Marcus L. Rowland (ffutures) wrote,
Marcus L. Rowland
ffutures

More Balance Pron

Don't read this if you're bored by scientific instruments...


Got myself another balance cheap on eBay last week - this one was a Precisa 2200g x 0.01g model, and I got it for 21 quid including postage with a "needs calibration and not warrantied in full working order" proviso. These cost about £600 new (the current 3200g model that replaced it is £800), so I thought it was worth taking a chance.

It turned up by parcel post today and my first discovery was that it hadn't been transit locked - as a result one of the metal strips that makes it work had broken. This is a bit of (I think) steel about 3cm long and 0.1mm thick or thereabouts, with a couple of bendy sections, so that it is flexible but doesn't stretch, and breaks without damaging the rest of the balance if the balance is overloaded.

So I phoned Precisa and asked if it was possible to buy a replacement bit of metal - the answer, of course was no, they'll fit one as part of a repair and recalibration, but the minimum cost for that is likely to be a couple of hundred pounds. So I declined the offer (without actually laughing hysterically, which I regard as an achievement considering that petty cash currently owes me more than that and I've no hope of getting it until April) and decided to try to build one.

Attempt one was a piece of nichrome wire - it worked reasonably well, about 99.5% accuracy, but stretched when I tried a heavy load on the balance and stopped working. Back to the drawing board.

Attempt two was a piece of steel from a blood sampling lancet, which turned out to be too stiff and degraded the accuracy to 95%

Attempt three was a piece of nickel foil, shaped as much like the original component as I could manage; this only gave about 96% accuracy and snapped under load, which didn't do my temper much good.

Attempt four is two bits of nichrome wire, and works well enough for most purposes, though not good enough for A-level chemistry, with accuracy obstinately stuck on 99.5 even after repeated recalibration. I've decided to set it to read to 0.1g rather than 0.01g, add it to our pool of general purpose balances, and leave it at that. Unless someone can suggest an alternative material that might work better and is more flexible than nichrome - I'm vaguely thinking of a piece of springy bronze or brass. Anyone got any suggestions?

So not a wholly unqualified success, but I've got a 2200g x 0.1g balance for 21 quid, it's accurate enough for most purposes, and it can also be set to read in kilos or newtons and has a computer interface, so ought to be fairly useful. Since I'd expect to pay £250 or so for something with that sort of specification I really can't complain.


On the strength of this scientific triumph I've decided to make a new icon - myself in mad scientist mode (one of the few occasions in recent years I've worn a lab coat, during trials for a new chemistry course a few years ago).
Tags: science, work
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