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Bob Thompson, our resident Make: Science Room lab geek, answered a question in the comments for the "Setting Up a Home Science Laboratory Part II - Gearing Up" topic, about buying digital scales. I thought it was worth posting here for the benefit of others.
Cynthia asked:
What would you recommend in the way of a digital scale for intermediate/high school science? I was thinking of purchasing one that was a 1000 g capacity with a 0.1 sensitivity. Could this both serve chemistry and physics, etc.?
Bob's reply:
Good question. The two big trade-offs in buying a balance are capacity and resolution. Ideally, we'd all like an inexpensive balance with 0.0001 gram resolution, but unfortunately, there aren't any such animals.
The balance I chose two or three years ago for my own home lab is the desktop MyWeigh iBalance 201, which has 200 gram capacity and 0.01 gram (centigram) resolution. That's still a current model, and is available in Maker Shed and elsewhere. However, it's also a $100+ balance.
If you're looking for something a bit less pricey, Maker Shed also carries a portable $33 electronic balance (on sale through 10/31 for $29) that has the same 200 gram capacity and 0.01 gram resolution. I have one of those as well, and it's a very nice little scale. I suspect it probably isn't quite as durable as the i201, but OTOH, it's less than a third the price. (It's also useful around the house. My wife just used it yesterday to see if she needed to put a second stamp on an envelope.)
My take on this is that 200 g is sufficient capacity. Almost any experiment you do that would use the 1000 g capacity of the balance you're considering can be scaled down to work within the 200 g capacity of these balances. OTOH, having 0.01 g resolution instead of 0.1 g resolution is very nice, particularly for chemistry.
It'll also save you money on chemicals. For example, if you need to make up a solution to a particular accuracy, being able to weigh out (say) 7.87 g of the chemical and making up 100 mL of solution is cheaper than having to weigh out 78.7 g of the chemical and make up 1,000 mL of the solution.
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Resolution is meaningless without accuracy. I wonder how accurate the inexpensive scales are, especially at low readings.
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Accuracy is actually not crucial. Precision is crucial.
But you can deal with a scale that always reads 1.203g higher than it should. (Or whatever the error in accuracy is.)
But precision would mean the reading on the scale is fluctuating, and the statistics to begin to counter that can be fairy.
What I would like to hear about is a mechanism that can be enhanced in a home environment though. There was a post somewhere recently about an excellent knife balance with a pretty astounding precision.
That is: is there some method to extend the available accuracy of a -digital- scale? The only method that comes to my mind would require a "hook scale" or "inline scale" operated in the inverted position - where the scale is hooked to one end of an unequal-arm pan balance.
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"Fairly Hairy" doesn't use the contraction fairy. What. the. heck.
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A digital scale would never read 1.2g high, as it is zeroed out each time it is turned on.
There is gain error, ie the scale always reads 1.1x what it should. This can be calibrated using an accurate weight (usually at full scale).
But there is also linearity error (10g might read 10g, while 20 reads 21g), temperature drift, etc.
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Too bad an accurate balance is probably illegal in Texas, along with laboratory glassware, hot/stir plates, bunsen burners, anything else that a Mad Scientist might have in their home lair.
You know, because those things are used for making illegal drugs. Thanks for saving me from myself.
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