+------------------------+------------------------+
| Centigrade | Fahrenheit |
+-------------+----------+-------------+----------+
| | Accuracy | | Accuracy |
| Temperature | Plus or | Temperature | Plus or |
| Range | Minus | Range | Minus |
| | Degrees | | Degrees |
+-------------+----------+-------------+----------+
| 200- 500 | 0.5 | 392- 932 | 0.9 |
| 500- 800 | 2 | 932-1472 | 3.6 |
| 800-1100 | 3 | 1472-2012 | 5.4 |
| 1100-1600 | 15 | 2012-2912 | 27 |
| 1600-2000 | 25 | 2912-3632 | 45 |
+-------------+----------+-------------+----------+
Mercurial Pyrometers--At atmospheric pressure mercury boils at 676
degrees Fahrenheit and even at lower temperatures the mercury in
thermometers will be distilled and will collect in the upper part of the
stem. Therefore, for temperatures much above 400 degrees Fahrenheit,
some inert gas, such as nitrogen or carbon dioxide, must be forced under
pressure into the upper part of the thermometer stem. The pressure at
600 degrees Fahrenheit is about 15 pounds, or slightly above that of the
atmosphere, at 850 degrees about 70 pounds, and at 1000 degrees about
300 pounds.
Flue-gas temperatures are nearly always taken with mercurial
thermometers as they are the most accurate and are easy to read and
manipulate. Care must be taken that the bulb of the instrument projects
into the path of the moving gases in order that the temperature may
truly represent the flue gas temperature. No readings should be
considered until the thermometer has been in place long enough to heat
it up to the full temperature of the gases.
Expansion Pyrometers--Brass expands about 50 per cent more than iron and
in both brass and iron the expansion is nearly proportional to the
increase in temperature. This phenomenon is utilized in expansion
pyrometers by enclosing a brass rod in an iron pipe, one end of the rod
being rigidly attached to a cap at the end of the pipe, while the other
is connected by a multiplying gear to a pointer moving around a
graduated dial. The whole length of the expansion piece must be at a
uniform temperature before a correct reading can be obtained. This fact,
together with the lost motion which is likely to exist in the mechanism
connected to the pointer, makes the expansion pyrometer unreliable; it
should be used only when its limitations are thoroughly understood and
it should be carefully calibrated. Unless the brass and iron are known
to be of the same temperature, its action will be anomalous: for
instance, if it be allowed to cool after being exposed to a high
temperature, the needle will rise before it begins to fall. Similarly, a
rise in temperature is first shown by the instrument as a fall. The
explanation is that the iron, being on the outside, heats or cools more
quickly than the brass.
Calorimetry--This method derives its name from the fact that the process
is the same as the determination of the specific heat of a substance by
the water calorimeter, except that in one case the temperature is known
and the specific heat is required, while in the other the specific heat
is known and the temperature is required. The temperature is found as
follows:
A given weight of some substance such as iron, nickel or fire brick, is
heated to the unknown temperature and then plunged into water and the
rise in temperature noted.
If X = temperature to be measured, w = weight of heated body in pounds,
W = weight of water in pounds, T = final temperature of water, t =
difference between initial and final temperatures of water, s = known
specific heat of body. Then X = T + Wt ÷ ws
Any temperatures secured by this method are affected by so many sources
of error that the results are very approximate.
Thermo-electric Pyrometers--When wires of two different metals are
joined at one end and heated, an electromotive force will be set up
between the free ends of the wires. Its amount will depend upon the
composition of the wires and the difference in temperature between the
two. If a delicate galvanometer of high resistance be connected to the
"thermal couple", as it is called, the deflection of the needle, after a
careful calibration, will indicate the temperature very accurately.
In the thermo-electric pyrometer of Le Chatelier, the wires used are
platinum and a 10 per cent alloy of platinum and rhodium, enclosed in
porcelain tubes to protect them from the oxidizing influence of the
furnace gases. The couple with its protecting tubes is called an
"element". The elements are made in different lengths to suit
conditions.
It is not necessary for accuracy to expose the whole length of the
element to the temperature to be measured, as the electromotive force
depends only upon the temperature of the juncture at the closed end of
the protecting tube and that of the cold end of the element. The
galvanometer can be located at any convenient point, since the length of
the wires leading to it simply alter the resistance of the circuit, for
which allowance may be made.
The advantages of the thermo-electric pyrometer are accuracy over a wide
range of temperatures, continuity of readings, and the ease with which
observations can be taken. Its disadvantages are high first cost and, in
some cases, extreme delicacy.
Melting Points of Metals--The approximate temperature of a furnace or
flue may be determined, if so desired, by introducing certain metals of
which the melting points are known. The more common metals form a series
in which the respective melting points differ by 100 to 200 degrees
Fahrenheit, and by using these in order, the temperature can be fixed
between the melting points of some two of them. This method lacks
accuracy, but it suffices for determinations where approximate readings
are satisfactory.
The approximate melting points of certain metals that may be used for
determinations of this nature are given in Table 8.
Radiation Pyrometers--These are similar to thermo-electric pyrometers in
that a thermo-couple is employed. The heat rays given out by the hot
body fall on a concave mirror and are brought to a focus at a point at
which is placed the junction of a thermo-couple. The temperature
readings are obtained from an indicator similar to that used with
thermo-electric pyrometers.
Optical Pyrometers--Of the optical pyrometers the Wanner is perhaps the
most reliable. The principle on which this instrument is constructed is
that of comparing the quantity of light emanating from the heated body
with a constant source of light, in this case a two-volt osmium lamp.
The lamp is placed at one end of an optical tube, while at the other an
eyepiece is provided and a scale. A battery of cells furnishes the
current for the lamp. On looking through the pyrometer, a circle of red
light appears, divided into distinct halves of different intensities.
Adjustment may be made so that the two halves appear alike and a reading
is then taken from the scale. The temperatures are obtained from a table
of temperatures corresponding to scale readings. For standardizing the
osmium lamp, an amylacetate lamp, is provided with a stand for holding
the optical tube.