How do scales that are capable of measuring incredibly tiny amounts of things actually work?
In: Technology
In the kitchen… Scales use sensors that change when pressure is applied: **Piezoelectric** Effect is the ability of certain materials to generate an electric charge in response to applied mechanical stress. The word **Piezoelectric** is derived from the Greek piezein, which means to squeeze or press, and piezo, which is Greek for “push”.
There are two sensors across a metal bridge that read ZERO when at rest, via electricity.
When something is put on the bridge, it bends. That produces a change in that electricity.
A computer calculates the number, via programming.
Generally, small pieces of metal that bends to the weight of an object on top, called load cells, are inside.
This load cells have small “bending sensors” stick to their sides, those are called “strain gauges”
A special “amplifier” chip is commonly used to read the signal of an arrangement of those sensors and give a value to those tiny amounts of deformation of the load cell.
Then a small “computer” chip can make a relation between the deformation and the corresponding weight, even taking many measurements, and finally display the information on a screen
I believe that most scales contain a piece of metal with a precise thickness/strength (called a load cell). Placing an item on the scale bends that piece of metal *very* slightly. By measuring how much the metal bends, you can figure out how much the item weighs. *How* that measurement is made is a little beyond ELI5, but it’s done electrically by passing voltage through a set of resistors attached to the metal whose value changes when they’re bent. By precisely measuring the voltage changes across those resistors, you can measure the distortion of the metal and thus calculate the weight.
To measure smaller things, you need a smaller load cell (that will bend more from less weight) and very precise calibrated electronics capable of accurately detecting equally tiny changes.
Using a mass spectrometer, you could measure things as small as atoms. Basically it looks like a slide with all of the tiny things sitting up top. Then they are all pushed with an equal force. The heavier items will take longer to get to the bottom while the lighter items will get there faster. Using a little math, after all of the atoms get to the end of the slide, you can measure the mass of the objects.
Besides the well known method of using strain gauges there is also the method called EMFR (Electro Magnetic Force Restoration) which is especially used in analytical scales to weigh really tiny amounts of material. Basically this method consists of a lever arm on which the item is placed and consequently deflects the lever arm. Then the current flowing through a coil is controlled very delicately to restore the equilibrium of the lever arm. The amount of current is used to deduce the weight. The design of an EMFR weigh cell is really ingenious when you see it the first time.
All the other answers are great; they describe practical scales that measure in the sub milligrams. But to give you something even deeper, some specialized scales can actually go to higher resolutions, measuring picograms, and more recently down to the yoctogram resolution (10^(-24) grams) which is basically the mass of a single proton.
They essentially work using a very basic law in physics. They have an oscillating beam (for the yg range, they use nanotubes for beams). Without getting too much into detail, if something is oscillating, it has a given natural frequency (a rate at which if you move it at, it will produce maximum deflection or amplitude). When you apply a force, no matter how tiny, to this oscillating structure, its natural frequency will change. This sounds insane to measure but it’s not, because measuring frequency is like counting, and we have very advanced technology for this (think atomic force microscopy). So now, if you put a mass on this oscillating beam, and you measure how much it’s natural frequency changed, you can calculate the mass of the object you added. This actually measures the inertial mass, but let’s not get into that.
Of course scales in your everyday scientific lab or kitchen or gold shop don’t use this technique, they use what other comments mentioned like magnetic methods.
Edit: English
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You know how a ruler works? Like that, but smaller.
When measuring incredibly tiny things, we use balances to find the mass. Modern day balances use magnetic force restoration. First you zero the balance. This tells the balance where the plate is without any additional mass added. You put your item you’re measuring on the plate. The balance then calculates how much mass would be required to return the plate to its original position using electromagnets. This allows us to measure very minute displacements of the plate. Meaning we can measure very tiny amounts of things 🙂
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