High temperature vacuum furnaces are used to manufacture silicon carbide tubes and rings, which are used in high quality bearings. Due to the high working temperature of up to 1675°C and the silicon atmosphere thermocouples can only last for a few cycles. As these thermocouples are made from Tungsten they are very expensive. To reduce the T/C wastage a pyrometer is used at higher temperatures. Normally T/C’s are used for temperatures up to 1200°C and pyrometers from 1100°C. This gives a 100°C switchover range between the T/C and the pyrometer. However, there can also be a requirement to use thermocouple control on the full cycle in certain processes which go above 1200°C.
A 2604 single loop programmer is used utilising its Toolkit blocks. Three PV inputs are used. The thermocouple is connected in parallel to two inputs, with one input (IP1) ranged 0-2000°C and the other (IP2) ranged 0-1200°C.
The Pyrometer input, which is a 4-20mA linear signal representing 1100°C to 2000°C is connected to the third PV input (IP3).
Using “Toolkit Blocks” we calculate Analogue 1 using the switchover function between IN2 and IN3. The result (AOP1) is then used as an input Analogue 2, which dependent on the state of a digital input will connect either IN1 or AOP1 to the actual process variable.
Other features include:
Alarms on PV1 at 1090°C to open the sight glass window for the Pyro, and at 1210°C to retract the T/C when using Pyrometer mode.
A 0-10Vac retransmission of PV1 to a chart recorder.
Digital inputs for run and reset.