Resistance thermometers Manufacturers online

Resistance thermometers (also known as RTDs for "Resistance Temperature Detectors") are temperature sensors that are based on the property of materials that their electrical resistance changes with temperature.

They are frequently used in industrial, scientific and technical applications for the precise measurement of temperatures.

Below you will find the functional principle of resistance thermometers, an overview of types and measuring circuits, their use and their advantages and disadvantages:

Operating principle:

The functional principle of a resistance thermometer is based on the dependence of the electrical resistance of a material on the temperature. With an RTD, such as the commonly used platinum resistance thermometer (PT100 or PT1000), the resistance increases with increasing temperature. The change in resistance is linear, which enables precise calculation of the temperature.

Example:

- PT100: Has a resistance of 100 Ohm at 0 °C.
- PT1000: Has a resistance of 1000 Ohm at 0 °C.

Types of resistance thermometers:

1. Platinum resistance thermometers (PT100, PT1000):
These are the most common because of their high accuracy, stability and wide temperature range.

2. Nickel resistance thermometers:
Less accurate and stable, but less expensive.

3. Copper resistance thermometers:
Inexpensive, but only suitable for low temperatures.

Measuring circuits:

1. Two-wire circuit:
Simplest method, but it is less accurate as the resistance of the leads is factored into the measurement.

2. Three-wire circuit:
Partially corrects for the influence of lead resistance and is often used in industrial applications.

3. Four-wire circuit:
Eliminates the influence of lead resistance, therefore most accurate, used in precision measurement applications.

Use and application:
Resistance thermometers are used in various fields:

1. Industry:
For monitoring processes, such as in the chemical or metal industry.

2. Science:
For precise temperature measurements in laboratories.

3. Household:
In household appliances such as ovens or heating systems.

4. Automotive:
For checking engine temperatures or for climate control.

Advantages:

• High precision: Platinum resistance thermometers in particular, such as PT100 or PT1000, offer high accuracy.
• Good stability: They offer long-term stable temperature measurement.
• Wide temperature range: Typically from -200 °C to +850 °C.
• Repeatability: They provide consistent measurements over long periods of time.

Disadvantages:

• Cost: Platinum RTDs are more expensive than thermocouples or NTC thermistors.
• Sensitivity to mechanical stress: Shocks or vibrations can affect measurement accuracy.
• Relatively slow response time: Compared to thermocouples, the response time is slower.

Resistance thermometers offer a precise and stable way of measuring temperature and are essential in a wide range of applications.

Here are the most important aspects to look out for when buying resistance thermometers:

When buying a resistance thermometer, there are various aspects to consider to ensure it is suitable for the desired application.

• Check that the temperature range of the resistance thermometer is suitable for the application. common RTDs such as PT100 measure temperatures between -200 °C and +850 °C.
• Industrial applications often require wide ranges, while laboratory applications may only measure low or medium temperatures.
• Tolerance classes: There are different accuracy classes (according to IEC 60751):
• Class A: Higher accuracy (±0.15 °C at 0 °C).
• Class B: Standard accuracy (±0.3 °C at 0 °C).
• Ensure that the thermometer has good long-term stability, especially in industrial processes where continuous and accurate measurements are required.
• Type of resistance thermometer: PT100 or PT1000:
• PT100 (100 Ohm at 0 °C): Widely used, good balance between accuracy and price.
• PT1000 (1000 Ohm at 0 °C): Higher sensitivity and often more suitable for long cable runs as the influence of cable resistance is lower.
• Platinum is the most common material, nickel and copper are common materials. Platinum has the widest temperature range and is the most stable.
• The measuring circuit determines the accuracy:
• Two-wire circuit: simple, but less accurate due to the influence of lead resistance. Suitable for applications where accuracy is less important.
• Three-wire circuit: Good compromise solution, often used in industry, as the lead resistance is partially compensated.
• Four-wire circuit: Highest accuracy as the influence of lead resistance is completely eliminated. Ideal for precise measurements.
• Pay attention to the sensor's casing to protect it from environmental conditions (e.g. chemical resistance, watertightness, high mechanical loads).
• In harsh environments or industrial applications, a protective tube (thermowell) may be required to protect the sensor from mechanical and chemical influences.
• Stainless steel is often used as the sheath material as it is robust and corrosion-resistant.
• The response time is determined by the design:
• Thin-film sensors: faster response, but may be less stable.
• Wire-wound sensors: More stable and accurate, but somewhat slower.
• If rapid temperature changes need to be measured, a short response time is crucial.
• Pay attention to the cable length and cable type. Long cables can affect the resistance, so PT1000 or four-wire circuits are recommended for longer distances.
• If the cable is exposed to extreme conditions (e.g. high temperatures or chemicals), it must be protected or insulated accordingly.
• Depending on the application, it may be advisable to use robust plug connections and cable guides.
• In environments with vibrations or mechanical stress, a robust sensor is required.
• In humid or corrosive environments, protection against moisture or chemicals is required, either through special coatings or hermetically sealed covers.
• Check whether the manufacturer offers calibration to the required standards.
• Check whether the sensor can be calibrated regularly and whether there is a simple procedure for recalibration.
• DIN/IEC 60751: Most platinum resistance thermometers should comply with this standard.
• Ex-protection (explosion-proof): Special sensors with Ex certification are required in hazardous environments (e.g. in the oil and gas industry).
• Make sure that the manufacturer works according to ISO 9001 or similar quality standards to ensure high product quality.
• Resistance thermometers are more expensive than thermocouples or NTC thermistors. Make sure that the higher price is justified by the requirements of the application (e.g. higher precision or stability).
• Consider maintenance, calibration and replacement costs when deciding on a resistance thermometer.

When purchasing a resistance thermometer, the most important factors are the measuring range, the accuracy, the type of thermometer (e.g. PT100 or PT1000), the measuring circuit (two-, three- or four-wire), the sheath of the sensor and the environment in which it will be used. The choice of the right sensor depends on the specific requirements of the application, such as the required precision, the environmental conditions and the price.

Resistance thermometers can be purchased from various suppliers. Comprehensive information about manufacturers and suppliers of Resistance thermometers from all over the world can be found on Exportpages.