Fixed-point cells – find manufacturers and suppliers

Fixed-point cells are high-precision reference systems for generating precisely defined temperatures based on physically exactly reproducible phase transitions of pure substances.

They enable an extremely low measurement uncertainty. In calibration laboratories, national metrology institutes, industrial reference laboratories and in quality assurance, they form the basis for traceable temperature calibrations. They are used in particular for calibrating resistance thermometers (Pt100, SPRT), thermocouples, digital temperature sensors and reference thermometers.

Below you will find the functionality of fixed-point cells, an overview of different types, their areas of application and their advantages:

How fixed-point cells work:

The way they work is based on a fundamental physical principle: pure substances have precisely reproducible phase transition temperatures at a defined pressure.
During the transition between solid and liquid or liquid and gas, the temperature remains constant as long as both phases are present at the same time.
This stable equilibrium temperature is referred to as the fixed point.

A fixed point cell typically consists of:

1. A high purity metal or ultrapure water.
2. A closed quartz or metal housing.
3. A thermowell for the thermometer to be calibrated.
4. A controlled environment to create the phase transition.

Controlled melting or solidification of the material creates an isothermal zone with an extremely stable temperature.
The thermometer to be calibrated is placed in the immersion sleeve and compared with the known reference temperature.
The accuracy can be in the millikelvin or even submillikelvin range, especially for cells based on the International Temperature Scale ITS-90.

Types of fixed-point cells:

1. Water three-point cells:
   The best known fixed point is the three-point of water at 0.01 °C. Ice, water and water vapor coexist simultaneously in a special glass cell.
   This point serves as a fundamental reference for the calibration of high-precision resistance thermometers.
2. Metal solidification point cells:
   These cells utilize the solidification or melting points of high purity metals such as:
   Gallium, Indium, Tin, Zinc, Aluminum and Silver.
   They cover temperature ranges from just above 0 °C to over 900 °C and are used in particular for industrial high-temperature calibrations.
3. Metal freezing point cells:
   The freezing process is specifically used here, as a particularly stable temperature plateau phase is created during solidification.
   This method offers high reproducibility.
4. High-temperature fixed-point cells:
   For very high temperatures, special high-temperature cells are used, which are operated in inert gas or furnace systems.
   They are often used in national metrology institutes.
5. Miniature fixed-point cells:
   Compact designs for use in industrial calibration laboratories.
   They offer lower uncertainties than comparison calibrators, but are easier to handle than classic ITS-90 cells.

Use of fixed point cells:

1. Calibration of standard platinum resistance thermometers (SPRT):
Fixed-point cells are used as a reference to define the characteristic curve of high-precision reference thermometers.

2. Traceable industrial calibration:
In accredited laboratories, temperature measuring devices are calibrated using fixed points to meet international standard requirements.

3. Validation of temperature measuring chains
In industries such as pharmaceuticals, chemicals, energy or aviation, complete traceability is mandatory. Fixed point cells ensure this metrological traceability.

4. Checking temperature baths and calibration furnaces
They are used to regularly check secondary calibration systems.

5. Research and development
In metrological institutes, they are used to realize the international temperature scale and to develop new reference methods.

Advantages of fixed-point cells as calibration equipment:

• Highest accuracy: fixed-point cells provide extremely low measurement uncertainties and are reference systems of the highest accuracy class.
• Excellent reproducibility: Phase transitions of pure substances are physically clearly defined and comparable worldwide.
• Long-term stability: With proper handling, fixed-point cells retain their accuracy for many years.
• Conformity to standards: They allow calibrations according to international temperature scales such as ITS-90.
• Independence from electronic reference systems
  The temperature is physically realized, not electronically simulated.
• Wide temperature range: Different metals allow temperature ranges from near 0 °C to over 1000 °C to be covered.
• Minimization of measurement uncertainty: Compared to dry-block or liquid calibrators, fixed-point cells offer significantly lower uncertainties.

Fixed-point cells are the most precise means of calibration in the field of temperature measurement technology. They are based on clearly defined physical phase transitions and enable extremely accurate, reproducible and internationally comparable reference temperatures. Whether water three-point cell or high-temperature metal fixed point - they form the basis for traceable calibrations in laboratories, industry and metrology institutes. Thanks to their high accuracy, stability and conformity to standards, they are indispensable for applications where the smallest temperature deviations are crucial.

Here are the most important aspects to look out for when purchasing fixed-point cells:

Fixed-point cells are among the most precise calibration devices in temperature measurement technology. They are used in accredited laboratories, industrial reference laboratories and research institutes to calibrate temperature measuring devices with the highest accuracy. Due to their central role in metrological traceability, the professional purchase of fixed-point cells is a demanding process. In addition to selecting the right fixed point, purity, uncertainty, design, long-term stability, conformity to standards and service aspects must be carefully evaluated. A wrong decision can increase the measurement uncertainty or jeopardize traceability.

• The procurement process begins with a precise definition of the temperature range to be covered. Fixed-point cells are based on defined phase transitions of pure substances. Depending on the application, for example, water three-point cells for 0.01 °C or metal fixed-point cells such as gallium, indium, tin, zinc or aluminum are used.
• The purchasing department must clarify whether individual reference points are sufficient or whether several fixed points are required for a complete calibration curve. Several ITS-90 fixed points are required for standard platinum resistance thermometers in particular.
• A key criterion is the purity of the material used. The accuracy of a fixed point depends directly on the chemical purity of the material.
• High-precision cells use metals or water with extremely high purity, often in the range of 99.999 % or higher.
• Impurities lead to temperature shifts and increased uncertainty. Purchasing should therefore ask for the material certificates and specifications for purity.
• Equally important is the specified measurement uncertainty of the fixed-point cell. Manufacturers typically specify the achievable uncertainty under defined operating conditions.
• This information must be coordinated with the requirements of your own calibration laboratory. Accredited laboratories must ensure that the uncertainty specifications are documented and traceable. A transparent indication of the uncertainty budget components is a quality feature.
• The design of the fixed-point cell plays a practical role. Classic ITS-90 cells are usually larger and designed for maximum accuracy.
• Miniature or secondary fixed-point cells are more compact and easier to handle, but offer slightly higher uncertainties.
• The purchasing department should check whether the highest metrological accuracy is required or whether a practice-oriented solution is sufficient. Mechanical stability, the quality of the quartz or metal housing and the finish of the immersion sleeve also influence the service life.
• Another decisive factor is compatibility with existing calibration systems. Fixed-point cells require suitable furnace or temperature control systems for the controlled generation of the phase transition.
• The purchasing department must ensure that the cell is mechanically and thermally compatible with existing calibration furnaces. The immersion depth and diameter of the thermometers must also be taken into account.
• Handling and maintenance should also be evaluated. Some fixed-point cells require special procedures for plateau generation, such as controlled freezing or melting.
• The supplier should provide clear operating instructions, training or support. The correct formation of the ice-water equilibrium zone is particularly important for water three-point cells. Errors in handling can significantly increase the measurement uncertainty.
• Long-term stability is a key selection criterion. High-quality fixed-point cells retain their properties for many years, provided they are operated properly.
• Purchasers should obtain information on ageing stability, recommended maintenance intervals and possible reconditioning. The availability of repair or cleaning services is also relevant.
• Conformity with standards and traceability are essential, especially for accredited laboratories. The fixed-point cell should meet the requirements of the International Temperature Scale ITS-90.
• Certificates, calibration certificates and documentation must be available in full. If the laboratory works in accordance with ISO/IEC 17025, the documentation supplied must meet the requirements of the accreditation body.
• An economic aspect is the consideration of life cycle costs. High-precision fixed point cells are expensive to purchase. However, they offer stable reference temperatures with minimal drift over many years.
• Purchasing should not only evaluate the purchase price, but also take into account accuracy, stability, service support and long-term usability.
• The manufacturer's experience and reputation play an important role. Fixed point cells are highly specialized metrological products. An established supplier with proven experience in temperature metrology generally offers higher quality assurance and better technical support.
• References from accredited laboratories or national metrology institutes can provide additional security.
• Finally, the logistical handling should also be considered. Fixed point cells are sensitive precision devices. Safe transportation, suitable packaging and, if necessary, climate-controlled delivery are important in order to avoid damage or contamination.

The professional purchase of fixed-point cells as calibration equipment for temperature measuring devices requires a careful and structured evaluation of technical, metrological and economic criteria. Decisive factors are the appropriate fixed point range, maximum material purity, low measurement uncertainty, conformity to standards in accordance with ITS-90 and compatibility with existing calibration systems. Companies and laboratories that systematically take these factors into account and work together with specialized manufacturers create the basis for high-precision, standard-compliant and long-term stable temperature calibrations.

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