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Construction of a simple Balance Beam Scale
Beam-balance scale that was built for accurate weighing of propellant constituents.
I had recently been investigating the properties of dextrose as a possible alternative to sucrose as a fuel and binder. This investigation involved preparing many propellant samples, often with different oxidizer/fuel (O/F) ratios. The batch size of these samples were typically small, in the order of 50 to 100 grams. The scale that I initially used to weigh out the constituents was a Pelouze digital (postal) scale, with a 2 kg. capacity. Even though this scale is very accurate, the problem is that it has a 2 gram resolution. A batch size of 100 grams, for a 65/35 O/F ratio, requires 35 grams of dextrose. Thus, at 2 gram resolution (i.e. the scale would indicate readings in 2 gram steps: 34, 36, etc.) this could lead to appreciable error, as well as variation in actual O/F ratio obtained, from batch to batch.
After giving this some thought, I soon realized that what I really needed was a balance scale. Since I wanted to measure ratios of one quantity with respect to another (oxidizer-to-fuel), rather than specific quantities, the function of the scale is greatly simplified. Calibration is not necessary with this balance scale.
Principle of the Balance Beam Scale
The scale that is presented here is an equal arm balance beam scale. In order to measure a certain mass of sample, weights having a total mass equal to the required sample mass is placed in the weights pan. A quantity of the sample is then slowly added to the sample pan until the arm is perfectly balanced, as indicated by the pointer. Just about any small identical (= mass) objects can be used as the weights. Perhaps the most convenient is coins. I used Canadian one-cent coins, of recent vintage, Those dated 1986-1997 are identical in mass, according to the RCM, but I limited the ones I used to 1990-1997 -- those with no significant wear. The approximate mass of this coin is 2.5 grams, with this value obtained by weighing 100 coins on the Pelouze scale. Note that it is not necessary to know the precise mass of the coin, but it is useful to know the approximate mass, to determine how many coins are required to obtain a sample of a certain mass.
For example, to obtain a sample of roughly 100 grams of 60/40 O/F propellant, 100/2.5 = 40 coins equivalent mass. To weigh out the oxidizer sample, use 0.6 x 40 = 24 coins, and 0.4 x 40= 16 coins to weight out the fuel sample. This results in an exact 60/40 (=24/16) O/F ratio.
Tiny washers may be used as fractional weights (e.g. 5 washers = 1 coin).
Resolution of this balance beam scale is potentially 1/5 gram, or better.
Front view of Beam-balance scale
Side view of Beam-balance scale
Basic dimensions (front view). Dimensions are in millimeters.Basic dimensions (side view). Dimensions are in millimeters.
The Support Structure and Base are made from 15 mm particleboard, fastened together with #8 x 50 mm particleboard screws .
The Sample Pan is a stainless steel bowl, of 1/2 litre capacity; the Weights Pan is a 250 ml "tin" can. Both are attached to the beam with fine wire tied to an electrical "eye" connector, secured to the beam with a #6 screw.
The Beam is formed steel C-channel, as shown below. This is made from a length of "stiffening channel" that is used as a component of a steel-stud framing for house walls (available at "Home Depot", et al).
At one end of the beam, a length of threaded rod is epoxied. This is to facilitate a number of nuts, used for fine balancing of the beam (aligning to the zero mark).
The beam rests on the balance blade. This is a length of utility knife (Olfa) blade, mounted with the cutting edge up, epoxied to the beam support. The sharp edge is slightly dulled with very fine (320 grit) emery paper, sufficient to make it safe to the touch. Two lengths of #10-32 threaded rod are used to "lock" the beam in position when the scale is not in use, by tightening the stop/lock nuts slightly. When in use, the nuts are backed off a few turns. These rods fit through 10 mm holes drilled in the beam.
The beam has vee notches filed into the beam flanges at the exact midpoint between the two pan attachment points. The completed beam assembly (beam, fine balance nuts, sample pan, weights pan) is placed on the blade and carefully balanced by adjusting the balance nuts.
The pointer is made from a length of steel wire, 1.3 mm (.050") in diameter. It is epoxied to the top surface of the beam exactly above the vee notches.
Last updated June. 21, 2001
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