Selecting a Vacuum Purging Oven
There are many vacuum purging ovens available on the market today. With the recent influx of companies, designs, and sizes, we wanted to create a guide to understanding these devices so you can make the best purging and equipment decisions for your needs.
How a Vacuum Oven Works
As the name implies, there are two basic functions of the vacuum oven: The vaccum, and the oven. While these each seem like simple enough concepts by themselves, it can present quite a challenge when attempting to coordinate both so they work well with each other.
Just like deep space, a vacuum exists in the absence of matter. A good vacuum oven is able to easily remove the air inside while allowing the operator to precisely control their vacuum level. This may be done in a variety of ways, but most commonly the vacuum level is controlled with a simple ball-valve. The ball-valve is very robust from a mechanical perspective, and can be designed to hold a deep vacuum level without leaking. The valve operates by rotating a ball 90-degrees: this ball has a hole through the center, allowing airflow to pass while the hole is exposed and preventing flow when closed. Another popular mechanism is the electrovalve- this valve operates by pushing or pulling a spring-loaded piston with an electronic coil. While the electrovalve tends to require more maintenance than a ball valve, it is commonly used for convenience and in conjunction with process control.
For ultimate vacuum control, some ovens feature an inert gas backflow valve. Commonly controlled by a flow-adjustable needle valve, this will allow operators to connect their vacuum to a source of inert gas such as nitrogen for repressurization of the chamber. This method is increasingly popular as it allows for the completion of a purge cycle without reintroducing atmospheric moisture.
A vacuum oven’s heating performance is, by far, the trickiest part of the oven to understand. This is because in physics, a vacuum acts as a heating insulator. So, half of the device (the vacuum) is working against the other half (the oven). How can this problem be overcome?
The most common way to heat a vacuum oven is by slowly radiating heat into the chamber from around the walls. This is certainly the most popular heating method in the market as it provides for a good balance between operational flexibility and performance. Heating elements around the outside of the chamber are PID-controlled to slowly pour heat into the system, which is then distributed through the interior shelving by physical contact. By heating the chamber from around the outside walls, operators are allowed the flexibility to adjust or configure the chamber interior to their choosing.
Another popular heating method is the direct-heat shelf. Ovens with this configuration do not apply heat around the walls of the vacuum chamber. Rather, several fixed shelves are installed with heating applied directly to each shelf. This method offers the fastest heat-up times and the highest degree of shelf control at the expense of flexibility, as heated shelves typically cannot be removed or adjusted within the chamber.
There are as many ways to measure the performance of a vacuum oven as there are people selling vacuum ovens. Often times, these results can be presented in confusing, unclear, or inconsistent manners, preventing consumers from truly understanding what it is they’re getting. In effort to help clarify, we want to explain some of our measurement methods and how they’re helpful.
This is a word commonly thrown around when discussing an oven’s performance. Overshoot refers to the temporary overheating of an oven during its initial heat-up phase. Because most ovens are regulated with a PID-controller, the actual heating curve of the oven may include a brief period where the internal temperature of the oven shoots over the desired temperature. A wide variety of factors may contribute to this effect, and higher-quality ovens are engineered to mitigate overshoot. Depending on how much a particular oven overshoots, it may be critically important to avoid placing samples in the oven until after the oven’s temperature has stabilized.
Heat Saturation / Preheat Time
Another term often heard regarding a vacuum oven is the heat saturation. Just like the oven in your kitchen, a vacuum oven requires a certain time to pre-heat. As the heating elements warm up, this energy is radiated to the surrounding material in the walls and shelves of the chamber. Because the vacuum is working against the heat as we mentioned earlier, it will take longer for a vacuum oven to preheat when compared to a conventional oven. This is one of the most commonly overlooked factors among novice technicians, as waiting the proper amount of time for an oven to reach saturation has the potential to greatly improve results.
Stability and Uniformity
The measurement of oven stability and uniformity are by far the least understood and most important oven performance factors, as they inform the user of an oven’s accuracy and precision.
Stability measures how stable the oven’s temperature is at any given point once it’s already reached temperature. For example, an oven with ±1⁰ stability should keep a 100-degree oven between 99 and 101 degrees while it operates under normal conditions.
Uniformity refers to the temperature distribution throughout the oven’s interior, and is typically expressed as a percentage. So, an oven that exhibits 5% temperature uniformity means once it reaches a temperature of 100 degrees, the hottest point inside the oven will be no further than 5 degrees away from the coldest point inside the oven. You may be thinking, “but why not make the whole oven exactly the same temperature throughout the interior?” Again, the vacuum mechanism is constantly working against uniform heat.
One solution which was mentioned earlier is to use direct-shelf heat. Ovens with shelf-heat exhibit much more uniform temperatures than ovens with wall-heat. Depending on your personal requirements, a solid understanding of heat stability and uniformity can help make these critical configuration decisions.