A Matter of Balance: A Look at Analytical Balances.

Scottish Chemist Joseph Black (1728-1799)
Scottish Chemist Joseph Black (1728-1799)

An Introduction to Analytical Balances

An analytical balance is used in the laboratory as a precision instrument to determine weight of an object of 100g to within +/- 0.0001g.

Until the 1950s laboratories used two-pan balances to determine weight. The single-pan balance was used after the 1950s and modern laboratories use electronic balances today.

Weight is the force exerted upon an object by gravity while mass is the quantity of matter the object is composed of. Weight is different at different locations while mass stays invariant.

So while the mass of the object in grams is what is being measured, the term weight and weighing has become the terminology used.

Analytic balances use reference masses to be the substitute weights in two-pan and single-pan balances, and is used for calibration purposes with electronic and pan balances.

Analytical weights above one gram are made of brass and bronze and plated with either chromium or lacquer coating. These weights range from 1 gram to 100 grams.

Smaller analytical weights are called fractionals and are made of aluminum or platinum. These weights range from 500 mg to 5 mg.

The National Institute of Standards and Technology have two classifications for analytical weights. Class M is used for high precision and Class S is used primarily for calibration.

Two-Pan Balance
Two-Pan Balance

Two-Pan or Equal-Arm

Two-pan or equal-arm balances are frist class levers where a fulcrum lies between two arms of equal length. (I1 = I2) Pans are suspended from the arms.

The object being weighed or M1 is placed on the left pan while a known mass or M2 is placed on the right pan. Both M1 and M2 are attracted to the earth due to gravity.

The operator adjusts M2 until a pointer is facing the fulcrum. At this point M1 = M2.

The precision and accuracy of two-pan balances reached maximum efficiency when the Scottich Chemist Joseph Black (1728-1799) introduced three prism formed "knife-edges" where the fulcrum and two arms are placed.

Each of the prism formed "knife-edges" is made of hard yet brittle agate.

Two-Pan Source of Errors:

1.) I1 and I2 have to be equal in length. If one arm is 1/100,000 longer the measurement will be 1/100,000 off.

2.) An increasing load can bend the beam slightly over the knife edges leading to slight errors in measurement.


Single Pan Balance Diagram
Single Pan Balance Diagram

Single-Pan or Unequal-Arm

Also called a constant-load balance the single-arm balance has two instead of three knife edges with two arms of unequal length.

The smaller arm is the balance pan and has a full complement of weights suspended. The longer arm holds a constant counter weight with a damping device built into the beam.

When the object is placed on the pan, the suspended weights are removed from the shorter arm. This type of weighing is called weighing by substitution and leaves a constant load on the beam.

When the sum of the weights removed is 0.1 g the weight of the object , the beam is released.

A reticle, or a scale etched on glass, displays its reading onto a reading display and the weight of the object would be taken.

Single-pan balances use a tare device, where the weight of the container can be removed from the weight of the object being weighed, by subtracting the weight of the container from the total weight.

Electronic balances
Electronic balances

Electronic

When a current is passed through a wire that is placed between the two poles of a permanent magnet, a force is generated. This system is called an electromagnetic servo system.

In an electronic balance this force is used to move a wire outside the magnetic air gap and generate a reading used to formulate weight.

When the force of gravity of an object is coupled mechanically to the servo motor an opposing magnetic force is generated.

A null-indicator checks the position of the wire in the magnetic field. This indicator could be optical, a vane attached to the beam, a small lamp, or a photo detector.

When the force of the object is at equilibrium of that generated by the opposing magnetic force an "error" indicator moves to the reference position.

When the beam is displaced the rapid change in current through the coil the "error" signal generates a correction current. This correction current is measured and is equal to the mass of the object.

Electronic balances need to be calibrated with known masses prior to use.

Errors in Weighing

Three ways to avoid error in your measurements are:

1. All samples that can take up water should be covered for the measurement.

2. All glass vessells need to be extremely dry prior to measurement.

3. Make sure that the object being measured is the same temperature as the balance.


More by this Author


Comments 10 comments

whonunuwho profile image

whonunuwho 3 years ago from United States

jhamann. you are a brilliant individual. I suspect Leonardo, Edison, and Einstein, may have all been made from the same mold as you, my friend. Keep writing these wonderful knowledge filled works for us all to digest. whonu


jhamann profile image

jhamann 3 years ago from Reno NV Author

Thank you whonunuwho for such a great compliment, I appreciate your feedback and hope to have more for you to read. Jamie


Mhatter99 profile image

Mhatter99 3 years ago from San Francisco

Fascinating. Thank you for this.


jhamann profile image

jhamann 3 years ago from Reno NV Author

Thank you Martin, I hope all is well. Jamie


Eiddwen profile image

Eiddwen 3 years ago from Wales

So very interesting indeed;not a subject I usually read but I found it very interesting.

Have a great weekend.

Eddy.


jhamann profile image

jhamann 3 years ago from Reno NV Author

Thank you Eddy. Jamie


midget38 profile image

midget38 3 years ago from Singapore

Very interesting,Jhamann. Now we know how they took accurate measurements before the advent of the weighing machines that we know today! Thanks for sharing!


jhamann profile image

jhamann 3 years ago from Reno NV Author

I am glad you found this interesting I was afraid this topic may have been a little too bland. Jamie


GusTheRedneck profile image

GusTheRedneck 3 years ago from USA

Howdy Jamie (jhamann) - I enjoyed your fine article about balances. In between school semesters one summer (1949) I worked at "The American Balance Corporation." Didn't know a thing about making balances when they hired me. They taught me all sorts of precision machine shop stuff, some of which I can still remember even now. One interesting thing about production of those analytical balances, Jamie, was that the parts made for one of them fit only that one and not any of the others. We calibrated the little weights against a costly set that was certified by the Bureau of Standards (as I recall). For slightly overweight weights, we carefully removed some of their metal by rubbing them gently with something "smooth," like maybe a piece of paper. The lightweights were brought up to weight by unscrewing the little "handles" and dropping in a tiny morsel of whatever-it-was, but it was surely not much at all.

An interesting summer, that one.

Gus :-)))


jhamann profile image

jhamann 3 years ago from Reno NV Author

Gus-It makes my day to know that my little hub on balances could bring back such a vivid memory. I have a lot of respect for the process of manufacturing precision equipment, and I respect the fact that you played a role in this production. Thank you for sharing your memories and stopping by to read my hub. Hope you have a good one. Jamie

    Sign in or sign up and post using a HubPages Network account.

    0 of 8192 characters used
    Post Comment

    No HTML is allowed in comments, but URLs will be hyperlinked. Comments are not for promoting your articles or other sites.


    Click to Rate This Article
    working