Let’s consider capacity. What is the heaviest weight you think you will have on the scale at one time? This must include the weight of any container used to hold the unknown weight, plus packaging material or anything else that must be on the scale at the time the unknown weight is measured. For example, if your unknown weighs 800 grams but must sit in a crucible that weighs 400 grams, then you need 1200 grams of capacity. When you do the weighing you will tare the 400 grams of the crucible (re-zero the scale with the crucible on the weigh pan) and then weigh the unknown. If you know you need 1200 gram capacity, and then go for at least 2000 gram capacity in your scale.

What accuracy do you need? This requires a little thought to answer. You might be happy if you could measure the 800 gram unknown on a scale with accuracy to 0.1g. However, if you also know that you have to measure unknowns that weigh as little as 50.0 grams you should opt for a scale with a resolution to 0.01g. A good rule of thumb is that if you are looking for a precision scale with capacity between 600g and 4000g (4 kilograms) you should always select a model that provides a resolution of 0.01g. It is also very important to pay close attention to the linearity. On a 1000g (1kg) scale with resolution of 0.01g the linearity should be +/- 0.02g. A precision scale with a resolution of 0.1g should have linearity of +/- 0.2g.

You should also consider the need for automatic calibration as one of the factors in your decision making process. Automatic internal calibration allows the scale to calibrate itself whenever the temperature changes or as time passes. On none legal for trade models these parameter can be configured and controlled. Automatic calibration comes at a premium but it may be worth the investment as the scale becomes essentially maintenance free and you do not have to worry about calibration the balance with an external calibration weight. All TORBAL Internal Calibration scales are equipped with a calibration mechanism which has a high precision calibration weight built in.

Now we need a scale with 2000 gram capacity, an accuracy of 0.01grams, we are selling the things we weigh by their weight, and we don’t want to spend more than $1200 for the scale. What now? Scales that are used to sell things in commerce must be NTEP certified, that is to say the scale has been named on a Certificate of Conformance granted by NTEP (National Type Evaluation Program). This is to protect the consumer from fraud in weighing.

Scales can be grouped into 3 categories by accuracy. The most accurate are Analytical scales, usually used in research projects. These are accurate to 1 part in one million parts or better. This would be i.e. 200g capacity with a resolution of 0.0001g - most likely more than you need and more expensive than you would like. The next category is Laboratory scales, the workhorse of the lab, and these are accurate to 1 part in 100,000 parts or better. This would be i.e. 100 gram capacity with a resolution of 0.001g. You can probably find several that are NTEP approved and within your price target. The third category is Strain Gauge scales, and these are accurate to roughly 1 part in 10,000 parts or better. This scale would most likely not be accurate enough, nor have NTEP certificate for many precision applications (i.e. prescription weighing), but it would be the least expensive.