Weights and weighing machines
Weighing scales, digital balances, jewellery scales, pocket scales and approved balances for diamonds, gemstones, jewellery, scrap, coins and bullion.
I sometimes refer to weighing machines as 'scales' and sometimes as 'balances'.
THE BASICS: CAPACITY AND RESOLUTION
The readability (resolution) is the finest weight on the display of a digital balance, or the finest divisions marked on a spring balance. The most sensitive balances have a resolution of 0.001g (i.e. they 'go down to' 0.001g, they 'read' in 0.001g divisions); the least sensitive balances have a resolution of 100g (i.e. they 'go down to' 100g, they 'read' in 100g divisions). For jewellery, scrap and coins 0.1g is best; for silver 1g is OK; for both, stick to 0.1g.
The capacity is the maximum weight the balance will weigh. This can be anywhere from 10g to 10,000g. For weighing only gold, anywhere from 200g to 500g is fine; for weighing silver jewellery, between 500g and 2000g; for weighing silverware (cutlery, teapots, trays), between 3000g and 8000g.
Confused by decimal points? Before ordering please do think for just a second:
0.001g = a piece of paper the size of a pin head
0.01g = a small staple
0.1g = a couple of small matches
1g = two or three small paperclips
10g = a one pound coin (actually 9.5g)
100g = nearly half a pack of butter, a large chocolate bar (not the 'giant' bar)
1000g (1Kg) = a small bag of sugar or two small packs of pasta
10000g (10Kg) = maximum for a case taken in hand luggage on a plane.
- diamonds. Switch the scales to carat (100 points = 1 carat, 20 carat = 1 gram). Whether you get an accurate (true) reading to the nearest 1 point will depend on the quality of the scales. If you pay £20.00 or £30.00 - unlikely. If you pay about £50.00 they will probably be OK. If you pay closer to £100.00 they should be good.
- testing diamonds and gemstones using a specific gravity tester.
0.01g scales are for:
- gemstones (but not diamonds)
- powder for ammunition
- testing gold using a specific gravity tester.
0.1g scales are for:
- Gold. To see the price of 0.1g of 9ct gold, click here - then select from the three drop-down boxes on the top left: GOLD, GBP and g. Multiply the answer (far top left) by 0.375. This is the price of 0.1g of 9ct gold. This is the 'middle' (the 'official') price, you will never get this much when selling scrap, you will never pay this little when buying jewellery, full details in The Gold & Silver Buyer's Handbook.
- other precious metals, platinum and Palladium
- silver isn't so valuable so you can get away with a resolution of 1g, but if you buy a high-capacity 0.1g balance, you can use it for gold and silver.
- checking the weight of coins to see if they are genuine, look up the weights of coins in The Gold & Silver Buyer's Handbook (if the coin is the wrong weight, it is not genuine).
- catalysts for adhesives; photographic chemicals; medicinal herbs; eggs (to see which are likely to be fertilised); rare foods (some mushrooms, truffles, saffron).
1g scales are for:
- silver. To see the price of 1g of Sterling (-.925) silver click here - then select from the three drop-down boxes on the top left: SILVER, GBP and g. Multiply the answer (far top left) by 0.925. This is the price of 1g of Sterling silver. This is the 'middle' (the 'official') price, you will never get this much when selling scrap, you will never pay this little when buying jewellery, full details in The Gold & Silver Buyer's Handbook.
- the bulk-counting of coins (110g of pre-decimal silver coinage has a face value of £1.00)
- letters, packets and parcels.
- food processing / crop picking
There are two ounces. The Ounce Avoirdupois (often called the "English Ounce") = 28g and is for weighing food and goods; the Troy Ounce, ozt = 31.1g and is for weighing precious metals. The Pennyweight (20 dwt = 1 ozt) was discontinued, here in the UK, over 40 years ago but is still used in the U.S.A. The sub-division of the pennyweight is the grain (2gr - 1 dwt) originally based on a grain of wheat,and is still used for weighing powder for ammunition. A carat (metric carat) is 0.2g, used for weighing diamonds (divided into 100 points, pts).
Do not, here in the UK, confuse carat weight (sometimes called metric carat) with carat as a measure of the purity of gold (24ct = pure gold). Life is simpler in the U.S.A, "carat" is spelt "karat". See a conversion chart, useful for weighing gold and silver. Or there's a conversion chart that will convert dozens of different weighing systems.
MECHANICAL OR DIGITAL?
Mechanical spring balance. A tube with a loop on top (to hold) and a hook at the bottom (for the goods), operated by a spring.
Mechanical Beam balance. You balance the goods on one side with weights on the other side, until they balance. Out of fashion, nobody, these days, wants to be bothered with weights.
Mechanical hanging balance, originally a Victorian design for weighing letters but also OK for weighing light items of gold.
Mechanical v. digital. The advantage of mechanical balances is that they are difficult to break and you will never get a flat battery. The advantage of digital balances is that they are easier to use and there is more choice: from 1g to 0.001g resolution; from 10g to 10,000g capacity; from matchbox size to dinner plate size
APPROVED FOR USE IN TRADE
"Approved" means approved by the Weights & Measures section of the Trading Standards department.
An approved balance must be used when buying or selling on a weight basis (how much you will pay per gram or per ounce). This applies whether you work from a high street store, market unit, antiques fair, boot sale or your front room. Even auction houses must use approved balances if the the weight is included in their listings.
If it is a tube with a spring we call it a spring balance (they weigh down to about 1g or down to about 0.1g). One variation is a 'digital spring balance' - you hang the item on the hook and read a digital display (it works by deforming plates of metal rather than stretching a spring, but the principle is the same). Another variation is a semi-circle of metal with a hook and a loop (no springs), popular as a letter balance. A good name to describe all these is 'hanging balance'.
If it has a battery and a digital readout we call it a digital (or electronic) balance. Since these have flat weighing platforms, they are sometimes known as 'platform balances', as opposed to digital 'hanging balances'.
Technically-speaking none of the above are 'balances' at all, it is only a balance if you 'balance' the goods against weights, e.g. two weighing pans balance over or under a beam (a beam balance). However, today 'balance' and 'scale' are taken to me the same: a weighing machine.
- Readability (resolution) and accuracy
Put a 100g. weight on two digital scales, you might get two different readings, both have the same readability (resolution) but one might be more accurate than the other.
Nothing is 100% accurate.
Accuracy is measured in 'percentage'. Multiply the percentage by the weight (any weight you like, any weight you are likely to weigh) to calculate the possible error. I've never been able to find out the percentage accuracy of digital pocket balances but, from my experience of manufacturing them many years ago, I would guess it varies from 0.1% for low-cost (under £100.00) scale to 0.01% for an expensive (over £300.00) scale.
An example. You weigh 10g, the calculation is 10 X 0.001 = 0.01g possible error. On a scale with a resolution of 0.1g. you would never notice an error of 0.01g. But now try a heavier weight and you will see that the error is noticeable, e.g. if you weigh 3000g the calculation is 3000 X 0.001 = a possible error of 3g. There's no point in complaining that at 3000g the scale reads 2997g (or maybe 3003g) because this is simply the accuracy you will get with a low-cost scale.
In practice, it's not that bad. Even the cheaper (under £100.00) balances can sometimes (and with careful calibration - see below) weigh 3000g and get a reading of 3000g ±1g. Basically, the more you pay, the better, you would not expect the same accuracy on a £20.00 balance as you would with a £450.00 balance. Be wary of really cheap balances (e.g. under £10.00), if you find it isn't accurate, please do not complain, throw it away and buy something better.
The reading on a measuring device (any measuring device, e.g. thermometer, weighing machine, clock...anything) must match the units it is measuring (e.g. degrees, grams, minutes). This matching-up is called calibration. Electronic machines go out of calibration, it is the nature of electronics. Most can be recalibrated. In the scientific industry all machines need to be recalibrated regularly (usually once a year), a technician will call to do this and his call-out charge will be £150.00.
Nearly all balances include calibration instructions which involve pressing buttons and placing a weight (sometimes two weights) on the weighing platform. The weight is rarely included when you buy the balance. Tip: buy a weight at the time of buying the balance, then you will save on postage. There is an article about calibration.