Fine-Tuning Hot Air Equipment: Mastering The 2-Pot Calibration

Calibration is an important process for hot air equipment to ensure accurate temperature control and optimal performance. This guide will focus on the 2-pot calibration process for hot air equipment, specifically the Hakko 851 hot air station, a popular choice among hobbyists and professionals alike.

Calibration procedure for hot air equipment

Overview

This document outlines the procedure for calibrating hot air equipment, specifically a hot air oven and a hot air rework station. Calibration is essential to ensure accurate temperature control and optimal performance.

Hot Air Oven Calibration

The following is a standard operating procedure (SOP) for calibrating a hot air oven, which should be followed by a microbiologist or a Quality Control Officer/Executive.

• Ensure the power supply to the oven is switched off and unplugged before beginning.
• Clean the interior and exterior surfaces of the oven with a dry, lint-free cloth daily to remove dust particles.
• Once a week, wipe all parts and the outer surface with a wet lint-free cloth soaked in purified water.
• Check that the ventilation knob is open and placed on top of the oven.
• Switch on the power supply and ensure the electronic temperature controller displays the chamber temperature.
• Set the required temperature by pushing the "PUSH" switch and rotating the coarse potentiometer knob until the desired temperature is reached.
• Make fine adjustments to the temperature using the fine potentiometer knob.
• Ensure the Indicator Bulb glows, indicating that power to the heater is on.
• Switch on the fan for air circulation.
• Allow the oven to reach the set temperature and note the time.
• Once the cycle is complete, switch off the oven and the power supply.
• Record the cycle period in the daily usage logbook.
• After one hour of starting the oven, begin the calibration procedure.
• Set the oven to the desired temperature.
• Place a standard thermometer (ranging up to 300°C) on the upper shelf of the oven and close the door for 30 minutes.
• After 30 minutes, open the door and read the temperature on the thermometer.
• Match the observed temperature with the display controller's temperature.
• Repeat the above procedure by placing the thermometer on the lower shelf for 30 minutes.
• Record the observed temperature in the calibration record of the hot air oven.
• Ensure that the observed temperature on both shelves is within a ±2.0°C tolerance limit of the set temperature value.
• Perform this calibration procedure once a month.
• Additionally, an external calibration by a third party should be conducted annually.

Hot Air Rework Station Calibration

The following is a procedure for calibrating a hot air rework station, such as the Hakko 851, to ensure accurate temperature control:

• Obtain a digital thermometer and a thermocouple probe rated to the appropriate temperature (e.g., 700°C).
• Ensure the main temperature dial and the trim pot on the back of the station are set to the maximum temperature.
• Place the nozzle of the hot air station close to the tip of the thermocouple probe.
• Compare the temperature reading on the thermometer with the expected temperature specified by the manufacturer.
• Make adjustments as needed, such as checking the type of probe used and its suitability for the task.
• Test the probe by trying to dial in a temperature that will melt solder.
• Additionally, you can use temperature-indicating crayons that melt at a specific temperature to verify the accuracy of the hot air station.

Safety Precautions

• Always ensure the power supply is switched off and unplugged before cleaning or performing any maintenance on hot air equipment.
• Do not open the door of the oven while it is in operation.
• Use asbestos gloves when removing objects from the oven to protect against high temperatures.

Maintenance

Regular maintenance and cleaning of hot air equipment are crucial to ensure accurate calibration and optimal performance. Refer to the equipment's user manual for specific maintenance instructions.

External Calibration

For more complex issues or annual external calibration, engage a qualified third party or external party to perform the calibration and any necessary repairs.

Using a standard thermometer

To adjust a 2-pot calibration on hot air equipment using a standard thermometer, follow these steps:

Firstly, ensure you have a standard thermometer that can measure temperatures up to 300°C. This thermometer will serve as the reference point for your calibration. It is important to use a thermometer with known accuracy to ensure the calibration process is effective.

Next, prepare your hot air equipment by switching it on and allowing it to reach a stable temperature. Set the desired temperature for your equipment, which should be within the range of the standard thermometer. Place the standard thermometer inside the hot air equipment, preferably on an upper shelf or rack, and close the door. Leave the thermometer in place for around 30 minutes to ensure an accurate reading.

After 30 minutes, open the door and carefully remove the standard thermometer. Take the temperature reading from the thermometer and compare it to the set temperature of the hot air equipment. If the temperatures differ, adjust the settings on the equipment accordingly.

For example, if the standard thermometer reads 20°C higher than the set temperature, adjust the hot air equipment settings to compensate for this difference. This may involve turning a knob or dial to increase or decrease the temperature until it matches the reading on the standard thermometer.

Repeat this process by placing the standard thermometer on a lower shelf or rack within the hot air equipment for another 30 minutes. Again, compare the temperature readings and make any necessary adjustments. Ensure that the temperature in both locations is within a ±2.0 tolerance limit of the set temperature.

By following these steps and using a standard thermometer, you can accurately calibrate your hot air equipment to ensure it maintains the desired temperature. This process is crucial for equipment used in applications such as food safety, laboratory work, and manufacturing, where precise temperature control is essential.

Setting the oven temperature

To set the oven temperature, you must first determine the oven's average temperature. Place an oven thermometer on the middle rack and set the oven to 350°F. Allow the oven to preheat and then wait for at least 10 minutes for the temperature to stabilise after the initial spike from preheating. Take the average of two or more temperature readings, as the oven temperature fluctuates throughout the baking cycle. The more readings you take, the more accurate your average will be. Appliance repair technicians often take a reading every minute for 10 minutes.

The difference between 350°F and the average temperature you measured is the offset you will use to recalibrate your oven. For example, if the average temperature was 330°F, the offset is 20°F. Most ovens can be recalibrated by about 35°F in either direction. If the average temperature is off by more than 35°F, you may need to replace the temperature sensor or the thermostat.

Ovens with Push-Button Controls

• Press the BAKE button until the temperature displays the current offset setting, which is usually 0 or 00 unless you have previously recalibrated the oven. On some ovens, it may be the TIME/TEMP button instead.
• Use the arrow keys to increase or decrease the offset to match the difference between the average you found and 350°F.
• Press the BAKE button to finish or CANCEL to exit without saving.

Ovens with Knob Controls

• Remove the TEMP knob. On the back of the knob, you will find a small adjustment screw in a track labelled "Cooler" at one end and "Hotter" at the other.
• Loosen the screw just enough to allow movement.
• Move the screw towards "Cooler" if your oven overcooks, or towards "Hotter" if it undercooks. Each audible click represents approximately 10°F.
• Tighten the screw and replace the knob. Repeat the process of checking the actual average temperature to determine if further adjustment is required.

Reading the temperature

One way is to use the boiling point method. This involves heating a pot of water to a rolling boil and then inserting a thermometer. It is important to make sure that the entire sensing area of the thermometer is submerged, but that the thermometer does not touch the pot. You should then wait for the temperature reading to stabilise. If the temperature reading does not match the boiling point of water in your area, you will need to adjust your thermometer. Note that the boiling point of water varies depending on altitude and is generally between 190 and 212°F (88–100°C).

Another way to check the accuracy of your thermometer is the freezing point method, also known as the ice point method. This involves filling a glass with ice water and letting it sit for a couple of minutes so that the temperature settles at 32°F (0°C). You can then immerse your thermometer in the water, being careful not to let it touch the glass, and wait for the temperature reading to stabilise.

You can also use a standard thermometer to calibrate your hot air oven. This thermometer should be placed on the upper shelf of the oven for 30 minutes, and then the temperature should be noted and matched with the observed temperature of the display controller of the oven. This process should then be repeated with the thermometer placed on the lower shelf. The observed temperature should be recorded in the calibration record of the hot air oven.

Recording the temperature

Understanding Calibration

Calibration is the process of ensuring that your temperature-measuring device, such as a thermometer, is accurate. Over time, thermometers can become inaccurate due to age, extreme temperature changes, or rough handling. Calibration involves testing the thermometer's accuracy and making adjustments to match a known temperature. This process is particularly important in food service, where even a few degrees' difference can affect food safety.

Calibration Methods

There are two common methods for calibrating thermometers: the boiling point method and the freezing point method.

The Boiling Point Method

This method is suitable for thermometers used with hot foods. First, determine the boiling point of water in your area, which varies depending on altitude. Then, heat a pot of water to a rolling boil and insert the thermometer, ensuring the sensing area is fully submerged. Wait for the temperature reading to stabilise. If the reading does not match the boiling point, adjust the thermometer accordingly.

The Freezing Point Method

Also known as the ice point method, this is a safer and simpler method, often used for thermometers that measure cold foods. Fill a glass with ice water and let it sit for a few minutes to reach a temperature of 32°F (0°C). Immerse the thermometer in the water without letting it touch the glass. Wait for the temperature reading to stabilise, then adjust the thermometer if needed.

Adjusting the Thermometer

Many thermometers have a nut under the temperature dial for adjustments. For digital models, there is usually a reset button. Consult the package instructions for specific details on how to calibrate your thermometer.

After making adjustments, record the temperature readings. This step is crucial for verifying that your equipment is functioning correctly. Here's a detailed breakdown:

• Placement: Ensure the thermometer is placed correctly within the equipment. For ovens, place the thermometer on the upper shelf for a set period, such as 30 minutes, and then repeat the process on the lower shelf. For hot air rework stations, position the nozzle close to the thermocouple probe.
• Stability: Allow sufficient time for the temperature reading to stabilise. This can take at least 30 seconds to a few minutes, depending on the method used.
• Matching Temperatures: Compare the temperature reading on the thermometer with the known temperature of the substance (either the boiling point or freezing point of water).
• Adjustment: If the readings do not match, adjust the thermometer as instructed in the previous section.
• Final Verification: After adjustments, record the final temperature readings to ensure they now match the known temperature.
• Tolerance Limits: In some cases, there may be a tolerance limit specified. For example, in an oven calibration procedure, the observed temperature should be within ± 2.0 tolerance of the set temperature value.
• Documentation: Record all temperature readings and observations in a calibration record or log book. This documentation is essential for future reference and to demonstrate compliance with safety standards.

By following these steps for recording the temperature during calibration, you can ensure the accuracy and safe operation of your equipment. Regular calibration and temperature recording will help maintain the functionality and longevity of your devices.

Frequently asked questions