﻿4 BULLETIN 1136, U. S. DEPARTMENT OF AGRICULTURE. 



types are the Harvard trip, which has the beam located under the 

 pans and is provided with a scale beam and rider sensitive to 0.1 

 gram, and with a 10-gram capacity; the torsion balance, with beams 

 below the "pans; and the multiple-beam balance, with only one pan 

 suspended from the beam, which is provided with sliding weights. 



DRYING OVENS. 



Several makes of drying ovens can be bought. All of these are 

 electrically heated and provided with thermostatic control, which 

 keeps the temperature accurately at the desired point. Steam- 

 heated ovens are convenient and free from trouble and will be found 

 excellent where high-pressure steam is continuously available. Ovens 

 of this kind are usually homemade. The walls and doors can be of 

 galvanized iron, made hollow with a 1^-inch space filled with mineral 

 wool, and the heating element can be conveniently made of 1-inch or 

 lj-inch pipe. Ventilators should be fitted to the top, and provision 

 made under the steam pipes for the entrance of fresh air. The 

 temperature is usually regulated by means of a reducing valve on the 

 steam line and dampers on the ventilators. For each cubic foot of 

 volume above the heating coils in the oven there should be at least 

 1£ square feet of heating surface and six square inches of ventilator 

 area. Shelves should be provided for the moisture sections. Plate I 

 illustrates one of the steam drying ovens used by the Forest Products 

 Laboratory. 



There are available various kinds of hot plates used in place of 

 ovens for drying out moisture sections. It is customary to use very 

 thin sections with these hot plates and to leave them on only a short 

 time — 15 to 45 minutes. These hot plates fill a need in that they 

 are cheap and used by those who do not care to buy a regular oven, 

 and in the hands of a skillful operator can be made to yield good 

 results. They can not be recommended except as makeshifts. 



It is very helpful, except in the simplest kinds of drying, to know 

 how the moisture is distributed throughout the cross section of the 

 board or stick, and for this purpose " moisture distributions " are 

 made. The moisture section is cut in the usual manner, but instead 

 of weighing it as a whole, it is cut or split so as to separate the core 

 or center from the shell or outside, and separate moisture determina- 

 tions are made on the core and shell. The latter will usually be in 

 two or four pieces, which can be most conveniently weighed as a 

 single unit. For thick stock it may be desirable to divide the sec- 

 tions into three units, a shell, an intermediate zone, and the core. 

 The procedure is precisely the same as before, the pieces of the inter- 

 mediate zone being weighed as a unit just as are those of the shell. 

 To secure satisfactory results, these "moisture distributions" must 

 be made accurately, and an analytical or torsion balance sensitive to 

 0.01 gram should be used. The capacity of this balance need not be 

 over 100 grams. A larger balance should also he available for the 

 heavier work of weighing regular moisture sections. 



Figure 1 illustrates the method of cutting the moisture and dis- 

 tribution sections. While it is the usual practice to cut a full section 

 and a distribution section whenever a distribution test is to be made, 

 it is not absolutely necessary, since the average moisture content may 



