Are balances better than scales?

[NeoPTLD]

So today, real "balances" are far and few in between with one of the few exceptions being the beam balance used at doctors' office. They've been almost universally replaced by scales.

Scales don't use any reference masses, so they have to be frequently calibrated with masses. They also have issues with sensitivity being non-linear across the span.

Even with balances, there was an issue with differences in load affecting fulcrum friction, but someone came up with this awesome idea in mid 20th century that keeps the fulcrum at a constant load the whole time.

It looks something like this:
diagram

If the capacity is 200g, then there is 199grams of mass(made of a bunch of little masses) sitting above the weighing hanger.
If you're weighing 149.2522g, you put the beam on a heavy duty fulcrum, then turn the dials to remove about 149g of weight until it becomes about level.

At this point, you put the beam on the delicate fine weighing fulcrum, then turn the fine tuner (like moving the finger on a triple beam) until it becomes level, then you read it out. Add it to 149 and you get 149.2522g.
If you were measuring 49.2522g, then you remove the 100g weight from the top, but the fulcrum won't see the difference, since it's always under "full load".

These things never need calibration and the response is linear, as long as the "reference masses" are very accurate.

So, aren't these things better than an electronic one that tries to measure from 0.1mg to 150,000mg (1 to 1.5 million ratio) and needs constant calibration?

 

[CycloWizard]

How do you define "better?" In a modern lab, things get weighed out all the time. If we were still doing the weighing manually, as with the device you suggest, then we wouldn't get anything done. Thus, every lab uses electronic balances. The nice thing about newer electronic balances is that the electronics are sophisticated enough that they can easily deal with nonlinearities and any other undesirable feature. These things are simply treated in software. Thus, while the design you posted is possibly technically superior, it's just not practical. Electronic balances achieve a similar degree of accuracy without the hassle. I can plop something down on an electronic balance in my lab and know its weight to within 10 micrograms in about five seconds - that's good enough for me.

 

[Paperdoc]

I agree that modern electronic balances are at least as good as the older beam balances, probably better. The largest source of malfunction in the constant-load beam balance was the knife edges that support the beam. The sharp edge, and the plate on which it sits, wear over time and become points where the movement of the beam is impeded. The effect is subtle at first and hard to detect, but becomes more obvious as the damage gets worse. The only solution is replacement of these parts, which requires a really good balance technician. Of course, that's no different from the technical prowess needed to diagnose and repair electronic balances.

The constant-load beam balances I've worked with did not have any way to adjust linearity. For all digits but the last three, they depended entirely on super-accurate weights built into the unit. Realistically, those weights only could be changed by accumulation of dust and / or finger oils, and that was not too hard to avoid. Infrequent proper cleaning can reduce that problem. The last three digits often were provided by a moving indicator system attached to the beam. The beam did NOT remain at a constant position, but swung though a small angle which was supposed to be linearly related to the difference from the nearest fixed weight. Those scales, often projected on a screen, had an adjustment for span from 0.0 to 100.0 to represent one unit of the next-most-significant digit. But within that span there was no adjustment for linearity.

A lot of modern electronic weighing devices are based on measuring magnetic field intensity - well, actually, current to produce the field. They really are "balances", though. The classic constant-load beam balance compared the weight of the sample with a set of fixed weights within the balance by putting them on opposite sides of a moving beam and adjusting the fixed weights to return the beam to its initial position. In many current electronic balances, a magnetic field is generated to oppose the force of gravity and return the balance pan (including sample weight) to its original position, and we really do have a balance of electromagnetic force against gravitational force. The current necessary to achieve this balance is quantified and the massaged as necessary digitally to establish zero, span, and linearity throughout the entire range. The accuracies and precision of current regulation, measurement and digitization available now are better than the old beam balances, and the calibration capabilities through software are quite impressive!

 

[NeoPTLD]

Originally posted by: CycloWizard
How do you define "better?" In a modern lab, things get weighed out all the time. If we were still doing the weighing manually, as with the device you suggest, then we wouldn't get anything done. Thus, every lab uses electronic balances.

The nice thing about newer electronic balances is that the electronics are sophisticated enough that they can easily deal with nonlinearities and any other undesirable feature. These things are simply treated in software. Thus, while the design you posted is possibly technically superior, it's just not practical. Electronic balances achieve a similar degree of accuracy without the hassle. I can plop something down on an electronic balance in my lab and know its weight to within 10 micrograms in about five seconds - that's good enough for me.[/quote]
The mechanical balances came in anywhere from 1 to 100 microgram resolution and if you know what you're doing, measuring takes less than 30 seconds.

Better as in always providing repeatable result and holding accuracy. The hang-down design was hardly affected by corner load, unlike today's top loading design.

Looks like this

Originally posted by: Paperdoc
A lot of modern electronic weighing devices are based on measuring magnetic field intensity - well, actually, current to produce the field. They really are "balances", though. The classic constant-load beam balance compared the weight of the sample with a set of fixed weights within the balance by putting them on opposite sides of a moving beam and adjusting the fixed weights to return the beam to its initial position. In many current electronic balances, a magnetic field is generated to oppose the force of gravity and return the balance pan (including sample weight) to its original position, and we really do have a balance of electromagnetic force against gravitational force.

Ok, but just like the bathroom scale, it has a defined internal parameter which defines how much force corresponds to how much mass, so it's not actually comparing physical mass against physical mass. They both work on the same principle that xx amount of electrical signal corresponds to a known mass, which must be corrected frequently. Also, when the density of air changes due to barometric pressure difference, with the old machines, there was a corresponding change in buoyancy in the masses used inside, so it didn't need a correction. Load cell based electronic scales don't use any masses to weigh, so they go out of calibration and needs correction time after time.


There was a company that used torsion beam balance principle to get around the fragile knife edge problem though.

The current necessary to achieve this balance is quantified and the massaged as necessary digitally to establish zero, span, and linearity throughout the entire range. The accuracies and precision of current regulation, measurement and digitization available now are better than the old beam balances, and the calibration capabilities through software are quite impressive!

The software simply performs frequent calibration, using actual weights of known mass and software simply performs the process using a motorized device to place the mass on the sensor. They don't have a knife edge, but they have many flexible bearings and shims inside. Although, they're probably cheaper to produce than making mechanical ones now a days.

 

[Gibsons]

Originally posted by: NeoPTLD
The mechanical balances came in anywhere from 1 to 100 microgram resolution and if you know what you're doing, measuring takes less than 30 seconds.

Better as in always providing repeatable result and holding accuracy. The hang-down design was hardly affected by corner load, unlike today's top loading design.

Looks like this

I've used those things many times and hated them. Tareing (sp?) them was a real pain. don't miss 'em.

 

[CycloWizard]

Originally posted by: NeoPTLD
The mechanical balances came in anywhere from 1 to 100 microgram resolution and if you know what you're doing, measuring takes less than 30 seconds.

I've used electronic balances with this resolution as well. They're not in most labs because they usually need to be mounted on marble/granite tables. Mechanical balances at this level of precision probably should be, too.

Better as in always providing repeatable result and holding accuracy. The hang-down design was hardly affected by corner load, unlike today's top loading design.

How much accuracy do you really need? If you're relying on the accuracy of the last digit or two, then you're probably not using an appropriate technique to measure what you're trying to measure. It's always a little tricky separating meaningful results from noise at the extreme low end of the dynamic range.

 

[NeoPTLD]

It should at least within 10d accuracy wise, but precise to 5d and not require a 3 hour warm up and a calibration before each use.

I think I want a mechanical one

 

[Pulsar]

Hmmm.

We use electronic scales for that level of accuracy. Ours are certified once a quarter with known weights, calibrated one a day. Mounted on granite, right next to our Zeiss CMM, and Hommel surface finish gages (also on granite).

The room is temperature controlled through in-floor/ceiling/wall electrical elements because air currents cause us all sort of grief.

I trust the shunt-calibration feature of the scale along with the software drift prediction (based on how long it's been since the last certification + a known drift established over the last 6 months of use) as much as I trust springs, balances, bearings and friction.

 

[NeoPTLD]

It will require calibration with barometric pressure change too. A real balance is "apparent mass vs. 8.0g/cm^3" so when the air pressure changes, the buoyant force changes proportionally on both sides.

An electronic scale does not inherently compensate for buoyancy change caused by humidity or barometric pressure change. It will think that the apparent mass of sample is increasing with increasing air density due to increase in buoyancy on the sample, but not an equal increase in counter-force.