deskthority

If you lack a scale, I might have something better than the nickel method, particularly for places that don't have nickels.

What is easy to get, easy to measure, and has a standard, known density?

Water.

Using a very thin plastic cup (the kind used to hold sauces when you get take-out) and a bit of foam for a spacer, put the foam on a key and the cup on the foam. Slowly and carefully add water until the mechanism starts to "give". At this point you'll want to be rather careful not to splash when the key goes down, but that's why I used a cup designed to hold 100 mL. I ended up putting in 55 mL. Alternatively, if you're worried about pouring over a keyboard, put in enough water to buckle the mechanism, then use an eyedropper or something like that to take water out. If you're a real klutz, maybe use a small ziplock plastic bag instead of a cup so you can't spill.

Afterward, measure the water however is convenient. I used a marked cup that came with a bottle of cough syrup. Then, since you're mucking about with an unknown mass in your cup and your spacer already, just assume the density of the water is 1 g per 1 mL. It will be slightly less at any temperature that's comfortable for performing the test, but not enough to be worth worrying about.

You'd need to weigh the cup and key cap as well. Also, it's probably best to pipette it in drop by drop at some point; the force of impacts can and will skew the results. Also also, try to reseat the weight on top of the switch after it's been activated. Do this as gently as possible, to eliminate errors.

I myself use a variety of coins and weights to stack on the switch, which I then weigh afterwards on one of the analytical balances in the lab (+/- 0.0001 gram).

Chyros wrote: You'd need to weigh the cup and key cap as well.

This is a method for the people who do not have a scale that can weigh in this range at all, or not precisely enough to give a meaningful answer. The cup and foam I used are far less than 5 grams combined. The only reasonably appropriate scale I have only handles one gram. I don't think that's going to help. I have a much more precise one that only goes to 130 mg with a precision of 0.5 mg. That's really not going to help here.

Fair enough, I just wanted to point out that ALL the weigh on the switch, including the key cap, should be taken into account when measuring weights. Even if it's not a lot, it only introduces extra errors that really shouldn't have to be there .