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05 October 2001

How to Built a Super Sensitive Manometer

by C. L. Stong
Excerpted from "The Amateur Scientist", December 1969.

Substantial changes in pressure can be measured fairly accurately by a simple manometer: a U-shaped tube of glass containing a fluid of known density, such as water.

One arm of the tube is connected to the vessel containing gas under unknown pressure. If the gas pressure is higher than atmospheric pressure, fluid moves downward in that arm and upward in the other arm. The pressure is equal to the difference in the height of the fluid in the alms multiplied by the specific weight of the fluid. The height can be measured with reasonable accuracy to within .5 millimeter. In the case of water .5 millimeter is equal to .0014 pound per square inch.

By a simple modification of the manometer the sensitivity of the instrument can be increased by a factor of several thousand. Details of the modification are explained by Kipling Adams, who is associated with the General RadioCompany in West Concord, Mass. Adams writes:

"The sensitive manometer consists of two cylindrical containers interconnected near the bottom by a pair of tubes. One of them is a capillary made of glass or clear plastic and therefore transparent. Each tube contains a stopcock. The assembly is filled with fluid. The movement of the fluid is indicated by a bubble of air trapped in the middle of the capillary.

"One cylinder is closed at the top by a gastight lid containing a pipe nipple through which the manometer is connected to the source of unknown pressure. The top of the second cylinder is exposed to the air in the room. The principle of the device is as simple as its construction. A difference in pressure that acts on the surface of fluid in the containers causes fluid to flow through the capillary toward the container of lesser pressure, as is indicated by the movement of the bubble.

 

Figure 1: Figure 6: Elements of the manometer designed by Kipling Adams

"The dimensions are not critical, but, they must be known as accurately as possible for calibrating the instrument. For instance, assume that the cylindrical containers have a diameter of five inches and that the capillary has a bore of .05 inch. If we now apply a pressure of 36 millionths of a pound per square inch to the surface of the water in the closed container, the water level will fall .00.5 inch and will flow through the transparent tube until the level rises an equal amount in the opposite container. (A cubic inch of water weighs about .036 pound.) The ratio of the cross-sectional area of the cylindrical containers in this example is 10,000 times the cross-sectional area of the bore of the capillary. The bubble will therefore move five inches toward the left. Hence a bubble displacement of 1/8 inch indicates a change in pressure of a millionth of a pound per square inch.

"A practical device can be made with a pair of coffee cans, two brass stopcocks, some rubber tubing and the capillary. Glass tubes with a bore of from .01 inch to several inches are available from distributors of scientific supplies. Assemble the apparatus as indicated in the accompanying illustration. Place it in operation by closing the stopcock connected to the glass tube and opening the other stopcock. Fill the cans about three-quarters full of water. When the levels have equalized, close that stopcock and open the other one. With a medicine dropper add a few drops to one reservoir until water pushes most of the air out of the glass tube. A few drops added to the second reservoir will bring the air bubble back to the center of the tube. If the reservoirs are five inches in diameter, each drop will move the bubble about 1.5 inches in a tube with a bore of .05 inch.

"The multiplying factor of 10,000 may prove to be too large for some measurements and the pressure range too small. A reduction factor of, say, 100 may be required. You can make this reduction in sensitivity by shrinking the effective diameter of the open reservoir from five inches to .5 inch. Reducing the diameter this substantially is not as difficult as it might seem. You can accomplish it by putting a nonfloating cylinder 4.741 inches in diameter in the open reservoir. This 'dividing' plug does not need to be placed concentrically in the reservoir. It will work in any position, provided that it does not block the outlet tubes and provided also that the level of the water in the reservoir does not reach the top or bottom of the plug.

"The instrument can also be used as a highly sensitive tilt indicator. The 10,000-to-1 sensitivity figure applies to the liquid levels, so that raising one reservoir from the balance position 10 millionths of an inch displaces the bubble almost 1/16 inch. To demonstrate the sensitivity of the device, place a piece of wood two feet long and two by four inches in cross section on supports two feet apart. Stand one reservoir on the center of the board and the other reservoir above one of the supports. The weight of a finger placed on the board near the center will cause the wood to bow downward about .0001 inch and will displace the bubble .5 inch!"