Pyrometers





Armor plate makers sometimes use the copper ball or Siemens' water

pyrometer because they can place a number of the balls or weights on

the plate in locations where it is difficult to use other pyrometers.

One of these pyrometers is shown in section in Fig. 109.



SIEMENS' WATER PYROMETER.--It consists of a cylindrical copper vessel

provided with a handle and containing a second smaller copper vessel

with double walls. An air space a separates the two vessels, and

a layer of felt the two walls of the inner one, in order to retard

the exchange of temperature with the surroundings. The capacity

of the inner vessel is a little more than one pint. A mercury

thermometer b is fixed close to the wall of the inner vessel,

its lower part being protected by a perforated brass tube, whilst

the upper projects above the vessel and is divided as usual on the

stem into degrees, Fahrenheit or Centigrade, as desired. At the

side of the thermometer there is a small brass scale c, which

slides up and down, and on which the high temperatures are marked

in the same degrees as those in which the mercury thermometer is

divided; on a level with the zero division of the brass scale a

small pointer is fixed, which traverses the scale of the thermometer.






Short cylinders d, of either copper, iron or platinum, are supplied

with the pyrometer, which are so adjusted that their heat capacity at

ordinary temperature is equal to one-fiftieth of that of the copper

vessel filled with one pint of water. As, however, the specific heat

of metals increases with the temperature, allowance is made on the

brass sliding scales, which are divided according to the metal used

for the pyrometer cylinder d. It will therefore be understood that

a different sliding scale is required for the particular kind of

metal of which a cylinder is composed. In order to obtain accurate

measurements, each sliding scale must be used only in conjunction

with its own thermometer, and in case the latter breaks a new scale

must be made and graduated for the new thermometer.



The water pyrometer is used as follows:



Exactly one pint (0.568 liter) of clean water, perfectly distilled

or rain water, is poured into the copper vessel, and the pyrometer

is left for a few minutes to allow the thermometer to attain the

temperature of the water.



The brass scale c is then set with its pointer opposite the

temperature of the water as shown by the thermometer. Meanwhile

one of the metal cylinders has been exposed to the high temperature

which is to be measured, and after allowing sufficient time for

it to acquire that temperature, it is rapidly removed and dropped

into the pyrometer vessel without splashing any of the water out.



The temperature of the water will rise until, after a little while,

the mercury of the thermometer has become stationary. When this

is observed the degrees of the thermometer are read off, as well

as those on the brass scale c opposite the top of the mercury.

The sum of these two values together gives the temperature of the

flue, furnace or other heated space in which the metal cylinder

had been placed. With cylinders of copper and iron, temperatures up

to 1,800 deg.F. (1,000 deg.C.) can be measured, but with platinum cylinders

the limit is 2,700 deg.F. (1,500 deg.C.).



For ordinary furnace work either copper or wrought-iron cylinders

may be used. Iron cylinders possess a higher melting point and have

less tendency to scale than those of copper, but the latter are

much less affected by the corrosive action of the furnace gases;

platinum is, of course, not subject to any of these disadvantages.



The weight to which the different metal cylinders are adjusted is

as follows:



Copper 137.0 grams

Wrought-iron 112.0 grams

Platinum 402.6 grams



In course of time the cylinders lose weight by scaling; but tables

are provided giving multipliers for the diminished weights, by

which the reading on the brass scale should be multiplied.





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