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]