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Sometimes the presentation of data is simplified, often at the expense of. flexibility and coverage, by setting up tables covering limited and specific an(j refrigerant conditions, in which total, sensible and latent capacities are set forth directly. Some of the methods now used require trial and error solutions and others are of very questionable accuracy, although the error may be small when applied within narrow limits of the several variables. It is hoped that investigations now in progress will lead to a convenient and reasonably accurate method of rating which can be widely adopted. In rating dehumidifying coils, there are two requirements.. The total capacity must be determined and the proportion of sensible and latent heat transfer ascertained. These determinations often involve an average coil surface temperature. This may be determined experimentally by the use of thermocouples or calculated theoretically from other test data. Sometimes, a fictitious temperature is used, determined by the point of intersection between the saturation curve on the psychrometric chart and a straight line drawn through points representing the entering and leaving air conditions. The total cooling capacity is determined in a variety of ways, of which the following are the most usual: 1. The use of surface or overall heat transfer coefficients in conjunction with dry-bulb mean temperature differences. The result is corrected for dehumidification by means of functions for: (X) the temperatures differences, (2) the expected total to sensible load ratio, and (3) empirical factors determined from test. 2. The use of surface or overall coefficients for combined sensible and latent heat removal with a wet coil, using as the driving force the difference between heat content of the entering air and that of saturated air at either the surface or the refrigerant temperature. 3. The calculation of sensible and latent capacities separately. The sensible is based on dry-bulb mean temperature difference and heat transfer while the latent is deter mined using a dew-point mean difference and a corresponding latent heat transfer value. 4. The use of a contact factor or ratio of heat removed to heat removable. This factor, is a function of coil depth and air velocity, is experimentally determined for each design' and used in.conjunction with a so-called surface temperature. The total capacity is influenced by the factors already enumerated and, as with dry cooling or heating coils, the air velocity correction is an exponential function of the mass air velocity. Unless the sensible and latent capacities have been separately calculated as in item 3 above, various means are used to determine these. The slope of a straight line on the psychrometric chart connecting points representing the entering and leaving air conditions gives a direct means of fixing the proportions of sensible and latent heat removal. Such a line is termed the load-ratio line. 'For given entering air conditions and a required proportion of sensible and latent cooling, the load ratio line may therefore be drawn, its slope being determined by the temperature and moisture content coordinates of the psychrometric chart usl. To satisfy the performance requirements, the point representing the' leaving air conditions for a coil chosen to give the necessary total cooling capacity must also fall on this line. 507