Heaters and thermocouples

Various types of heater are used, depending on the requirements made of individual HR system elements and their design (see Chapter 4). They are manufactured and made available by specialist firms or by the manufacturers of standard mould bases with special recommendations for their installation and use. 

A heater with a coil of rectangular cross-section (b), or more often a flattened oval cross-section, has a greater contact area with the nozzle, which allows some of the heat to be transferred by conduction. This reduces the heat load and improves the thermal balance. The thermal density used is 6-15 W/cm.  

The non-heated zone at the beginning of the heater in the connection zone is at least 55 mm (tube length). At the end of the heater (near the face of the nozzle), where the thermocouple is located, a short non-heated zone is again used (10-15 mm). 

Methods of fastening cylindrical heaters [2] (a, b, c) and conical heaters (d) which facilitate their removal 

Many companies recommend using an aqueous magnesium oxide solution rather than paste this not only conducts heat very well when the water has evaporated, but also helps to get the heater out. Both this agent and others, especially those preventing corrosion (used even where the gap is very small) are available in spray form. 

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A hot press was made using a hydraulic 20 ton shop press, and two homemade aluminum heating plates. Each heating plate was drilled to accept an electric cartridge heater and a thermocouple. A temperature controller was connected to the heater and thermocouple on each heating plate. 

Pressure was generated with a diamond anvil cell (DAC) employing beveled anvils with central flats ranging from 20 to 100 jim and flat diamonds with 200-500 pm culets. Two types of DAC were used modified (to match a continuous flow He cryostat) Mao-Bell cell for operations at room and low temperatures [41] and a Mao-Bell high-T external heating cell [42]. The latter one is equipped with two heaters and thermocouples. Four experiments were performed at RT aiming to highest pressure and the final pressures varied from 180 to 268 GPa. For low-temperature measurements we used a continuous-flow He cryostat, which allowed infrared and in situ Raman/ fluorescence measurements. More details about our IR/Raman/fluorescence setup at the NSLS are published elsewhere [41]. 

Figure 1 Matrix apparatus as used in Nottingham (1) valve power supply, (2) helium pressure connections to compressor, (3) silicon diode and heater lead-through, (4) pump-out port, (5) gas inlet port, (6) cold station, (7) heater and thermocouple wires, (8) stainless steel tube, (9) silicon diode, (10) very cold station and heater sleeve, (11) cell, (12) photolysis and spectroscopic windows, for an experiment the cell is turned by 90° towards the windows, (13) vacuum shroud, (14) large half of cell, (15) small half of cell, (16) IR optical windows, (17) lead seal, (18) indium window seal, (19) circlip retainer groove, (20) bolt, (21) steel washer, (22) spring washer, (23) nut, (24) femal-femal union, (25) steel tubing, (26) to very cold station, (27) threaded copper stud, (28) threaded hole in large half of cell to receive stud, (29) indium joints, (30) circlip, which fits in groove and retains windows.

Cone-carrying metal foil at its truncated apex. Foil has one or several leaks through which the gas and ions enter the pumping and electrode chamber. 9. Heater and thermocouple wells for temperature control of ion source. 10. Auxiliary electron gun for gas purity determinations. 11-19. Electrodes focusing ion beam into magnetic mass analyzer. Note in later versions of the apparatus the distance from the alpha source to the ion exit slit was shortened, which increases the effective intensity. 

Figure 14. Micrometer positioning device for the DAC in an FTIR Nicolet instrument. The ample available space permits inclusion of the micrometer instrument, the heater and thermocouple leads with ease.

The electric leads for heaters and thermocouples are located in grooves milled in the mould plates, and normally run to the top of the mould, on which a standard socket-contact is fastened. 

Note. When conductive agents are applied it is necessary to protect the connecting terminals of heaters and thermocouples, as well as existing connections, from being covered by them otherwise there is the risk of a short circuit. 

Parallel hot wire method (BS EN 993-15 1998 and ISO 8894-2)— To extend the range of conductivites measured, the parallel hot wire method was devised. In this method, as its name suggests, the heater and thermocouple wires are arranged in parallel. This modified arrangement allows thermal conductivities up to 25 W/mk to be measured. 

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