Yesterday I purchased a vintage Mettler balance, circa 1961, if I estimate correctly from what little information I can find about them. It is the B5 model, which was first released in 1955, and manufactured through the 60s. Official Mettler service for them was discontinued in 1972, though it appears they had already stopped selling them by then. It measures to the .0001 gram, which is more precise than what I need right now. As long as it keeps working, I should never need to buy another balance!
After I got it home, I leveled out its feet, turned it on, and found that the display wasn’t aligned. A view of the misaligned display
Seeing this, I decided to open up the top (which pops right off, no screws necessary) and see if I could diagnose the problem with my limited knowledge of these balances and some logical reasoning.
Top view with the lid off. Annotated.
Balances like this have more in common with old timey double pan balances than with modern analytical balances. The heart of this balance is the white balance beam that runs down the middle. On the back end of it is a large counterweight, and on the front end is the pan and the set of dial weights. The counterweight equals the mass of the pan plus the combined mass of all of the dial weights.
When weighing an object, the object is placed in the pan, and the dials are turned to remove weights from the pan until equilibrium is reached. The set of weights covers through the tenths of a gram, and the last three decimal digits are read off of the display once the rough mass is dialed in. This is where it gets interesting.
The display is backlit by a lightbulb all the way in the back of the machine. Light from this bulb shines through a prism, which directs it through a lens set in the balance beam on which a scale is printed. The light is then directed through another prism and projected onto the screen at the front of the balance, so that the scale can be easily read. The slightest motion of the pan sends the scale flying. A moveable prism, adjusted by the knob on the side, is used to finely tune the balance, by setting the projected scale even with the printed zero line on the display when the pan is empty.
I figured out that one of the stationary prisms inside was slightly misaligned, which was why the display didn’t look right. One quick adjustment later and it was good to go.Reading of a 5 gram standard weight. 5.0000 grams, +/- .0001 g.
I’m still trying to figure out how exactly to read the last decimal digit, though since that’s way more precision than I need, I’m not going to bother with it. Having a balance that accurately measures masses in milligrams is already enough for me!