Document ID: SOP-TEMP-001 (Example)

Revision Number: 01 (Example)

Effective Date: 2024-03-08 (Example)

Author: [Your Name/Department]

Approver: [QA/QC Department]

1.0 Purpose

This Standard Operating Procedure (SOP) outlines the procedure for calibrating thermometers and thermocouples using the ice point method. This method provides a reliable and practical way to establish a reference point at 0°C (32°F) for accurate temperature measurements. This calibration is essential for ensuring the accuracy and reliability of temperature readings obtained using these instruments in various applications, including but not limited to, environmental monitoring, stability studies, and manufacturing processes.

2.0 Scope

This SOP applies to all personnel responsible for calibrating thermometers and thermocouples within [Organization Name/Department]. It covers the calibration of liquid-in-glass thermometers, electronic thermometers, and thermocouples used in [List relevant areas/equipment].

3.0 Definitions

• Calibration: The process of comparing a measuring instrument to a known standard to establish the relationship between the indicated value and the true value.
• Ice Point: The temperature of a mixture of ice and water at equilibrium under standard atmospheric pressure (101.325 kPa or 760 mmHg). This temperature is defined as 0°C (32°F).
• Thermometer: A device used to measure temperature. This SOP covers liquid-in-glass thermometers and electronic thermometers.
• Thermocouple: A temperature-sensing device consisting of two dissimilar metals joined at one end, producing a voltage proportional to the temperature difference.
• Reference Standard: A thermometer or thermocouple of higher accuracy used as the calibration standard. This standard must be traceable to a national or international standard.
• Uncertainty: A measure of the dispersion of the values that could reasonably be attributed to the quantity being measured.

4.0 Responsibilities

• Calibration Technician: Responsible for performing the calibration according to this SOP and documenting the results.
• Supervisor: Responsible for overseeing the calibration process and ensuring that it is performed correctly.
• Quality Assurance (QA) Department: Responsible for reviewing and approving the calibration records and ensuring compliance with this SOP.

5.0 Procedure

5.1 Preparation:

1. Materials:

   • Crushed ice (made from potable water)
   • Distilled or deionized water
   • Insulated container (e.g., Dewar flask or insulated bucket)
   • Reference standard thermometer or thermocouple (with current calibration certificate traceable to a national or international standard)
   • Thermometers or thermocouples to be calibrated
   • Thermometer or thermocouple holder/clamp
   • Stirring rod or spoon (non-metallic)
   • Data recording sheet/calibration logbook (See Appendix A)
   • Protective gloves
   • Safety glasses (optional but recommended)

2. Environmental Conditions: The calibration should be performed in a stable environment with minimal temperature fluctuations. Record the ambient temperature in the calibration log.

3. Ice Bath Preparation:

   • Fill the insulated container approximately halfway with crushed ice.
   • Add distilled or deionized water slowly until the ice and water mixture reaches a slushy consistency. Ensure that there is enough water to fully immerse the sensing portion of the thermometers/thermocouples being calibrated.
   • Allow the ice bath to stabilize for at least 15 minutes before starting the calibration. The mixture should be thoroughly mixed and have a consistent temperature throughout. Add more ice if necessary to maintain the slushy consistency.

5.2 Calibration Steps:

1. Reference Standard Check: Verify that the calibration certificate for the reference standard is valid and traceable. Record the reference standard’s information (serial number, calibration date, etc.) in the calibration log.

2. Thermometer/Thermocouple Preparation: Clean the thermometers/thermocouples to be calibrated. Visually inspect them for any damage or defects. Record the identification details (serial number, model, etc.) of each thermometer/thermocouple in the calibration log.

3. Immersion:

   • For liquid-in-glass thermometers: Immerse the sensing portion of the thermometer (bulb) into the ice bath, ensuring that it is not touching the sides or bottom of the container. The immersion depth should be consistent with the manufacturer’s recommendations.
   • For electronic thermometers and thermocouples: Immerse the sensing probe into the ice bath, ensuring that it is not touching the sides or bottom of the container. The immersion depth should be sufficient to cover the entire sensing element.

4. Stabilization: Allow the thermometers/thermocouples to stabilize in the ice bath for at least 5 minutes (or longer if necessary, depending on the type of instrument). Gently stir the ice bath periodically to maintain temperature uniformity.

5. Reading:

   • For liquid-in-glass thermometers: Carefully read and record the temperature reading. Estimate readings to the smallest graduation on the thermometer scale.
   • For electronic thermometers and thermocouples: Record the temperature reading displayed on the device.

6. Reference Reading: Immediately after reading the thermometer/thermocouple being calibrated, read and record the temperature indicated by the reference standard.

7. Repeat Readings: Repeat steps 5.2.5 and 5.2.6 at least three times for each thermometer/thermocouple. This helps to assess the repeatability of the readings.

8. Calculations: Calculate the difference between the readings of the thermometer/thermocouple being calibrated and the reference standard for each set of readings. This difference represents the correction factor.

9. Documentation: Record all readings, including the date, time, thermometer/thermocouple identification, reference standard reading, and calculated correction factor, in the calibration log (Appendix A).

5.3 Acceptance Criteria:

The acceptance criteria for the calibration should be defined based on the intended use of the thermometers/thermocouples. Typically, the correction factor should be within a specified tolerance range. This tolerance range should be documented in a separate SOP or specification document. If the correction factor exceeds the acceptance criteria, the thermometer/thermocouple should be adjusted or removed from service.

6.0 Documentation

The calibration log (Appendix A) must be completed for each calibration performed. The log should include:

• Date and time of calibration
• Calibration technician’s name/initials
• Thermometer/thermocouple identification (serial number, model, etc.)
• Reference standard information (serial number, calibration date, etc.)
• Readings from the thermometer/thermocouple being calibrated
• Readings from the reference standard
• Calculated correction factor
• Acceptance criteria
• Calibration result (pass/fail)
• Any deviations from this SOP

7.0 Quality Control

• The calibration process should be reviewed periodically by QA personnel to ensure compliance with this SOP.
• The reference standard should be recalibrated at regular intervals by a certified calibration laboratory.
• Any out-of-tolerance readings should be investigated and documented. Corrective actions should be taken to address the issue.

8.0 Safety Precautions

• Wear appropriate personal protective equipment (PPE), including gloves and safety glasses (optional).
• Handle glassware carefully to avoid breakage.
• Be cautious when working with ice and water to prevent slips and falls.

9.0 Revision History

Appendix A: Calibration Log

(Example)

Note: TC = Thermocouple

This SOP should be reviewed and updated periodically to ensure its continued accuracy and effectiveness. All personnel