Geoscience Reference
In-Depth Information
Table 3.7 Some Melting Point Standards for Thermocouple Calibration
Material
Temperature (
C)
Tin
231.9
Lead
327.4
Antimony
630.5
Sodium chloride
801.0
Silver
a
960.8
a
Calibration strongly impaired by minor amounts of dissolved oxygen.
Source: From Ref.
[21]
.
computer. With a rack-type system of 10
20 vessels, such a system can record run
temperatures, in 1 s, of each vessel several times.
The pressure in hydrothermal experiments is measured by different means. In the
initial experiments in a given system, standard calibrated bourdon gauges are used
and are recalibrated at regular intervals against either a dead weight tester or master
gauge. Pressure inside a vessel can be monitored through a pressure transducer to
separate the corrosive contents of the vessel from the pressure gauge. It is useful to
have a valve between the vessel and the transducer to act as a micropressure genera-
tor to check for hysteresis in the transducer and to isolate the vessel during evacua-
tion at the start of the run. A stainless steel or titanium capillary tube is used for the
connection with the pressure gauge because of its easy installation and the small vol-
ume of trapped fluid that it contains. For routine pressure measurements, Kennedy's
PVT diagram and Bain's PVT data are used because of their simple procedure
[21,22]
. Whether heating or cooling, identical pressure values should be observed,
and differences of a few tenths of a bar indicate either hysteresis in the pressure
transducer, or that a gas consuming or evolving reaction is taking place. Automatic
pressure controllers and recorders are also available commercially. For example, in
the Tuttle cold-cone-seal autoclaves unit (Tem-Press, Leco Corp., USA;
Figure 3.7a
and b
), the required pressure is fixed in the gauge which will maintain the pressure at
that level for a fairly long duration. However, pressure measurements with a temper-
ature gradient have not been studied in detail, but it is certain that, under hydrother-
mal conditions, there can be no pressure gradient. Hence, the local temperature and
local density anywhere in the system would determine the pressure of the entire
system. With a computer, the experimental pressures can be continuously monitored
by the EMF (Electromagnetic Field) signal of a pressure transducer (i.e., strain gauge
or manganin cell).
In addition to monitoring pressure and temperature, computers offer the poten-
tial to control run conditions. As an example, the computer could check the milli-
voltage signal from the thermocouple of each vessel and compare the signal to that
of the desired temperature. The control action could be provided through on/off
relays controlling power to the furnaces. Thus, the computers can make the record-
ing and controlling more handy in hydrothermal experiments in a racklike system.
