Apparatus - Hydrothermal Technology - page 87

Table 3.6 Some Common Thermocouple Wires and Their Temperature Ranges

Wires Temperature

Range ( C)

Maximum
Temperature ( C)

Chromel Alumel (90% Ni 10% Cr)
(95% Ni, 5% Al, SiMn)

200 to 1200

1350

Pt Pt 90 Rh 10

0 to 1450 1700

Pt Pt 87 Rh 13
Pt Pt 80 Rh 20

Copper Constantan (60 45% Cu, 200 to 350 600

40 55% Ni)

Chromel Constantan
Iron Constantan

200 to 750 1000

Source: From Ref. [20] .

experimental temperature, temperature gradient, heating and cooling rates, and so
on. A variety of alloys are used for furnace windings to reach the desired tempera-
tures in the hydrothermal range. The most common are nichrome and Kanthal.
These are more durable and are less subject to damage if sheathed in magnesia
insulation plus Inconel or similar high-temperature alloys. The ratio of maximum
voltage over the required amperage equals the necessary resistance of the furnace
winding. The necessary resistance divided by the length of the winding provides
the unit length with which the furnace should be constructed. Tables of resistance
per unit length with measurements for each of the available commercial heater
wires are often supplied by the manufacturer. To control temperature gradients in
the reaction vessel, the windings should be more closely spaced near the ends of
the furnace, especially at the end where more pressure tubing is connected to com-
pensate for the higher heating fluxes from these areas.
The temperatures in hydrothermal experiments are measured using
suitable thermocouples that are classified according to the pair of metals as noble
metal thermocouples and base metal thermocouples. They are made of chromel and
alumel or platinum and rhodium alloys owing to their stability and resistance to
oxidation throughout the hydrothermal temperature range. Table 3.6 gives the list
of some commonly used thermocouple wires and their temperature ranges [19] .
Thermocouples are best inserted into holes drilled into the walls at both ends of the
vessel to allow temperature gradients in the vessel to be monitored. Such external
temperature measurements should be calibrated by inserting a thermocouple
directly into a closed bottom, hollow tube extending into the vessel. The thermo-
couple calibration is accomplished by measuring the elements at the melting points
of reference materials. Table 3.7 shows the melting points of standards used for
thermocouple calibration [20] .
A computer can be used to record the experimental temperatures uninterrupt-
edly, and it requires an analogue-to-digital converter (ADC) to transform the ther-
mocouple millivolt signal to a digital signal to be processed and displayed by the