Case 1--Horizontal pipe. Water flows at bottom. If perforations in nozzle are too near bottom of pipe, water piles against nozzle, flows into calorimeter and gives false reading. Case 2--If nozzle located too near junction of two horizontal runs, as at a, condensation from vertical pipe which collects at this point will be thrown against the nozzle by the velocity of the steam, resulting in a false reading. Nozzle should be located far enough above junction to be removed from water kept in motion by the steam velocity, as at b. Case 3--Condensation in bend will be held by velocity of the steam as shown. When velocity is diminished during firing intervals and the like moisture flows back against nozzle, a, and false reading is obtained. A true reading will be obtained at b provided condensation is not blown over on nozzle. Case 4--Where non-return valve is placed before a bend, condensation will collect on steam line side and water will be swept by steam velocity against nozzle and false readings result.] Fig. 19 indicates certain locations of sampling nozzles from which erroneous results will be obtained, the reasons being obvious from a study of the cuts. Before taking any calorimeter reading, steam should be allowed to flow through the instrument freely until it is thoroughly heated. The method of using a throttling calorimeter is evident from the description of the instrument given and the principle upon which it works. [Illustration: Babcock & Wilcox Superheater] SUPERHEATED STEAM Superheated steam, as already stated, is steam the temperature of which exceeds that of saturated steam at the same pressure. It is produced by the addition of heat to saturated steam which has been removed from contact with the water from which it was generated. The properties of superheated steam approximate those of a perfect gas rather than of a vapor. Saturated steam cannot be superheated when it is in contact with water which is also heated, neither can superheated steam condense without first being reduced to the temperature of saturated steam. Just so long as its temperature is above that of saturated steam at a corresponding pressure it is superheated, and before condensation can take place that superheat must first be lost through radiation or some other means. Table 24[20] gives such properties of superheated steam for varying pressures as are necessary for use in ordinary engineering practice.