Oven Temperature Controls
Automatic Single Temperature Control
An automatic or single setpoint type of control is used to precisely control the oven at one selected temperature. The operator sets the control to the desired temperature and the oven heats up to this level automatically. No supervision by the operator is necessary. A fluorescent digital display is provided for easy viewing of the actual setpoint temperature values at all times.
Both automatic and programmable controls are closed loop systems. In a closed loop situation, the temperature sensed by the thermocouple gives a feedback signal directly to the microprocessor control that regulates the power to the heating elements.
16-Segment Programmable Temperature Control
A programmable control of this type operates very similarly to a four-segment control. The main difference is that this control is supplied with sixteen programmable segments, including eight ramp and eight dwell segments. In addition, a bidirectional RS-232 communication port allows the operator to control the oven operation from a computer.
8-Segment Programmable Temperature Control
A programmable control allows the operator to program the rate of increase and decrease (RAMP), and the period of time the temperature holds at specific levels (DWELL). The eight segments include four ramp and four dwell segments.
Accuracy
Control accuracy is the measurement of overall error that the control device can add to the entire control loop of the heating system. It is not a measurement of the entire thermal system but only of control itself. This measurement is usually expressed as a percentage of the operating span (range) of the control. Other factors of the entire thermal system can add additional error to the overall performance of the oven's ability to maintain set temperature. These factors include:
- The selected operating temperature.
- Size and density of material being heated
- Position of the material inside the heating chamber
- Line voltage variations
- Thermal lag during the heat up
- Temperature gradients within the heating chamber
The selected operating temperature is the most important factor in determining the accuracy of the oven operation. It is difficult to design an oven to operate with the same level of accuracy over the entire operating range. Operation below 260°C will require considerable time for any type of control to provide a stable temperature; variations above and below the setpoint may be 20°C or more.