On This Page On This Page Manage & Measure Your Autoclave’s Performance Our log sheet template will help you maintain a detailed record of sterilization cycles and optimize future cycles. Get the Template How to Calibrate Your Autoclave in 12 Simple Steps Written by: Scott Mechler BS Mechanical Engineering, Mechanical Engineer Calibration is an important but often misunderstood maintenance requirement for steam sterilizers. Done correctly, it ensures that the temperature your autoclave displays is the same as the temperature the chamber actually reaches, which has a direct and measurable effect on sterilization efficacy. In this article, we’ll explain what calibration is, what equipment you need to calibrate your autoclave, how to choose the right calibration method, and more. Key Takeaways: Calibration aligns an autoclave’s temperature sensors with a NIST-traceable reference standard. A 2°C (35.6°F) difference in chamber temperature can reduce cycle lethality by nearly 20%. This variability is more common than operators might realize. Your calibration standard should be five times more accurate than the sensor you’re calibrating. For a sensor accurate to ±0.5°C, that means a standard accurate to ±0.1°C. Multipoint calibration is more reliable than single- or two-point calibration because it reveals non-linearity and averages out measurement errors. Calibration and validation are related but distinct: Calibration confirms sensor accuracy, while validation confirms whether a specific load achieved sterility. Both are necessary for a fully qualified sterilizer. What Is Autoclave Calibration? At its most basic, calibration is the process of bringing an autoclave’s sensor response — whether a temperature transducer, thermocouple, or pressure sensor — to within a specified range relative to a primary reference standard. Calibration compares the sensor’s output to a previously calibrated device (the “standard”) whose accuracy is traceable to a national reference standard. In the U.S., the National Institute of Standards and Technology (NIST) is responsible for maintaining that standard. As a general rule of thumb, the standard should be five times as accurate as the sensor you’re calibrating. For example, when calibrating a temperature probe with a desired accuracy of ±0.5°C, the autoclave calibration standard should be accurate to ±0.1°C. What Is the Difference Between Calibration & Validation? The terms “calibration” and “validation” are sometimes used interchangeably but actually refer to separate-yet-related procedures, both of which are critical to autoclave performance: Calibration confirms that the autoclave’s sensors provide an accurate reading of the chamber temperature. Calibration says nothing about whether a load inside the autoclave has achieved sterility; it only establishes that the measurements used to control the cycle are correct. Validation confirms that a load, processed in a specific way, achieved the required conditions for sterilization throughout. Full validation, which typically involves heat distribution and heat penetration studies, biological indicator testing, and load-specific documentation, demonstrates that steam has reached the surface of all items in a load for the required time and temperature. Calibration is a prerequisite for validation, as you cannot validate a sterilizer whose sensors are out of calibration, and validation is not a substitute for calibration, as a validated autoclave can still drift out of calibration over time. Both are essential to a proper steam sterilizer maintenance program. Why Should You Calibrate Your Autoclave? The success or failure of an autoclave cycle depends on the actual chamber temperature, and an uncalibrated or out-of-calibration controller display can skew results. To put this into more concrete terms, a cycle that runs at 120°C (248°F) for 15 minutes has only 82% of the theoretical lethality as a cycle running at 122°C. Most laboratory autoclaves do not require temperature accuracy above ±0.1°C, which means this kind of variability is more common than many operators realize. Calibrated temperature sensors close that gap. Meet the People Behind Our Autoclaves > How Often Should You Calibrate Your Autoclave? Annual calibration is the baseline for autoclaves in routine laboratory use. However, those in high-throughput environments or used to process critical loads in pharmaceutical, biotech, or healthcare settings may require more frequent calibration. In some cases, it may even be mandated. Beyond scheduled intervals, the following events should trigger an out-of-cycle calibration: A temperature sensor or transducer is replaced The autoclave undergoes repair or significant that might disturb the sensors A biological indicator failure or other sterilization monitoring anomaly that cannot be explained by load or cycle factors The autoclave is moved or reinstalled The as-found calibration data at the start of a scheduled calibration shows the sensors drifted outside acceptable limits; this is a clear sign to shorten calibration intervals Record both the as-found and as-left data after any calibration. The former is especially valuable, as it tells you how far the sensors have drifted since the last calibration, which is the best available evidence for determining whether your current calibration interval is appropriate. Autoclave Calibration Methods Calibration procedures vary based on the number of calibration points they measure. The three main approaches, listed below, differ in terms of both the effort required and the confidence they provide: Single-point calibration is valid only within the accuracy window at that one specific temperature. For facilities that sterilize goods exclusively at 121°C (250°F), this is workable, but it provides no information about how the sensor behaves at temperatures above or below that specific point. Any process excursions, however brief, fall outside the validated range. Two-point calibration measures readings at two temperatures and uses them to calculate the slope (gain) and y-intercept (zero offset) of the sensor’s response. It can be faster and more convenient than multipoint calibration, while also being more reliable than single point. The limitation is that, by definition, two points define a single line, meaning two-point calibration reveals nothing about any non-linearity in the probe’s reading. Additionally, any errors in the two measurements might not be immediately apparent. Multipoint calibration, which consists of three or more measurement points, is generally considered the most reliable method. It can reveal non-linear behavior in a probe’s response (which itself is a reason to replace the probe, rather than calibrate it) and averages out measurement errors across the greater number of data points via linear regression.The measurement points for any multipoint calibration should bracket the full working range of the sterilization cycles you run, not just the target temperatures. A facility running exclusively at 121°C should calibrate at 116° (241°F), 121°C, and 126°C (259°F) for a three-point calibration. This ensures the calibration accounts for the sensor’s behavior across the full range of conditions the autoclave will experience, including approach and overshoot.For facilities running cycles at multiple temperatures, calibration should extend from 5°C (41°F) to 5°C above the maximum temperature used. A useful rule of thumb is to calibrate against at least the number of temperature decades in your operating range plus one. For example, a facility that runs cycles at 115°C (239°F), 121°C, and 134°C (273°F) should use four calibration points, such as 110°C (230°F), 120°C, 130°C (266°F), and 140°C (284°F). Autoclave Calibration Equipment The proper way to calibrate an autoclave is with an NIST-traceable device such as a dry block, oil bath, or temperature probe. If you use a dry block or oil bath, confirm that it’s designed to hold a constant temperature to ±0.1°C. If the dry block or oil bath is not NIST-traceable, or its calibration has expired, use a NIST-traceable temperature probe with a minimum accuracy of ±0.1°C, instead. If neither is available, boiling water can serve as a fallback single-point calibration reference at approximately 100°C (212°F). However, it’s important to keep in mind that boiling water varies with atmospheric pressure, and at any elevation above sea level, water will boil below 100°C. Before using this approach, check the current atmospheric pressure at your location or use an absolute pressure manometer to obtain the exact pressure reading, then calculate the pressure-corrected boiling temperature of water. If your sensor reading is within 1°C of the theoretical boiling temperature at your location’s pressure, no calibration is needed. If it is not, you’ll need to perform a full calibration using a dry block or oil bath. Be sure not to overlook your autoclave’s internal clock, as this is also a calibrated parameter. The standards governing steam sterilizers include accuracy requirements for the process control clock, and a clock that runs fast or slow will cause the sterilizer to under- or over-report exposure time. Sample Autoclave Calibration Procedure Use the following as a guide when calibrating your autoclave: Record the as-found calibration data (zero and gain) for each sensor to be calibrated. With appropriate caution, shut off the steam supply and wait for the chamber pressure to reach zero before removing any sensors. Remove the sensors from the sterilizer, leaving their cables connected to the control system. Set the zero and gain to 0 and 1, respectively. If using a NIST-traceable dry block or oil bath place, the sensor in the dry block or oil bath. If using a NIST-traceable temperature probe, place the temperature probe into a central position in the dry block or oil bath and the sensors as close to it as possible. Measure at the selected temperatures and record the standard and transducer data. You should wait for at least one minute at each temperature to allow the measurement to stabilize. Don’t rush this step. Perform a linear regression on the collected data using the standard’s readings on the y-axis and the sensor’s readings on the x-axis. The intercept() and slope() functions in Microsoft Excel make this straightforward. Run a correlation as well using the correl() function. The result should be at least 0.999999 — a very straight line. A lower value is a signal that the probe may need to be replaced rather than calibrated. Enter the zero and gain values from the regression into the sterilizer controller. Verify the calibration using one or more points. This step gets you an exact, documented statement of sensor accuracy at the temperatures that matter for your application. Reinstall the sensors in the sterilizer. Frequently Asked Questions Q: What is autoclave calibration? A: Autoclave calibration is the process of verifying that an autoclave’s sensors are reading accurately by comparing them against a NIST-traceable reference standard and adjusting the controller settings to bring them within the required tolerance. Q: How often should an autoclave be calibrated? A: An autoclave should be calibrated at least once a year, however, certain applications or environments may require more frequent calibration. Beyond scheduled intervals, you should calibrate your autoclave after any sensor replacement, after significant maintenance or repair, if the autoclave is moved or reinstalled, and whenever a sterilization monitoring anomaly cannot be explained by load or cycle factors. Q: What equipment do I need to calibrate an autoclave? A: The equipment you need to calibrate an autoclave include a NIST-traceable dry block, oil bath, or temperature probe accurate to ±0.1°C (which is five times more than the ±0.5°C tolerance most laboratory autoclave sensors are held to). If this equipment isn’t available, boiling water can serve as a single-point fallback reference, but you’ll need to correct its temperature for local atmospheric pressure before use. Don't Buy An Autoclave Until You Read This Find out the 17 questions you need to ask before buying your next autoclave. Download eBook Tweet Like Share
How to Calibrate Your Autoclave in 12 Simple Steps Written by: Scott Mechler BS Mechanical Engineering, Mechanical Engineer Calibration is an important but often misunderstood maintenance requirement for steam sterilizers. Done correctly, it ensures that the temperature your autoclave displays is the same as the temperature the chamber actually reaches, which has a direct and measurable effect on sterilization efficacy. In this article, we’ll explain what calibration is, what equipment you need to calibrate your autoclave, how to choose the right calibration method, and more. Key Takeaways: Calibration aligns an autoclave’s temperature sensors with a NIST-traceable reference standard. A 2°C (35.6°F) difference in chamber temperature can reduce cycle lethality by nearly 20%. This variability is more common than operators might realize. Your calibration standard should be five times more accurate than the sensor you’re calibrating. For a sensor accurate to ±0.5°C, that means a standard accurate to ±0.1°C. Multipoint calibration is more reliable than single- or two-point calibration because it reveals non-linearity and averages out measurement errors. Calibration and validation are related but distinct: Calibration confirms sensor accuracy, while validation confirms whether a specific load achieved sterility. Both are necessary for a fully qualified sterilizer. What Is Autoclave Calibration? At its most basic, calibration is the process of bringing an autoclave’s sensor response — whether a temperature transducer, thermocouple, or pressure sensor — to within a specified range relative to a primary reference standard. Calibration compares the sensor’s output to a previously calibrated device (the “standard”) whose accuracy is traceable to a national reference standard. In the U.S., the National Institute of Standards and Technology (NIST) is responsible for maintaining that standard. As a general rule of thumb, the standard should be five times as accurate as the sensor you’re calibrating. For example, when calibrating a temperature probe with a desired accuracy of ±0.5°C, the autoclave calibration standard should be accurate to ±0.1°C. What Is the Difference Between Calibration & Validation? The terms “calibration” and “validation” are sometimes used interchangeably but actually refer to separate-yet-related procedures, both of which are critical to autoclave performance: Calibration confirms that the autoclave’s sensors provide an accurate reading of the chamber temperature. Calibration says nothing about whether a load inside the autoclave has achieved sterility; it only establishes that the measurements used to control the cycle are correct. Validation confirms that a load, processed in a specific way, achieved the required conditions for sterilization throughout. Full validation, which typically involves heat distribution and heat penetration studies, biological indicator testing, and load-specific documentation, demonstrates that steam has reached the surface of all items in a load for the required time and temperature. Calibration is a prerequisite for validation, as you cannot validate a sterilizer whose sensors are out of calibration, and validation is not a substitute for calibration, as a validated autoclave can still drift out of calibration over time. Both are essential to a proper steam sterilizer maintenance program. Why Should You Calibrate Your Autoclave? The success or failure of an autoclave cycle depends on the actual chamber temperature, and an uncalibrated or out-of-calibration controller display can skew results. To put this into more concrete terms, a cycle that runs at 120°C (248°F) for 15 minutes has only 82% of the theoretical lethality as a cycle running at 122°C. Most laboratory autoclaves do not require temperature accuracy above ±0.1°C, which means this kind of variability is more common than many operators realize. Calibrated temperature sensors close that gap. Meet the People Behind Our Autoclaves > How Often Should You Calibrate Your Autoclave? Annual calibration is the baseline for autoclaves in routine laboratory use. However, those in high-throughput environments or used to process critical loads in pharmaceutical, biotech, or healthcare settings may require more frequent calibration. In some cases, it may even be mandated. Beyond scheduled intervals, the following events should trigger an out-of-cycle calibration: A temperature sensor or transducer is replaced The autoclave undergoes repair or significant that might disturb the sensors A biological indicator failure or other sterilization monitoring anomaly that cannot be explained by load or cycle factors The autoclave is moved or reinstalled The as-found calibration data at the start of a scheduled calibration shows the sensors drifted outside acceptable limits; this is a clear sign to shorten calibration intervals Record both the as-found and as-left data after any calibration. The former is especially valuable, as it tells you how far the sensors have drifted since the last calibration, which is the best available evidence for determining whether your current calibration interval is appropriate. Autoclave Calibration Methods Calibration procedures vary based on the number of calibration points they measure. The three main approaches, listed below, differ in terms of both the effort required and the confidence they provide: Single-point calibration is valid only within the accuracy window at that one specific temperature. For facilities that sterilize goods exclusively at 121°C (250°F), this is workable, but it provides no information about how the sensor behaves at temperatures above or below that specific point. Any process excursions, however brief, fall outside the validated range. Two-point calibration measures readings at two temperatures and uses them to calculate the slope (gain) and y-intercept (zero offset) of the sensor’s response. It can be faster and more convenient than multipoint calibration, while also being more reliable than single point. The limitation is that, by definition, two points define a single line, meaning two-point calibration reveals nothing about any non-linearity in the probe’s reading. Additionally, any errors in the two measurements might not be immediately apparent. Multipoint calibration, which consists of three or more measurement points, is generally considered the most reliable method. It can reveal non-linear behavior in a probe’s response (which itself is a reason to replace the probe, rather than calibrate it) and averages out measurement errors across the greater number of data points via linear regression.The measurement points for any multipoint calibration should bracket the full working range of the sterilization cycles you run, not just the target temperatures. A facility running exclusively at 121°C should calibrate at 116° (241°F), 121°C, and 126°C (259°F) for a three-point calibration. This ensures the calibration accounts for the sensor’s behavior across the full range of conditions the autoclave will experience, including approach and overshoot.For facilities running cycles at multiple temperatures, calibration should extend from 5°C (41°F) to 5°C above the maximum temperature used. A useful rule of thumb is to calibrate against at least the number of temperature decades in your operating range plus one. For example, a facility that runs cycles at 115°C (239°F), 121°C, and 134°C (273°F) should use four calibration points, such as 110°C (230°F), 120°C, 130°C (266°F), and 140°C (284°F). Autoclave Calibration Equipment The proper way to calibrate an autoclave is with an NIST-traceable device such as a dry block, oil bath, or temperature probe. If you use a dry block or oil bath, confirm that it’s designed to hold a constant temperature to ±0.1°C. If the dry block or oil bath is not NIST-traceable, or its calibration has expired, use a NIST-traceable temperature probe with a minimum accuracy of ±0.1°C, instead. If neither is available, boiling water can serve as a fallback single-point calibration reference at approximately 100°C (212°F). However, it’s important to keep in mind that boiling water varies with atmospheric pressure, and at any elevation above sea level, water will boil below 100°C. Before using this approach, check the current atmospheric pressure at your location or use an absolute pressure manometer to obtain the exact pressure reading, then calculate the pressure-corrected boiling temperature of water. If your sensor reading is within 1°C of the theoretical boiling temperature at your location’s pressure, no calibration is needed. If it is not, you’ll need to perform a full calibration using a dry block or oil bath. Be sure not to overlook your autoclave’s internal clock, as this is also a calibrated parameter. The standards governing steam sterilizers include accuracy requirements for the process control clock, and a clock that runs fast or slow will cause the sterilizer to under- or over-report exposure time. Sample Autoclave Calibration Procedure Use the following as a guide when calibrating your autoclave: Record the as-found calibration data (zero and gain) for each sensor to be calibrated. With appropriate caution, shut off the steam supply and wait for the chamber pressure to reach zero before removing any sensors. Remove the sensors from the sterilizer, leaving their cables connected to the control system. Set the zero and gain to 0 and 1, respectively. If using a NIST-traceable dry block or oil bath place, the sensor in the dry block or oil bath. If using a NIST-traceable temperature probe, place the temperature probe into a central position in the dry block or oil bath and the sensors as close to it as possible. Measure at the selected temperatures and record the standard and transducer data. You should wait for at least one minute at each temperature to allow the measurement to stabilize. Don’t rush this step. Perform a linear regression on the collected data using the standard’s readings on the y-axis and the sensor’s readings on the x-axis. The intercept() and slope() functions in Microsoft Excel make this straightforward. Run a correlation as well using the correl() function. The result should be at least 0.999999 — a very straight line. A lower value is a signal that the probe may need to be replaced rather than calibrated. Enter the zero and gain values from the regression into the sterilizer controller. Verify the calibration using one or more points. This step gets you an exact, documented statement of sensor accuracy at the temperatures that matter for your application. Reinstall the sensors in the sterilizer. Frequently Asked Questions Q: What is autoclave calibration? A: Autoclave calibration is the process of verifying that an autoclave’s sensors are reading accurately by comparing them against a NIST-traceable reference standard and adjusting the controller settings to bring them within the required tolerance. Q: How often should an autoclave be calibrated? A: An autoclave should be calibrated at least once a year, however, certain applications or environments may require more frequent calibration. Beyond scheduled intervals, you should calibrate your autoclave after any sensor replacement, after significant maintenance or repair, if the autoclave is moved or reinstalled, and whenever a sterilization monitoring anomaly cannot be explained by load or cycle factors. Q: What equipment do I need to calibrate an autoclave? A: The equipment you need to calibrate an autoclave include a NIST-traceable dry block, oil bath, or temperature probe accurate to ±0.1°C (which is five times more than the ±0.5°C tolerance most laboratory autoclave sensors are held to). If this equipment isn’t available, boiling water can serve as a single-point fallback reference, but you’ll need to correct its temperature for local atmospheric pressure before use. Don't Buy An Autoclave Until You Read This Find out the 17 questions you need to ask before buying your next autoclave. Download eBook Tweet Like Share
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