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## Introduction

Have you ever wondered what the accuracy and uncertainty specifications in our calibrations mean? If you require an ISO17025 certificate for your instrument (E.g., Thermal Chuck, temperature wafer, Oven, etc.), you will receive a certificate that mentions something like “ calibrated at k=2”. You may also wonder why the uncertainty value is larger than the accuracy value or why uncertainty seems to matter more than accuracy. This post intends to help you understand these three potential sources of confusion and will help you make the right decision when assessing your temperature calibration needs.

## Uncertainty vs. Accuracy

First, let’s begin by defining accuracy in the temperature calibration context. Accuracy may be defined as “a measure of a calibration product’s performance and quality” (Bucy, 2019). While accuracy may be the most common quality indicator, it is not often the best term to use. Sometimes it is better to use the term uncertainty to determine the quality of the calibration.

These definitions beg the question, should one use these two terms interchangeably? The short answer is no. Accuracy is the proximity a reading is to its actual value, whereas uncertainty relates to the outliers and anomalies that may skew accuracy readings (Bucy, 2019).

## Why Are Uncertainty & Accuracy Important?

Uncertainty is the degree of statistical dispersion of the temperature points one measure. Temperature is not the only parameter that is subject to uncertainty. All parameters are subject to uncertainty. So, why is determining accuracy so important?

As mentioned before, accuracy tells us how close a measurement is to its actual value, whereas uncertainty considers the outliers and anomalies that skew the accuracy readings. These outliers are products of “anomalies, adjustment, or other factors” (Bucy, 2019). These anomalies are not factored directly into the instrument’s accuracy to avoid misleading the reader. To provide a better indicator of an instrument’s performance, one should take the uncertainty values as a whole and calculate them as a component of accuracy.

In addition, one must calculate the deviation in a reading to determine measurement uncertainty better. The deviation is the difference between measure values and the actual or expected value. For instance, the limited resolution or error in reading leads to the measurement of uncertainty on display. The deviation essentially represents the random and systematic components of a measure. Since the accuracy is proportional to the deviation, one can expect that the greater the deviation, the higher the measurement uncertainty. Thus, the less accurately the instrument works.

At Sigma Sensors we are able to measure uncertainty and accuracy. Our Lab2Go program allows you to remotely do so. Learn more about Lab2Go here!

## ISO 17025;2017

We can calibrate anything with temperature! Our services are accredited by A2LA and ILAC. Here are some of the products we can calibrate:

• Ovens
• Freezers
• Environmental enclosures
• Oil and liquid baths
• Thermometers
• sensors
• Temperature systems with meter/logger
• Thermal chucks
• Surfaces
• Temperature wafers
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## Introduction

I believe most of us are familiar with ovens. Once heard, we think of it as equipment used for heating. This can be what most of us know, but for those exposed in manufacturing zones, a few usages of ovens are for quicker drying, sterilization, and reliability testing.
For example, in semiconductor industries, Oven tests the strength of a solder joint in a chip at a given temperature. Some laboratory apparatus used for testing products inside the lab are being dried and sterilized using the Oven in the food industry.

As we see, usage of ovens is fundamental where a precise temperature ought to be kept up. This accuracy can be determined through calibration. This blog will share how to calibrate an electric oven employing an advanced thermometer and a thermocouple wire. The blog will display a 3-point calibration set up in an electric oven with an analog temperature controller. Although this calibration will be conducted on a basic and small electric Oven, this method is appropriate to most single-layered industrial ovens used within the manufacturing field and the digital Oven, the more advanced type.

## Why Should You Calibrate Ovens?

The short answer is to guarantee the precision of the temperature interior of the oven enclosures. Most electric ovens are mechanically operated where each part experiences stress, particularly the thermostat section. This section is where the sensor is attached. The thermostat is operated by a dial knob that’s designed to be triggered to switch on and off once it detects the desired temperature settings. When this sensor is not accurate, the desired temperature will not be produced, influencing the quality of your product.

As usage is at maximum, the sensor loses its accuracy or efficiency, and therefore, a calibration should be performed to detect such in-efficiency or in-accuracy.

Temperature calibration is also performed after a repair is done to verify its accuracy.
Moreover, calibration is performed to determine the exact position and location of items placed inside the oven where the temperature is at a minimum or maximum to create proper adjustments or positioning of products.

## Electric Oven Calibration Procedure

We calibrate an electric oven to determine the accuracy of the temperature it produces at a specified set point or set value.
Other reasons we perform calibrations includes:
1. To determine the stability of temperatures.
2. To determine inhomogeneities or uniformity of readings.
3. To determine the most accurate or appropriate locations inside the chamber.
4.  To determine the accuracy of a particular test point as per-user settings.
Oven temperature calibration is simple, but it may be time-consuming when calibrating more than one test point. The need to stabilize it before taking the reading may make the process slow. Moreover, stabilizing times are not the same for every oven. Some ovens take an hour to reach the test point before stabilization.

## Calibration Method

The basic procedure to calibrate an electric oven is to compare the generated temperature of the oven through the set value in the dial knob to the reading of the reference standard, a digital thermometer with a thermocouple/RTD wire the sensors. Possible to use high temperature wireless thermo-shield data logger.

## Requirements

• Warm-up time (Unit Under Calibration): At least 1 hour for proper stabilization
• Room Temperature:   23± 5°C
• Room Humidity:         50 ± 30%
• Perform 3 trials for each point or range

## Reference Standard To Be Used

• Calibrated reference thermometer.
• Thermocouple/RTD wires (capability to withstand a temperature of at least 300 deg C).
• Thermo-hygrometer– for temperature and humidity
• Cleaning materials.

## Oven Calibration Set-Up

Open the oven and insert the sensor wires inside. For standard calibration, distribute the cables in three locations: one in the center, the middle right, and the middle left side, respectively. The middle sensor will be the reference sensor.
The more sensor wires, the better. This way, we can determine a good uniformity reading.

5pcs is the optimal number of sensors for an oven this size. The sensors should be evenly positioned across the four middle areas of each side of the oven and one at the middle.
A temperature recorder with five or more channels is the preferred reference standard to be used. This setup can be left alone to while it records the readings automatically.

## Calibration procedure

1. Take proper care and safety precautions; ovens generate a high temperature which can cause burns and damages.
2. Check the electric ovens for any visual defects that can affect their accuracy. Check the fan, the heating elements, and the thermostat dial functionality. Discontinue calibration if any defect is noted.
3. Prepare the measurement data sheet and record all necessary details or information (Brand, Model, serial #, etc.).
4. Determine the electric oven range, divide the range based on the scale, and choose at least 3 test points.
6. Turn on the electric oven and set the temperature to set point 1, starting from low to high temperature.
7. Wait for the readings on display to stabilize (approximately 20 min).
8. Record readings on the datasheet. Fill up Trial 1 for sensor 1, trial 2 for sensor 2, and trial 3 for sensor 3.
Sensore 1Sensor 2Sensor 3
200200.80200.20200.60200.500.50
300
400
9. Turn to the next set point and Continue steps 7 and 8 until all ranges are finished.
10. For stability, select the mid-range and determine the highest and lowest reading within 30 min. time interval
11. Check readings if within the accuracy defined by the manufacturer or as per user requirements, for example, accuracy = ±2 °C. If the readings are already within limits, update the corresponding record, do labeling and sealing; otherwise, do necessary repair or adjustment.

Through the calibration process, we are able to determine errors and perform adjustments. To get the correction, just subtract the average standard reading and the oven knob setting. Use the correction for each use of the oven setting to compensate for the error. Typically, applicable in case the oven is not adjustable. Measurement uncertainty can be assessed by advanced calculations. Based on the calibration result, there is a correction of 0.5°C, which means that for every setpoint of 200, add a 0.50 as the correction factor.

I have presented here a simple procedure to calibrate an electric oven by using a high-temperature wire sensor. Make sure to use the right thermocouple/RTD wires to avoid burning its coating.

#### Author information

Iftakharul Islam

Calibration Engineer

Sigma Sensors (TCL) GmbH

Germany