Scales and Distance
I always suspected counting and therefore mathematics started when Ugha the caveman asserted he had more rocks than cave neighbor, Effua. As long as the question remains how many rocks, homes or TEM grids one has, simple counting suffices. It’s only when we want to measure objects (my rocks are bigger than your rocks) do we run into problems.
The yard is a sub-division of the rod, which was based on the total length of the left feet of 16 God-fearing Englishmen. How do we know they were God-fearing men? Easy, they were the first 16 to run out of church one English Sunday.
I’d like to remind the metric crowd that there are a few meter stories out there too. The first attempt to define a meter was the length equal to one ten-millionth of the distance at sea level from the pole to the equator. Not the easiest to use and they scrapped out this definition. For a long time the meter was the distance between to scratches on a platium bar. TWO SCRATCHES?!? I’m measuring microscopic particles based on two scratches!?!
For a while the meter was defined as the length equal to 1650763.73 wavelengths of the light from the 2p10 and 5d5 electron orbital transition of the krypton 86 atom in a vacuum. This still wasn’t good enough, so the physics community is considering a meter in terms of the speed of light in a vacuum. The value for the speed of light, (299,792,458 meters per second) is the current ruler. We will not go into what is a second.
Ultimately we need to calibrate our measing devices, from stereomicroscopes to STEM if we wish to compare results. So what do microscopists use to calibrate their TEMs? It was pointed out to me that most TEM aren’t calibrated, but the imaging system should be. I remember a TEM being installed 30 years ago in which the magnification read out was calibrated to match the magnification on the film. Prints were made by setting the enlarger to a specific magnification as determined with a negative of a grating replicant. I’m concerned about the calibration of my imaging system because it allows me to do manual measurements.
So what do microscopists use? My informal survey is based on 24 responses:
63% (14 people) use a grating replica,
68% (15 people) use crystal spacing.
This gives me 131%, due to microscopists who use both gratings and lattice spacings at different magnifications. It seems a sensible approach.
Of the crystal spacing gang:
47% Silicon based crystal,
7% MoO3,
7% Graphite,
27% Catalase crystals,
7% Tobacco Mosaic Virus (possibly the most unique!),
7% unspecified.
Again I exceed 100% due to utilizing several different crystals as a function of magnification.
26% of the respondents indicated they use a commercial product called Mag*I*Cal. This is an ion milled silicon crystal orientated in the 111 direction. The spacings it produces are used to calibrate your scope. Only one person or 7% mentioned it or any other product was NIST traceable. It seem to be the only one on the market.
I agree that many standards are “God-traceable” and have well documented constant features in the scientific literature. Their validity in a court of law is another question. Still most of the people who use these type of standards can recognize a 111 from a 011 face. I can’t, so I use a grating replica.
No matter what, every calibration is still dependent on deciding where a specific feature starts and ends. Seems we’re still using two scratches as a standard.
The yard is a sub-division of the rod, which was based on the total length of the left feet of 16 God-fearing Englishmen. How do we know they were God-fearing men? Easy, they were the first 16 to run out of church one English Sunday.
I’d like to remind the metric crowd that there are a few meter stories out there too. The first attempt to define a meter was the length equal to one ten-millionth of the distance at sea level from the pole to the equator. Not the easiest to use and they scrapped out this definition. For a long time the meter was the distance between to scratches on a platium bar. TWO SCRATCHES?!? I’m measuring microscopic particles based on two scratches!?!
For a while the meter was defined as the length equal to 1650763.73 wavelengths of the light from the 2p10 and 5d5 electron orbital transition of the krypton 86 atom in a vacuum. This still wasn’t good enough, so the physics community is considering a meter in terms of the speed of light in a vacuum. The value for the speed of light, (299,792,458 meters per second) is the current ruler. We will not go into what is a second.
Ultimately we need to calibrate our measing devices, from stereomicroscopes to STEM if we wish to compare results. So what do microscopists use to calibrate their TEMs? It was pointed out to me that most TEM aren’t calibrated, but the imaging system should be. I remember a TEM being installed 30 years ago in which the magnification read out was calibrated to match the magnification on the film. Prints were made by setting the enlarger to a specific magnification as determined with a negative of a grating replicant. I’m concerned about the calibration of my imaging system because it allows me to do manual measurements.
So what do microscopists use? My informal survey is based on 24 responses:
63% (14 people) use a grating replica,
68% (15 people) use crystal spacing.
This gives me 131%, due to microscopists who use both gratings and lattice spacings at different magnifications. It seems a sensible approach.
Of the crystal spacing gang:
47% Silicon based crystal,
7% MoO3,
7% Graphite,
27% Catalase crystals,
7% Tobacco Mosaic Virus (possibly the most unique!),
7% unspecified.
Again I exceed 100% due to utilizing several different crystals as a function of magnification.
26% of the respondents indicated they use a commercial product called Mag*I*Cal. This is an ion milled silicon crystal orientated in the 111 direction. The spacings it produces are used to calibrate your scope. Only one person or 7% mentioned it or any other product was NIST traceable. It seem to be the only one on the market.
I agree that many standards are “God-traceable” and have well documented constant features in the scientific literature. Their validity in a court of law is another question. Still most of the people who use these type of standards can recognize a 111 from a 011 face. I can’t, so I use a grating replica.
No matter what, every calibration is still dependent on deciding where a specific feature starts and ends. Seems we’re still using two scratches as a standard.
1 Comments:
Amusing and informative blog...just for the record, I use replica gratings for lower magnifications. They are effective until about 70,000.
From 100,000 up I use asbestos line spacing. This is very good and consistent and fairly easily done. I do not use catalase and have tried but do not like TMV because they are biological samples subjust to more orientation and shrinkage problems than crystal line spacing.
The problem comes with the magnifications from about 70 to 100K. There is really no good calibration standard for these areas. We use a combination of line grating and catalase or TMV but none are really satisfactory. So we try to avoid these magnifications if accurate calibration is really needed.
Debby Sherman
Purdue University
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