steam at boiler pressure,
L = latent heat of steam at boiler pressure,
h = total heat of steam at reduced pressure after passing
orifice,
t_{1} = temperature of saturated steam at the reduced pressure,
t_{2} = temperature of steam after expanding through the orifice
in the disc,
0.47 = the specific heat of saturated steam at atmospheric pressure,
x = proportion by weight of moisture in steam.

The difference in B. t. u. in a pound of steam at the boiler pressure
and after passing the orifice is the heat available for evaporating the
moisture content and superheating the steam. Therefore,

H - h = xL + 0.47(t_{2} - t_{1})

H - h - 0.47(t_{2} - t_{1})
or x = --------------------------- (4)
L

Almost invariably the lower pressure is taken as that of the atmosphere.
Under such conditions, h = 1150.4 and t_{1} = 212 degrees. The formula
thus becomes:

H - 1150.4 - 0.47(t_{2} - 212)
x = ------------------------------ (5)
L

For practical work it is more convenient to dispense with the upper
thermometer in the calorimeter and to measure the pressure in the steam
main by an accurate steam pressure gauge.

A chart may be used for determining the value of x for approximate work
without the necessity for computation. Such a chart is shown in Fig. 15
and its use is as follows: Assume a gauge pressure of 180 pounds and a
thermometer reading of 295 degrees. The intersection of the vertical
line from the scale of temperatures as shown by the calorimeter
thermometer and the horizontal line from the scale of gauge pressures
will indicate directly the per cent of moisture in the steam as read
from the diagonal scale. In the present instance, this per cent is 1.0.

Sources of Error in the Apparatus--A slight error may arise from the
value, 0.47, used as the specific heat of superheated steam at
atmospheric pressure. This value, however is very nearly correct and any
error resulting from its use will be negligible.

There is ordinarily a larger source of error due to the fact that the
stem of the thermometer is not heated to its full length, to an initial
error in the thermometer and to radiation losses.

With an ordinary thermometer immersed in the well to the 100 degrees
mark, the error when registering 300 degrees would be about 3 degrees
and the true temperature be 303 degrees.[19]

The steam is evid