Calibrating a weighing machine

[Jack Merlin]

A discussion came up on this forum about the choice of weighing machines which led on to a discussion about the accuracy of electronic scales.

I was wracking my brains to try to remember how to calibrate scales. I have an old set of pig weighers here which needed re-calibrating so I contacted the manufacturer. What they told me reminded me of what I learnt in physics at school.

Some scales measure weight by measuring how much a spring stretches as a load is applied. Stretch ought to be proportional to the weight applied, until too much weight is applied the the spring cannot recover to regain its original length due to metal fatigue.

Electronic scales work on the same principle but electronics are used to measure a change of resistance as the spring is stretched. Someone suggested that these scales are never accurate enough for falconry.

Well, here's a way to check! You need two known weights, one near the bottom of the range and another near the top. Now a draw a graph plotting the actual weight against the reading off the scale. As you will now have two points on the graph, you can join them with a straight line.

Now, if your scales are accurate, every weight you put on the scales should show a reading off the dial to a value that corresponds to the reading off the graph. Never mind if this reading is what it actually weighs!

Provided all the test weighings show a reading on the scale which corresponds with a reading off the straight line on your graph, your scales are accurate!

"But, Ah!", you say, "My weight weighs 500 grams but the scales says it is 510 grams! It must be inaccurate!"

Not so, grass hopper. What is wrong is the CALIBRATION of your machine. There will be a small screw or potentiometer somewhere (probably inside the machine) which can be used to adjust the read out to the correct value. Do that and your machine is as accurate as it can be.

Hope that all makes sense.

[OutFlying]

Jack,
I calibrate scales / weighing systems for a living. Electronic scales will be accurate and calibration correct (easily altered) if you buy scales that are suitable for TRADE use (class 3 machine) i.e butchers scales etc. Not the cheap rubbish bought from Argos and the like. You usually test at minimum. halfway, maximum weight - also lift and replace the same weight 3 times to check if it repeats the same weight.

I'd test your scales with a known weight just above and below your range your using to weigh the hawk i.e if your hawk weighs 2 lb - test at 2lb, 2 1/4 lb and 1 3/4 lb. The most important check if always weighing on the same scale is repeatability, if doesn't matter if the calibration is out as long as it is always out by the same amount.
Outflying.

[Varmint]

I'd test your scales with a known weight just above and below your range your using to weigh the hawk i.e if your hawk weighs 2 lb - test at 2lb, 2 1/4 lb and 1 3/4 lb. The most important check if always weighing on the same scale is repeatability

Sounds like the way i test electronic scales! but very impressed with the magnitude of thought that went into that one Jack!

Your not getting £20 that easy! :wink: :lol:

[OutFlying]

Varmint,
I take that £20 everyday off you. A 50000 kg weighbridge will repeat to within 2kg over 3 repeats with a 46000 test load during testing, sometimes with no error if the wagon drives to the exact same place on each return. But my electronic scales at home read exactly the same over 3 weighs, there not from Argos though :lol: :lol: :lol:

Outflying

[Moritz]

Where do I get good electonic scales class 3.? How much are they?

Thx

Moritz

[OutFlying]

Moritz,
Type in Avery scales on Ebay, plenty on there at moment. The class 3 means that they fit for trade i.e accurate enough to sell off - so you need to look for scales that have been used in shops etc. Get one that measures either to 1/8 of an ounce or 2 grams increments.
Outflying.
p.s you'll need to make a small perch to go on the flat surface of the scale. Their are probably other makes available try using "shop scales" as a search on Ebay.

[Spudy]

i borrowed this idea from a couple of falconers i spoke to..
buy a set of cheepish kitchen scales from a know high street seller(digi ones)
then as soon as you get them home grab one of the tins of beens outta your kitchen cupboard and waigh it first before any thing else wright this waight on the tins top and put it safethen befoe every use waigh the tin first and check that it still waighs the exact same(as the scales may move but the tin will never increase or decrease in mass)if they are out take them back bin them or keep as an ornement but allways use the same tin of beans(remember to convert the scales to have a pearch first) but as most digi scales have a zero out button you dont have to work out the difference.
hope you find this info useful guys :D

[OutFlying]

You need to know the scale is constant at the weight range your using. Unless your hawk flies at a weight near that of a tin of beans - it doesn't mean a lot.

[Varmint]

Also i have found that as birds step onto them they can auto Tare or alter for some reason?

inanimate object= Great
animate= Sh........te

[Spudy]

ok cheers guys i take back what i posted and wont use that idea when i get my bird in june dont want to risk him thanks again

[OhMyGod]

I got some scales from tesco for £15, they measure in 1g increments (electronic 9V)

Each day I check them with a 1kg mass, and every day it says 1000g

Then I put the perch on to check that and it's the same each day weather hot or cold, sunny or wet.

I don't trust them totally that's why I check them a few seconds before putting the bird on. I take her off and put her back on. Exactly the same each time. occasionally her weight will be different by 1g

Can't complain at £15 - £20

[Falconry Equipment International]

Hi Jack , this sounds pretty complicated, I use an old pair of avery balance scales ( Like gorcers/ gpo used to use) have one set of wights thta I use on a dayly basis and then test weights thta I check once a month/ week / what- ever, when I test I also add an extra 1/4 oz wieght to the original 1/4 oz wieght and check m y 1/2 oz wieght etc to do all my weights add together.Mind you about 15 years ago , I have to admit I managed a whole season once flying 2 gosses( not together of course) whithout weighing them once, just going on conditioning,their body language, both on bow and on glove withy careful consideration to covering on the keel & pectorals
rgerads
J

[Falconry Equipment International]

I Meant to say I would not recomend, not weighing, it was just a position I found mysylf in after a fire that had damaged amongst amany other things my scales. I had also been flying hawks & falcons for about 25 yrs by this time.
J

[OutFlying]

Hi Jack , this sounds pretty complicated, I use an old pair of avery balance scales ( Like gorcers/ gpo used to use) have one set of wights thta I use on a dayly basis and then test weights thta I check once a month/ week / what- ever, when I test I also add an extra 1/4 oz wieght to the original 1/4 oz wieght and check m y 1/2 oz wieght etc to do all my weights add together.Mind you about 15 years ago , I have to admit I managed a whole season once flying 2 gosses( not together of course) whithout weighing them once, just going on conditioning,their body language, both on bow and on glove withy careful consideration to covering on the keel & pectorals
rgerads
J

Screaming Jay, those are also class 3 weighing machines, I bet there's a small lead seal somewhere on it with a stamping of a crown on it - if it was previously used to sell off by weight.

[Hawkmaster]

Spudy you can still use your idea, just increase the amount of tins or the size of them.

[OutFlying]

forgot to say, the weight on the side of the can is the contents not the total weight of the can and contents (normally). So you still wouldn't know is your scales are spot on.

[Jack Merlin]

I have to admit I managed a whole season once flying 2 gosses( not together of course) whithout weighing them once, just going on conditioning,their body language, both on bow and on glove withy careful consideration to covering on the keel & pectorals

Falconry has been practised in these islands at least since the Saxons arrived. That's about 2000 years. Weighing machines have only been used here since the late 1950's. They seem to have managed OK without them!<g>

[Falconry Equipment International]

Screaming Jay, those are also class 3 weighing machines, I bet there's a small lead seal somewhere on it with a stamping of a crown on it - if it was previously used to sell off by weight.
I seem to remember there is. I also use avery weights that are all stamped
regrads
J

[Falconry Equipment International]

I have to admit I managed a whole season once flying 2 gosses( not together of course) whithout weighing them once, just going on conditioning,their body language, both on bow and on glove withy careful consideration to covering on the keel & pectorals

Falconry has been practised in these islands at least since the Saxons arrived. That's about 2000 years. Weighing machines have only been used here since the late 1950's. They seem to have managed OK without them!<g>
Hi Jack
precisely, although I don't know how some of the 'modern falconers' would get without the use of scale on their non indigeonous species ;o)) (only joshing lads & lasses)
J