Measure mass with high precision at home? Can I measure mass of an object with household means, when the precision must be around the weight of 0,1ml of water?
Background:
As a person with a lot on my mind, and a diabetic, I occasionally find myself looking at an insulin pen and wondering, whether I already injected myself or was just going to.
Just the case for yesterday when I had toothache and had to take a new insulin pen from the fridge for the evening shot. This morning I find I have left the pen by the kitchen table. Did I inject myself or did I not? Obviously, if I didn't, my blood sugar would be rising, but there are additional circumstances why it might be not, or why it will start rising later. This was the basal insulin, which I should only inject once per day. Injecting it twice would be dangerous. Not injecting it at all would be less bad, but also detrimental.
The pen in question is a plastic device of around 16cm in length and the total weight of a full one is around 22g (as shown by kitchen scale). 
Now, since I had started a new pen, I thought I could compare the weight by seeing the difference of weighing a new unopened pen, which should have 36 units of insulin more. As the pen has 300units/ml and holds 1,5ml (for a total of 450 units), the difference of 36 units would be the difference of weight of 0,12ml of the fluid. 
Unfortunately, of course, the kitchen scales show only difference as large as gram, which is too much to account for this amount.
Is there anything I can do with household means (buying some extra items in a DIY store or similar is also OK), especially in circumstances when the pen in question must have had at most 1 use, and I can compare it to an unused one?
 A: There are many ways to create a balance of great sensitivity by mechanical methods, but the main issue for your application is ease of use, I think, and a sensitive mechanical balance would not be quick to use. Therefore I would certainly recommend buying an electronic balance with the required sensitivity. I believe the cost would not be too great. Other practical ideas such as taping an ordinary pen to the insulin pen might help, but I agree that the idea of weighing would offer a useful further indicator. 
(I also am apt to forget whether I have done something which I have to do each day. I understand that it is indeed part of the way our brain works that such things may go into a very short-term form of memory but simply are not put into longer-term memory at all.)
A: Build a simple beam balance: a stiff wire about a foot long, perhaps bent a little at the center for stability, supported in the middle by a piece of string hanging from something, two identical plastic lids hanging from the ends of the wire by three strings. Adjust the balance by a moving support positions or adding additional small weights.
