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Fairbanks Scales, Ohaus, balance, weighing, poise, calibration

Platform Scales

Weighing Devices in the Shop

By Jock Dempsey

Fairbanks-Morse Platform scale In our shops we have myrid measuring devices. We measure distances large and small, pressure, weight and torque. Some shops have few measuring tools and others have many. Shops that do a lot of shipping will have at least one or more scales for shipping weight. A few may have scales for weighing materials to sell.

There are several basic types of scales. The equal balance like the "scales of Justice" or laboratory analytical scale. The steelyard, a hanging counter balance scale with calibrated beam and a hook for counterpoise. The proportional balance with sliding counterpoise such as the laboratory tripple beam scale. The mechanical platfom scale, an extension of the steelyard that is self supporting and convienient to use. The spring scale in either hanging or platform type like bathroom scales. The modern strain gauge based digital scale. Digital scales with their clearly read digits are rapidly replacing all other types in modern commerce.

Scales and standard weights are one of mankind's oldest mechanical inventions. First used in trade then in science and engineering they have a long history of mechanical advances. While scales steadily improved the units used by them have been varied and global standards slow to be adopted. In the US we primarily use pounds out side of scientific circles while the rest of the world is primarily metric. But in the area of weight there are still many local hold outs world wide. In trade the price of gold is still announced world wide in troy ounces (1/14th pound avoirdupois), an ancient unit originaly based on the weight of grains of wheat.

The Ancient Greeks understood the relationship between the cross sectional area of a taught string, the tension applied and the resulting frequency of vibration. If you doubled the tension you doubled the frequency of vibration. If you doubled the cross section of the string (and thus its weight) you halfed the frequency of vibration. Thus you could design a musical instrument with the optimum tension on each string by adjusting the string diameter. Later, during the steam era, pounds were converted to horsepower (Power = mass * distance * time) and then kilograms the same way. All this starting with two pans, some grains of wheat and something to trade.

In this article we are dealing primarily with the modern mechanical platform scale. We are using pounds and kilograms depending on the unit the specific scale was designed for.

In my shop I've managed to collect a number of weighing devices. They include.

0 - 310g by .010g Ohaus quadruple beam school and laboratory scale.
0 - 1 kg or 2 lb. by .002 lb. Digital scale
0 - 20 kg or 45 lb by 1g or .01 lb. Ohaus HD Solution Scale (dual units, with 2kg/5lb tare weight)
0 - 300 lb. by 1/4 oz. ( 1/64 lb. 6.8g ) Archaic antique Fairbanks counter top scale (0 - 3 lb beam)
0 - 500 lb. by 1/4 lb. Standard Fairbanks Platform scale (needing parts and repairs)
0 - 1,000 lb by 1/2 lb. Standard 1124 Fairbanks Platform scale.
0 - 20,000 lb. by 5 lbs. Dial crane scale.

The important fact about measuring devices used to measure force, ie. weight and pressure, is that the range or maximum capacity determines the the precision or smallest increment of measurement. The 10 Ton crane scale above can only be read in 5 pound increments at best. The 310 gram laboratory scale can be read in .010 grams (a fraction of the weight of a postage stamp). The 2 pound digital scale is only good down to .002 pounds or .005 kg. (1/2 gram).

The reason for the proportionality of units to capacity is due to a combination of the physics of materials and rules of measuration (based on numeration logic). A discussion of this is beyond the scope of this article. However, some scales are built to be more sensitive than others. The Ohaus laboratory scale is a quadruple beam scale with 1/31,000 sensitivity range. The Antique Fairbanks is 1/19,200 while common scales are only 1/2,000 (smallest unit vs. total capacity).

So you can't measure tons and grams with the same capacity scale. And the collary to that is you need the correct range scale to weigh anything with precision. In general you want a scale that you are using in about the middle to upper half of its range.

antique fairbanks scale The 1000 pound and 500 pound capacity scales have identical workings except for the final ratio and beam calibration. They are suitable for most common weighing tasks such as yourself, packages to be shipped by freight, bulk materials to buy or sell (other than high value substances such as gold or silver).

I bought this antique fairbanks platform scale for our forge burner testing project. This scale has an archaic beam calibrated to only 3 pounds with ounce markings down to 1/4 ounce or 1/64th pound (.015 lbs.). Most scales of this type have 5 pound or greater beams and measure in fractional parts of a pound using fractional pound poise. The three pound beam scale uses ounce and fractional pound poise.

This old scale is the optimum for our measuring forge fuel consumption (the common way to measure propane) from 30 and 100 pound bottles. Typical fuel usage is 1 to 3 pounds an hour with our small test forge in the middle at about 1.5 to 2 pounds per hour when manually operated for efficiency. We will be testing steady state which will likely be higher.

We will be able to put 20 to 100 pound propane bottles on this scale and measure flow in .015 lb/hr increments (in one hour tests). If we doubled the time the precision would double but at a cost of time (labor) and fuel. Before we use this scale we will need to calibrate it and check the sensitivity. In this case absolute accuracy is not important but sensitivity is critical.

How They Work

Simple scales have equal length arms and balance or zero with equal weights on each side. Proportional scales have unequal length arms so that smaller poises can balance the object being weighed. The rules of simple levers apply. Sliding poise

The real ingenuity in Fairbanks and other platform scales is how the platform is evenly supported by levers. Beyound that a simple lever system produces a multiplier so that a small poise or counterweight can balance a much larger mass accurately. Multipliers vary from 40:1, 50:1, 80:1, 100:1 up to over 1,000:1.

In the base of the scale the platform is supported by levers connected together by yokes. All the pivot points are what are known as "knife edge". In actuallity they are just very narrow edges. These ride in round holes so they cannot escape AND the slight "hill" toward the middle of the circle locate the knife edge. The location of the knife edges is important as these determine the length of each lever and the ratio of the levers. The last yoke in the base of the platform scale pulls down on a slender rod that applys load to the beam in the top of the scale.

The beam in the top of the scale is also supported on knife edges. On the beam there is an adjustable counter weight that balances the beam itself as well as its attachments. This counder weight is supported on a screw so that it can be moved to "zero" the scale when the sliding poise is at the zero marking on the beam.

Ohaus HD Heavy Duty Solution Scale - dual unit
Out of Production Ohaus HD Solution Scale with Dual English Metric Weights and Calibration

Each scale has its peculiarities and after a while you think you know what you are doing.

I recently bought the Ohaus 20Kg/45lb HD solution scale above. I had been looking for one for a long time and this was the first in my price range AND it looked to be in good condition (it is). This is a unique scale that Ohaus does not list anymore. It has both gram and pound calibration and weights. I had seen the Ohaus scale with the double set of weights but did not know they were dual Unit.

When I got it unpacked and setup the first thing I did was try to weigh one of the scale poises (weights). These are handy and usually very precise. It was late at night and not very well lit in the shop. But this should have been easy. Put the 2kg weight on the pan and move the slider to 2kg. Nothing worked close to the poise value. I fiddled with the scale and tried again then tried another weight. . . no good. So I went to bed frustrated.

In the middle of the night it dawned on me that unlike my Fairbanks platform scales with poises marked 1 lb./100 and 2 lb./200 which indicates the weight of the poise and the 100x value when hung on the poise hanger. Some scales have different multipliers but they are usually something fairly even, 10, 50, 100, 1000.. . The oddest I've seen are 30x and 40x.

However, on Ohaus double and triple beam scales the poise is a multiplier value plus/minus some amount that compensates for components of the scale. So, in the morning I grabbed one of my Fairbanks weights, plopped it on the Ohaus pan, slid the slider to 1 pound and viola' - perfect results (+/- .01 lb). My new toy worked as advertised.

The Ohaus solution scale is one of their professional scales. They were designed for accurate wide range weighing for small batch chemical compounding and production. The weighing platform and slide bars are stainless and the paint acid resistant. I bought it for its designed use and it filled a gap in our precision weighing capability.


Calibrating Shop Scales

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