The heater consists of a conducting sleeve a few mm long and 1-2 mm in diameter located inside the sample assembly, or two disks above and below the sample itself. Currently, three designs are extant, a low resistance heater made of carbon, a somewhat higher resistance one made of silicon carbide (used for disk heaters only), and one made of LaCrO3. A large DC current (up to 100 Amperes) is passed through the heater. The current flows from the upper anvil, through the sample assembly, and to the lower anvil. The side anvils are insulated from the upper and lower anvils. The upper anvil is isolated from the rest of the press.
Current is supplied by one of two regulated power supplies located inside the hutch; one is 10 volts at 250 amperes, the other is 40 volts at 60 amperes. Either one can be controlled by the computer, although the 40 volt one is currently disconnected. The voltage is measured directly across the anvils using channel 5 of the Keithley Digital Multimeter (DMM), and the current is measured with a Hall effect current probe, the output of which is measured by channel 6 of the Keithley DMM.
The sample thermocouples are usually type C (tungsten 5% rhodium - tungsten 26% rhodium), but can be type D (tungsten 3% rhodium - tungsten 25% rhodium) or type K (chromel-alumel) thermocouples. The thermocouple voltages are measured directly with channels 1 and 2 of the Keithley DMM, usually with no external cold junction compensators. The data is fed to the computer via a serial interface. The thermocouple EMF is converted to temperature using a sixth order polynomial based on the calibration data using a Epics User Transform. Both the EMF and the temparature are displayed in the logging file.
The thermocouple polynomial coefficients can be determined using the program TC_CALIB.EXE, which reads a file (TC_CALIB.DAT) which contains a set of EMF vs. temperature data. The output file (TC_CALIB.OUT) gives the coefficients. These seven numbers are entered into one of the appropriate lines of the IDL procedure load_thermoucouple_cal.pro. This data is loaded into the User Transform by entering load_thermocouple_cal index meter where index is the number of the thermocouple coefficient set, and meter is either 1 or 2, depending on which thermocouple you are using. This is done from the IDL command line.
Addendum March, 2006: When running the Startup program, the user is asked for which set of thermocouple parameters are to be used. When one of the 10 sets of parameters in load_thermocuople_cal.pro is selected, that set of parameters is automatically loaded into the system.
