Operating Range & Calibration for Analytical Balance

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Weighing balances,
1) Do we require to establish 'operating range' ; If yes, what are the principles/requirements and references outlining the same.

Thumb rule is that we should weigh only in the range of 0-80% of balance capacity + least count must also be accounted...
i.e., for a balance with 100g capacity(0.01g least count), we should use it for weighing 0.02g-80g

2) Can we use two standard weights for carrying out calibration. (viz., use 100mg + 50 mg weights to check a calibration of balance at 150mg value)
 
G

George Weiss

The calibration of a commercially produced analytical balance would normally have a manufacturer’s manual with it. Asking the manufacturer for a service/calibration manual is a second option. It is never best to second-guess the manufacturer’s method of calibrating their device. Each balance has it’s own special specifications, features, and requirements. In general devices have a maximum of range and an operational range. A device may have from 10% to 90% over-range capability, (which is beyond normal operational use). A 100gram scale might operate up to 110grams or higher. The device is commonly tested at several points in it’s range, (from 3 to 10 or more), including 0% & 100% of operational range. The use of several weights is common, and allows for several combinations of weight totals. A single generic procedure is a possible option, but reviewing a manufacturer’s procedure/manual is always a good first step.
 
D

DrM2u

I echo George's feedback related to using manufacturer's calibration procedures instead of re-inventing the wheel. As far as the calibration range, an auditor would probably expect the calibration to cover at least the use range. For example, if you have a floor scale that can measure up to 5000 kilos but you are not weighing anything more than 3000 kilos then the calibration range should cover at least 3000+ kilos.

Regarding the scale capacity, it is up to you what the 'rule of thumb' is. Like George said, most devices have a built-in design/safety factor of at least 10%, just in case someone decides to over-load the scale. You might not get a reading on the over-weight but the scale is protected from serious damage.

If you just want to verify (not calibrate) the scales, then you can use the weights in any combination necessary to cover the desired range. Let's face it, not many calibration outfits/labs haul around a 5000 lbs weight, instead they use a number of smaller weights. Keep in mind your desired level of accuracy in verification and what is the course of action when the verification finds that the scale is out of calibration.
 

BradM

Leader
Admin
Hello there! :bigwave:

Weighing balances,
1) Do we require to establish 'operating range' ; If yes, what are the principles/requirements and references outlining the same.

Thumb rule is that we should weigh only in the range of 0-80% of balance capacity + least count must also be accounted...
i.e., for a balance with 100g capacity(0.01g least count), we should use it for weighing 0.02g-80g

2) Can we use two standard weights for carrying out calibration. (viz., use 100mg + 50 mg weights to check a calibration of balance at 150mg value)

I am assuming by your questions that you are considering calibrating a balance. In the absence of any other guidance document, you should purchase Handbook 44 and refer to it.

You will need to assure that you have appropriate test weights, and that they are calibrated by a competent vendor.

Now, analytical balances typically have a sweet spot, around 50% of range. Hopefully this is the use range of the balance, and what you are interested in. So you definitely want to verify it there. Past that, you will want to verify the balance across its use range. Saying, focus on calibrating the balance where it is used. :) Also, you will want to perform corner load verifications on the balance.

My advice might be to at least for the first time, have a local competent vendor come in and calibrate it (ask for as-found/ as-left data). Then, assure you have a proper set of test weights, and perform in-house verifications on the scale before/after use. Be sure and record the results. :D

Also, depending on how old the balance is, it should have the ability for internal calibrations, and possibly, auto-balance and such.

Your second question is yes, you can use multiple weights. You'll need to be careful, though; and stack them on top of each other. Having a small weight placed next to a large weight could cause some issues.
 
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