Electric Drying Oven

Heat is the most common tool in a laboratory and the least examined. A sample is dried, a coating is cured, glassware is dried before use, and the conditioning step is written into a method as a line of text, which is where the trouble starts: the line says what the instrument was set to rather than what the sample experienced, and the two are only the same when the chamber is empty.

A laboratory oven is that line of text made physical, and the whole of its specification is an attempt to close the gap between the setpoint on the display and the conditions on the shelf.

Product overview

The range is built around two structures and seven chamber volumes. The desktop units heat from the bottom and circulate air from the bottom, and run from 16 litres up to 225 litres on a single-phase supply before the larger sizes require three phases. The vertical units stand the chamber above the controls, heat and circulate from the rear, and are stated more tightly on temperature.

Within each structure the chamber can be galvanised or stainless steel. The two are the same size in the same volume, so the choice is a question of what the sample gives off rather than of what will fit, and it can be made after the size has been settled. Two shelves are supplied with every chamber and each is rated at 15 kg.

How the chamber holds a temperature

Elements heat the air and a controller reads a sensor and holds it at the setpoint. That loop is what the display reports, and on this range it is stated to hold to a tenth of a degree on the vertical structure and to a degree on the desktop one. Neither figure says anything about the rest of the chamber, because a controller can hold its own sensor closely while a corner of the shelf runs several degrees away from it.

What closes that gap is air movement. The blower carries heat into the load and through the shelf space rather than letting the chamber stratify, which is why the range is specified with a percentage uniformity figure for the whole chamber as well as an accuracy figure for the sensor. The two numbers describe different things and both are needed. The material states the uniformity as a percentage without naming the reference temperature or the method used to establish it, so it is recorded as an open point rather than published as a test result.

How to select the right drying oven

Start with the structure, because it follows from the method. Where a written procedure names a temperature and a tolerated deviation, the vertical structure is the one stated to meet it, and its stand also allows a tray beneath a dripping load. Where the work is general drying and keeping things warm and dry, the desktop structure does the same job with fewer demands on the bench.

Then take the volume from the batch rather than from the shelf. A chamber loaded to its nominal capacity has no free space left for air to move through, and the uniformity the specification offers is measured in an emptier chamber than that. After the size, the interior material follows the chemistry, and the supply voltage follows the size, since 640 litres and above are three-phase only.

Measurement limitations and selection considerations

The figures on this page describe what the chamber can hold, not what a particular load will experience. Drying is dominated by the latent heat of the water in the load, and a chamber that recovers quickly when empty will still sit below its setpoint for as long as evaporation continues. The material does not state a recovery time, so no attempt has been made to quantify one.

Two further points are open and are the ones to settle before an acceptance test. The uniformity figure is quoted as a percentage with no reference temperature or method attached, and no independent over-temperature cut-out is described, which is the protection that matters most when a controller fails with the elements energised. Both are listed for confirmation rather than assumed.