Weights and Scales
We use scales all the time collection and trading meteorites, but they can be a little tricky.
Pictured is one of the HOS branded low cost scales with a calibration weight showing it to be true.
I was recently asked a question about the weights on Certificates of Authenticity being off a fraction of a gram. It was a really good question that I bet a lot of folks have so I wanted to share my answer here also.
This is pretty normal actually.
First is the scale. There are different qualities of scales, and most that we use are at the consumer level. Basically if it is "affordable" it is in this zone. Scientific scales don't have a lot of the problems I am going to discuss here, but are also much more expensive and are for research quality measurements.
So for most of our scales the last digit in a scale is to be taken with a grain of salt. This is for two reasons. First is that it is more or less rounding the imaginary next digit up or down. Second is that scales can very easily lose accuracy on the last digit and often need to be recalibrated. A quick temperature change, overloading it, or a bump can all cause a slight error on the tail end. Low batteries are also a BIG reason scales start to wander. If you have a scale be sure to find the calibration instructions and get the proper calibration weights to keep it tuned.
If you are talking very fine readings, like the WIW game, little details can cause fluctuation. At that sensitivity leaning on the table, uneven surfaces, or a breeze can mess it up too. A lot of scales have trouble at the "baseline" too. Like I know my WIW scale likes to add .003 on very light items but has no problem at like 20g. So I like to double check my measurement on small stuff by adding the 20g weight also to see if they match. I end up hitting "tare" and recalibrating a lot to make sure I get it as accurate as I can. Another thing to note is that placement on the tray can give some variation, meaning dead center vs edge may provide different readings.
Not having to do with the scale, here is a common reason for a small difference- the relative air humidity when and where it was weighed. I noticed it from doing the game. That is why I now put all specimens for the game in a dehydrator for a while before weighing so that it is at an accurate baseline, but I live near the coast so if it is hanging out in open air for a while after it will pick up some weight. Pretty crazy right?
At the collector level we use weight to help determine price but the most important thing is that it is close enough to tie it to the COA thus keeping the chain of provenance.
Summary or things that can alter the measurement:
-Temperature change of the scale itself.
-Uneven surface the scale is on
-Physical shock to the scale
-Drifting over time
-Low batteries
-Air flow
-Not centering the object being weighed
-Moisture content of the object being measured
Here are some more insights provided from other members of the meteorite community:
Pat Brown offers: Good write-up on scale uncertainty. One more factor not mentioned in your excellent discussion is the accuracy of the calibration weight. The low cost scales that we use on meteorites often come with a calibration weight of unknown accuracy class. Attached are a couple of documents from Troemner, a top manufacturer of calibration weights of many different accuracy classes: https://www.troemner.com/calibration-weight-guide
David "DrSolo" Peri suggests: Sometimes putting a tray on three scale before turning the scale on can provide another way to perform a secondary way of double or triple checking accuracy of weight.
once you turn the scale on, remove the tray it will provide you the - weight of the tray replacing the tray should return the weight to 00.000.
avoiding the TARE button all together. your scale may read different it both processes.
identifying a need to change batteries of the need of calibration.
Sean Mahoney says: One good reason to calibrate before quoting the weight of something and especially before writing the CoA. Calibration takes seconds. I do it multiple times every day when my scales are very active, like at trade shows or when doing multiple listings. Very affordable M1 calibration weight are accurate to +/-50mg in 1kg. Cheaper M2 weights are also fine being accurate to +/-160mg in 1kg. I am constantly shocked by how many people never calibrate their scales. How on earth do they know they are accurate? When you regularly calibrate you quickly learn how quickly scales can loose significant accuracy.
Michael Farmer adds: Every scale will weigh a tad different, everything affects accuracy. Altitude, temperature, even humidity
Jeroen Zuiderwijk give us this interesting insight: Location on earth matters, as earths gravity varies depending on where you are on earth. This can add a difference up to 0.7%. Another reason to calibrate scales, particularly if you travel around with them.
Pictures:
1. I have had these branded scales for a while but wanted to be sure they held up over time. For the price I have been impressed and will start selling/giving-away them soon.
2. Here is a low weight item and the process I use to double check its accuracy knowing that this particular scale can be tricky.
3. Here I have removed the dish, hit “tare”, and added a 20g weight with the specimen.
4. Finally I added the dish back on, hit “tare”, and put two 20g weights along with the specimen. Looks good!