(3) Pot-traps having bodies 6" in diameter and having 1½" or 1¼" connections may, however, be considered safe when they are not exposed to the repeated action of plunger water-closets of the largest water-capacity.
II. Experiments on Back Pressure.
These experiments were made on the basement floor, just above the horizontal run of the soil-pipe. They may be subdivided into (A) those in which the traps were tested without vertical extension of the inlet arm; and (B) those in which the traps had their inlet arms extended. The water-closet used was a Zane's trapped plunger-closet.
(A) Experiments on Traps without Vertical Extension.
(a) An S-trap having the ordinary length of inlet-arm, under the discharge of the water-closet alone. The first discharge threw the water out of the trap, projecting it several feet in the air, and broke the seal. The experiment was often repeated with the same result.
(b) The same result attended the discharge of the water-closet, simultaneously with the bath-tub, only that the greater power of the action threw out more water from the trap, leaving the level considerably below the top of the mouth of the inlet-pipe. Several repetitions of the test produced the same result.
(c) The above experiments were repeated with a trap-less plunger-closet. The results were substantially the same.
(d) A 4" pot-trap lost its seal in four discharges of the water-closet alone. The top of the inlet-arm stood 2" above the top of the seal.
(e) The same trap lost its seal in a single discharge of the water-closet and bath-tub together.
(f) The same traps were tested with a trapless plunger-closet, with substantially the same results. Figure 26 shows the manner in which the water is blown out of a large pot-trap by back-pressure.
Fig. 26 (580).
(g) An 8" pot-trap lost 2" of its seal in seven discharges of the trapped-closet discharged alone. The top of the inlet-pipe stood 3" above the top of the seal.
(h) The same trap lost its entire seal of 3¾" by the fourteen discharges of the water-closet and bath-tub together. (B) Experiments on Traps with Vertical Extension.
(a) An ordinary 1½" cast-lead S-trap with an extension of 1' 4" of 1½" lead-pipe attached to the top of its inletarm, making the top of the extension 22" above the top of the seal was tested. No water was thrown out of the trap by the discharge of the water-closet, either trapped or un-trapped, and whether alone or together with the bath-tub; but in all cases air was forcibly driven through the water forced into the inlet-pipe, because the volume of water in the trap was insufficient to outweigh the back-pressure.
(b) The same result attended the tests made with a 24" extension-pipe.
(c) An S-trap having 5" of seal without extension lost its seal in all cases; but with an extension of 1' 4" the water was not thrown out by discharges of the water-closet alone, or in combination with the bath-tub, and whether the closet was trapped or trapless. With this trap, moreover, the large volume of water was, with the extension sufficient to overbalance the pressure of the air, and no bubbles were driven through the trap.
(d) The same deep trap was then tested after half its seal had been removed, as by evaporation, or other accident. In this case the trap acted exactly as did the ordinary shallow-sealed, cast-lead S-trap before described, and always allowed air to be driven through it.
(e) A 4" pot-trap was then tested with the 1' 4" extension, bringing the top of the pipe 18" above the seal. No water was driven out of the trap, and no bubbles forced through the water under any of the four conditions under which the tests were made as described for the others.
(f) The same trap with a 6" extension bringing the top of the pipe 8" above the top of the seal lost its entire seal in two discharges of the water-closet and bath-tub together. The volume of the water in the trap was sufficient, but the pipe was not long enough to allow of the formation of a column sufficiently high to resist the air-pressure.
(g) An 8" pot-trap with 1' 4" extension lost no water, and allowed no air to pass under either of the four tests.
(h) The same results attended the tests on this trap, having an extension of only 12".
(i) The trap was next tried with 9" of extension, with the same results.
(j) The extension was finally reduced to 6" bringing the top of the pipe 9" above the top of the seal. In this case the water was driven out of the trap.
(k) A "Sanitas" trap was then tested, and the results were substantially the same as with the 8" pot-trap.
Deductions from the Experiments on Back-Pressure.
From these experiments we learn (1) that in traps which are unventilated back-pressure may be resisted by constructing them in such a manner that they shall contain a large volume of water, and by setting them far enough below the fixture to admit of the formation in the waste-pipe above the trap of a column of water large enough to outweigh the back-pressure of the air.
(2) That the back-pressure in the tests herein recorded was sufficient to balance a column of water between 9" and 12" long, plus the depth of the water forming the seal. Calling the depth in the average trap 3", our water-column was not less than 12" or 15" in height. This is equivalent to one thirty-second or one twenty-fifth of an atmosphere, (0.43 or 0.56 lbs. to the square inch.).
(3) The back-pressure likely to be encountered in properly-plumbed houses will probably never exceed that obtained in the tests above recorded, since all the conditions most favorable to produce it were here combined. Hence any trap may be considered safe against back-pressure which is so formed as to contain a body of water large enough to fill the waste-pipe full to a height of 12" or 15" (including its own seal), and which is so set as to admit of the formation of this column.