

Error = 100.5 °C−100.0 °C

Error = 0.5°C

0.5°C > 0.03°C

The error exceeds the tolerance, thus the RTD fails the tolerance test.

Several factors influence the tolerance and accuracy of RTDs:

• The purity of the platinum wire used in the RTD directly affects its resistance characteristics. Higher purity platinum ensures more predictable and stable resistance changes with temperature.
• Precision in the manufacturing process ensures that the RTD conforms closely to specified tolerance limits. This includes tight control over the dimensions and the welding of the platinum wire, as well as consistent winding techniques.
• Proper calibration against known temperature standards ensures that the RTD performs accurately.

• Exposure to high humidity levels can cause corrosion and insulation breakdown, affecting the accuracy and stability of the RTD.
• Mechanical vibrations can induce microcracks or alter the strain in the RTD element, leading to changes in resistance.
• Physical stress and strain can deform the RTD element, affecting its resistance characteristics.
• Over time, the properties of the RTD material can change due to prolonged exposure to high temperatures or other environmental factors, leading to a drift in resistance values.
• Gradual changes in resistance values over time can result from material fatigue, contamination, or other long-term effects, impacting the accuracy and tolerance of the RTD.
• Definition: Interchangeability refers to the ability to replace an RTD with another of the same type and accuracy class without recalibration.

• Impact: High interchangeability minimizes downtime and maintenance costs in industrial settings.
• Standards: RTDs meeting stringent standards (e.g., Class AA or A) have higher interchangeability due to tighter tolerance limits.
• High Precision: Applications like laboratory measurements, where even small deviations can be critical, require Class AA or A RTDs.
• Industrial Applications: Process control applications may suffice with Class B or C RTDs, balancing cost and precision.
• Harsh Environments: RTDs used in extreme conditions (high humidity, vibration, etc.) must be robust and possibly have protective coatings or housings.
• Temperature Range: The selected RTD should cover the entire temperature range of the application without exceeding its tolerance limits.
• Cost vs. Accuracy: Higher accuracy RTDs (Class AA or A) are more expensive. The decision should balance the accuracy required and the available budget.
• Usage: Monitoring and controlling temperatures in chemical processing, food and beverage production, and pharmaceutical manufacturing.
• Requirements: Moderate to high accuracy (Class B or A), robustness against environmental factors, and long-term stability.
• Usage: Temperature monitoring in heating, ventilation, and air conditioning systems to ensure comfort and energy efficiency.
• Requirements: Standard accuracy (Class B), reliable performance over a broad temperature range.
• Usage: Temperature sensing in medical diagnostics equipment, incubators, and patient monitoring systems.
• Requirements: High accuracy (Class A or AA), biocompatibility, and fast response time.
• Wire Wound RTDs: Made by winding a thin wire around a ceramic or glass core, providing high accuracy and stability but are more fragile.
• Thin Film RTDs: Constructed by depositing a thin platinum layer on a ceramic substrate, offering better shock resistance and compact size but slightly less accurate.
• 2-Wire Configuration: Simplest and most economical but prone to lead wire resistance errors.
• 3-Wire Configuration: Compensates for lead wire resistance, suitable for most industrial applications.
• 4-Wire Configuration: Provides the highest accuracy by completely eliminating lead wire resistance, used in high-precision applications.