Seasoning of Wood Part 26

EXAMPLE 9. To correct readings of the hygrometer for changes in barometric pressure:

A change of pressure affects the reading of the wet bulb.

The chart applies at a barometric pressure of 30 inches, and, except for great accuracy, no correction is generally necessary.

Find the relative humidity as usual. Then look for the nearest barometer line (indicated by dashes). At the end of each barometer line will be found a fraction which represents the proportion of the relative humidity already found, which must be added or subtracted for a change in barometric pressure. If the barometer reading is less than 30 inches, add; if greater than 30 inches, subtract. The figures given are for a change of 1 inch; for other changes use proportional amounts. Thus, for a change of 2 inches use twice the indicated ratio; for half an inch use half, and so on.

Example: Dry bulb 67, wet bulb 51, barometer 28 inches.

The relative humidity is found, by the method given in example 1, to equal 30 per cent. The barometric line--gives a value of 3/100H for each inch of change. Since the barometer is 2 inches below 30, multiply 3/100H by 2, giving 6/100H. The correction will, therefore, be 6/100 of 30, which equals 1.8. Since the barometer is below 30, this is to be added, giving a corrected relative humidity of 31.8 per cent.

This has nothing to do with the vapor pressure (concave) curves, which are independent of barometric pressure, and consequently does not affect the solution of the previous problems.

EXAMPLE 10. At what temperature must the condenser be maintained to produce a given humidity?

Example: Suppose the temperature in the drying room is to be kept at 150 Fahr., and a humidity of 80 per cent is desired. If the humidity is in excess of 80 per cent the air must be cooled to the dew-point corresponding to this condition (see example 4), which in this case is 141.5.

Hence, if the condenser cools the air to this dew point the required condition is obtained when the air is again heated to the initial temperature.

EXAMPLE 11. Determination of relative humidity by the dew-point:

The quant.i.ty of moisture present and relative humidity for any given temperature may be determined directly and accurately by finding the dew-point and applying the concave (vapor-pressure) curves. This does away with the necessity for the empirical convex curves and wet-and-dry-bulb readings. To find the dew-point some form of apparatus, consisting essentially of a thin gla.s.s vessel containing a thermometer and a volatile liquid, such as ether, may be used. The vessel is gradually cooled through the evaporation of the liquid, accelerated by forcing air through a tube until a haze or dimness, due to condensation from the surrounding air, first appears upon the brighter outer surface of the gla.s.s. The temperature at which the haze first appears is the dew-point. Several trials should be made for this temperature determination, using the average temperature at which the haze appears and disappears.

To determine the relative humidity of the surrounding air by means of the dew-point thus determined, find the concave curve intersecting the top horizontal (100 per cent relative humidity) line nearest the dew-point temperature.

Follow parallel with this curve till it intersects the vertical line representing the temperature of the surrounding air. The horizontal line pa.s.sing through this intersection will give the relative humidity.

Example: Temperature of surrounding air is 80; dew-point is 61; relative humidity is 53 per cent.

The dew-point determination is, however, not as convenient to make as the wet-and-dry-bulb hygrometer readings.

Therefore, the hygrometer (convex) curves are ordinarily more useful in determining relative humidities.

The Hygrodeik

In Figure 94 will be seen the Hygrodeik. This instrument is used to determine the amount of moisture in the atmosphere. It is a very useful instrument, as the readings may be taken direct with accuracy.

To find the relative humidity in the atmosphere, swing the index hand to the left of the chart, and adjust the sliding pointer to that degree of the wet-bulb thermometer scale at which the mercury stands.

Then swing the index hand to the right until the sliding pointer intersects the curved line, which extends downwards to the left from the degree of the dry-bulb thermometer scale, indicated by the top of the mercury column in the dry-bulb tube.

At that intersection, the index hand will point to the relative humidity on scale at bottom of chart (for example see Fig. 94). Should the temperature indicated by the wet-bulb thermometer be 60 degrees, and that of the dry-bulb 70 degrees, the index hand will indicate humidity 55 degrees, when the pointer rests on the intersecting line of 60 degrees and 80 degrees.

The Recording Hygrometer

In Figure 95 is shown the Recording Hygrometer complete with wet and dry bulbs, two connecting tubes and two recording pens and special moistening device for supplying water to the wet bulb.

This equipment is designed particularly for use in connection with dry rooms and dry kilns and is arranged so that the recording instrument and the water supply bottle may be installed outside of the dry kiln or drying room, while the wet and dry bulbs are both installed inside the room or kiln at the point where it is desired to measure the humidity. This instrument records on a weekly chart the humidity for each hour of the day, during the entire week.

[Ill.u.s.tration: Fig. 94. The Hygrodeik.]

The Registering Hygrometer

In Figure 96 is shown the Registering Hygrometer, which consists of two especially constructed thermometers. The special feature of the thermometers permits placing the instrument in the dry kiln without entering the drying room, through a small opening, where it is left for about 20 minutes.

[Ill.u.s.tration: Fig. 95. The Recording Hygrometer, Complete with Wet and Dry Bulbs. This instrument records on a weekly chart the humidity for each hour of the day, during the entire week.]

The temperature of both the dry and wet bulbs are automatically recorded, and the outside temperature will not affect the thermometers when removed from the kiln. From these recorded temperatures, as shown when the instrument is removed from the kiln, the humidity can be easily determined from a simple form of chart which is furnished free by the makers with each instrument.

The Recording Thermometer

[Ill.u.s.tration: Fig. 96. The Registering Hygrometer.]

[Ill.u.s.tration: Fig. 97. The Recording Thermometer.]

In Figure 97 is shown the Recording Thermometer for observing and recording the temperatures within a dry kiln, and thus obtaining a check upon its operation. This instrument is constructed to record automatically, upon a circular chart, the temperatures prevailing within the drying room at all times of the day and night, and serves not only as a means of keeping an accurate record of the operation of the dry kiln, but as a valuable check upon the attendant in charge of the drying process.

[Ill.u.s.tration: Fig. 98. The Registering Thermometer.]

[Ill.u.s.tration: Fig. 99. The Recording Steam-Pressure Gauge.]

The Registering Thermometer

In Figure 98 is shown the Registering Thermometer, which is a less expensive instrument than that shown in Figure 97, but by its use the maximum and minimum temperatures in the drying room during a given period can be determined.

The Recording Steam Gauge

In Figure 99 is shown the Recording Steam Pressure Gauge, which is used for accurately recording the steam pressures kept in the boilers.

This instrument may be mounted near the boilers, or may be located at any distance necessary, giving a true and accurate record of the fluctuations of the steam pressure that may take place within the boilers, and is a check upon both the day and night boiler firemen.

The Troemroid Scalometer

In Figure 100 is shown the Troemroid Scalometer. This instrument is a special scale of extreme accuracy, fitted with agate bearings with screw adjustment for balancing. The beam is graduated from 0 to 2 ounces, divided into 100 parts, each division representing 1-50th of an ounce; and by using the pointer attached to the beam weight, the 1-100th part of an ounce can be weighed.

[Ill.u.s.tration: Fig. 100. The Troemroid Scalometer.]

The percentage table No. II has a range from one half of 1 per cent to 30 per cent and is designed for use where extremely fine results are needed, or where a very small amount of moisture is present. Table No. III ranges from 30 per cent up to 90 per cent. These instruments are in three models as described below.

MODEL A. (One cylinder) ranges from 1/2 of 1 per cent to 30 per cent and is to be used for testing moisture contents in kiln-dried and air-dried lumber.

MODEL B. (Two cylinders) ranges from 1/2 of 1 per cent up to 90 per cent and is to be used for testing the moisture contents of kiln-dried, air-dried, and green lumber.

MODEL C. (One cylinder) ranges from 30 per cent to 90 per cent and is applicable to green lumber only.

=Test Samples.=--The green boards and all other boards intended for testing should be selected from boards of fair average quality. If air-dried, select one about half way up the height of the pile of lumber. If kiln-dried, two thirds the height of the kiln car. Do not remove the kiln car from the kiln until after the test. Three of four test pieces should be cut from near the middle of the cross-wise section of the board, and 1/8 to 3/16 inch thick. Remove the superfluous sawdust and splinters. When the test pieces are placed on the scale pan, be sure their weight is less than two ounces and more than 1-3/4 ounces. If necessary, use two or more broken pieces. It is better if the test pieces can be cut off on a fine band saw.