Are Brass weights obsolete? Should they be replaced by Steel weights?

[Bubba]

Once again, I feel the need to appeal to the knowledge base maintained within the cove to make up for my ignorance. My company is using a set of brass weights that are calibrated to NIST 105-1 Class F tolerances for the purpose of calibrating general purpose gram scales. We have a smaller ASTM Class 1 set that we use to calibrate our analytical balances.

One of the weights in the brass set has failed its scheduled calibration and needs to be replaced. I looked on the net and had a hard time finding replacement brass weights. I did, however, notice some commments from a weight manufacturer that basically said that brass weights were obsolete and should all be replaced.

Can some one clue me in to the reasons I should get rid of my brass weights and replace them with shiny steel ones? Is this a valid requirement, or just a way to get me to spend more money? I apologize for my ignorance, but I am certainly glad that I have a place like this to find the answers.

 

[Ken K]

Personally, if the brass weights are within tolerance there is no reason I would replace them. They may be hard to find, but what exactly would make them obsolete? Sounds like someone pawning his wares to me.

I find it interesting one weight is out of tolerance...even brass if it is stored in proper environments.

We still use brass weights for the majority of our calibrations. Had them for about 12 years and they are well within tolerance. We also have one 8 pound steel weight we recently bought. I prefer the brass...it feels lighter.

 

[Ryan Wilde]

Direct excerpt from NIST Handbook 105-1:

"Brass is no longer an acceptable material for weights: the metal is too soft for maintaining the required tolerance."

That said, I'd probably keep using my set and possibly shorten the calibration interval, and replace out of tolerance weights with shiny new steel ones

BTW, the NIST website has Handbook 105-1 online.


Ryan

 

[Jerry Eldred]

Although the answer has been covered, since I just received input on this, I'll post it...

My weight expert tells me (after he just returned from Christmas break) that he received an input from the state standards lab we use (government agency, considered a branch of NIST, and not a profit making entity) that brass weights may continue to be used as long as they remain in tolerance. However, when they do go out of tolerance, they should be discarded and replaced with stainless steel weights.

At first glance it does have the appearance of someone selling their wares. But with that input, apparently it is indeed true that brass weights as standards have been stated by NIST to not be desirable.

It does sound like in the case of the original poster, that he can continue using his brass set, and just replace the out of tolerance weight within the kit.

I know I'm being redundant. But after my weight person taking the time to research it, I wanted to post his input.

 

[Ken K]

I'll admit this is the first I have heard this but now I have a question...

Can someone explain to me why a "soft" material is no longer acceptable? Unless someone is using theirs for batting practice, I really cannot imagine the why behind this.

Does brass suddenly vanish without a trace? Help me out here... make me understand.

 

[Jerry Eldred]

I'm going to answer from somewhat a layman's perspective. Although I have about 20+ years in the industry..

My guess is that it is because we are talking about high precision (lab level, versus every day). Because this comes from NIST, I have to assume they have historical data to base this on. Remember that all the state labs around the country are considered a branch of NIST. Therefore, the data that NIST has on history of weights (in my opinion) is based on inputs from all of those labs as a minimum.

With that presupposition, there is probably something less stable about brass (as a softer metal) compared with stainless steel (harder). So for example (at a very high precision level) picking up and setting down brass weights over the course of years (still my guess), may cause enough abrasions on the weights to slowly allow them to drift out of tolerance. It is additionally possible that material properties allow them to a little more readily pick up contaminants which ingress their surface. Whereas with stainless steel being a harder material, may be more resistive to both of those.

When I recall that in handling high accuracy weights, you are supposed to handle them with cloth gloves, to prevent skin oils from getting on the weights, it seems a pretty reasonable possibility that differences in material properties between brass and stainless steel may well be enough to impact long term stability of the weights (due to abrasion, ingression, oxidation, etc.).

That's not an authoritative answer. But I believe is probably an educated guess. As I have some time I plan to look it up. I suspect there is a paper written (perhaps available at the NIST website) on this. If I can find it, or anything authoritative, I'll post it.

If anyone else here has authoritative details/data to post, please do so, as I'd like to understand better.

 

[Ryan Wilde]

Here's another layman answer:

Take a brass weight and rub it against a piece of paper. You will actually see the brass on the paper. It fares worse against metal, such as a scale tray.

Your technicians may be properly trained to never slide a weight and to never touch them with their bare hands. The problem is, for every company such as yours, there are 50 that use them improperly. Hence, NIST, who receives huge amounts of data from the state labs (who actually report directly to NIST on measurement data), found that brass weights were too sensitive to treatment for use in commerce.

You will find after several years of dealing with NIST that you have to take everything that they say and add the phrase "for legal trade". NIST is actually part of the Department of Commerce, and everything that they do boils down to making things "for legal trade". If you are calibrating scales that are used as final weight which determines price, you must follow their guidance. If weight is not the method that you use to determine price, take it as good practice, but within reason.

The bottom line is that no reputable weight manufacturer will sell you brass weights. But years of data that shows shift in weight (or the lack of) will tell you how long a particular weight should last, and when it is time to replace a weight, replace it with what is available - a steel weight.

Ryan

 

[Bubba]

I want to thank you all for your replies. The world makes sense again. I will do what has been suggested by replacing the offending weight with a stainless steel one while retaining the use of the "good" brass weights. Ya'll are wonderful!