A Heating Oven is a device for controlling temperatures in the range of -10°C to +300°C. Simple ovens are insulated boxes with an adjustable heater. More advanced ovens may also include the ability to lower the temperature below ambient (via refrigeration), and/or the ability to control vacuum.
Most ovens provide inner and outer doors often made of glass for transparency. Main controllers include a timer. Some can also be programmed to cycle through different temperatures. Ovens vary in size from tabletop to large floor standing units. Temperature distribution is secured either through gravity or forced convection by fan operation.
Most standard ovens are designed with outside air intake through a filter followed by heating and uniform distribution of the air throughout the chamber in order to secure even impact on the samples placed on (perforated) shelves anywhere in chamber volume. Exhaust air is vented to the outside through a port at the top of the unit. The parameter of interest is temperature and temperature range is typically -10 to +300°C. Samples are often placed in closed containers so cross contamination cannot occur.
Vacuum ovens are more complex in design as temperature along with pressure is monitored and controlled. To decrease pressure levels below normal ambience, a vacuum pump is fitted to the chamber and to secure homogeneity throughout the chamber typically a fan is moving the air. Vacuum ovens are primarily used for drying substances in open containers by forced evaporation.
Typical controlled parameters in ovens are:
• Temperature: -10 to +300°C (+/-2°C)
• Pressure : 10 mBar to 1100 mBar
Validation of ovens falls into three main categories.
Standard ovens with a temperature range of -10 to +100°C do not usually pose any special challenges because parameters to be controlled are relatively easy to match. As temperatures are modest and no pressure measurements are needed, the Lab logger series with temperature or temperature and humidity sensors can be used to simplify the operation. The Lab loggers contain an LED light indicator that will provide the operator with instant information on the logger status.
High temperature ovens with a temperature range of +100 to +300°C will require special equipment. When operating wireless loggers, high temperature sensors as well as a thermal barrier to protect the battery will be needed.
For (high temperature) vacuum ovens wireless loggers are practical and therefore the same sensors and protection of the battery applies. In some cases the standard water based thermal barrier cannot be used due to the additional evaporation created by the vacuum.
If loggers are fitted with SKY option, wireless real-time data can be processed but for very small and low volume standard ovens that contain access port(s), the use of thermocouples could be advantageous to reduce impact and chamber volume.
One should consider the amount of time needed to qualify/validate an oven. The logger sample rate will need to be assigned accordingly based on the logger memory capacity.
The DIN 12880 specifies performance requirements and tests for heating cabinets and incubators in laboratories. The standard applies for cabinets and incubators with a working temperature range from -10 °C to 300 °C and includes shaking incubators and vacuum ovens.
Validation of an oven consists of various steps regarding Installation Qualification (IQ), Operational Qualifications (OQ) and Performance Qualifications (PQ).
The IQ is conducted to demonstrate that the oven is inspected and contains all the required parts for its proper functioning.
During the OQ the performance of the oven and its operational procedure is verified. The OQ portion consists of temperature mapping the empty chamber and is performed to confirm that the empty chamber works within the specified limits throughout the chamber. The OQ also contains information about the calibration of measuring equipment and defines the location of the thermocouples and/or data loggers in the oven.
The PQ verifies that the loaded oven chamber is capable of reaching and maintaining the target temperature range throughout the chamber. The effect of opening doors for different lengths of time and the effect of a power failure on the oven’s ability to get back into the temperature profile are also evaluated.
According to DIN 12880 the temperature shall be measured in three horizontal measuring planes equally distributed over the inner chamber by using nine measuring points per plane (corners, center and mid sides). For smaller ovens less than 50 L, two measuring planes with four measuring points (corners) per plane plus one at the center per plane typically are sufficient. During the OQ, measuring points are fixed 15 mm above shelf surface in an empty chamber and during PQ they are placed into loaded samples. Ovens with forced air circulation shall be tested with fan switched on.
Test temperatures should be the following three:
- one within the lower fifth of the working temperature range but at a maximum of 20 °C above the minimum working temperature
- one within the upper fifth of the working temperature range but at a maximum of 20 °C below the nominal temperature
- one at a temperature in the middle of the working temperature range
When validating vacuum ovens the temperature measuring points shall be in direct contact with the shelves since heating of the samples in vacuum is largely due to thermal conduction via the shelves. The temperature should be measured at five measuring points (corners + center) in each measuring plane and pressure should be set at 20 mBar or below.
Temperature sample readings should be collected every ten seconds for one hour once equilibrium has been reached but no earlier than two hours after equipment has been switched on.
Ellab's variety of wireless data loggers are ideal for measuring temperature and pressure in high temperature and/or vacuum ovens. Even with most standard ovens that include access port(s), feeding thermocouples through the ports could disturb the integrity of the chamber atmosphere.
Ambient temperature during test shall be 22 ±3 °C.
For pharmaceutical and medical device clients, most applications will require the use of advanced software such as ValSuite™ Pro with its 21 CFR Part 11 compliance and sophisticated reports/features. The ValSuite Basic version of software may suffice in other industries (food) where electronic records are not necessary.
For validation purposes creating a Unit with the exact position of each measuring point documented by pictures is advantageous when handling many sample points. Furthermore, adding it all up in a Profile session including a Limit Report as well as a Statistical Report is useful and time saving.
At the end of each study report(s) can be signed by Electronic Signature(s), stored, printed, and distributed in secure and locked PDF format.
Using the built-in calibration functionality, users can calibrate the Ellab sensors at regular intervals. The ValSuite™ Pro software generates an easy to read calibration report that includes all measurements and tolerances that are defined by the user. It is recommended that Ellab sensors and probes are sent in at least once a year for factory calibration (which includes a traceable calibration certificate).