Most paints start with a white base color. Highly concentrated colorants and pigments are added to this base to produce the required color. To achieve the desired hue, the colorants must be mixed in an exact ratio. To achieve the desired color intensity, the exact amount of each colorant is important. Generally, the only practical method for measuring the batch is by weight. Formulas are determined by the weight of each of the ingredients. It is usually impossible to meter by volume or any other means.
The two major specifications required for a scale that is suitable for this application require a high capacity and high resolution and accuracy. Normally there is a tradeoff between these two terms. A scale that has a high capacity will have lower resolution, and a scale with high resolution will have a lower capacity. For example, a 10 lb capacity standard industrial scale may have a resolution of 0.002 lb. But a 100 lb capacity scale may only have a resolution of 0.02 lb. In each case, the accuracy is typically 1/10 of 1%.
It is best to show the difficulty by using an example. In this case, a 79 lb batch of green paint needs to be mixed. The ratio of mix colors are white (97%), blue (1.5%) and yellow (1.5%). Therefore the ideal amounts for each color are 76.63 lb white, 1.185 lb blue, and 1.185 lb yellow. The scale can measure the white paint to either 76.62 or 76.64. The overall accuracy of the white will be the general accuracy of the scale, that is 0.1%. That is pretty good. The blue can be measured to 1.18 lb, which represents any weight between 1.17 lb up to 1.19 lb. So the error can be as high as (1.185 lb – 1.17 lb) which equals .015 lb. This represents an error of 1 ¼ %. The same holds true for the yellow. So the ratio of blue to yellow can be wrong by as much as 2 ½ %, which will result in a very poor color matching.
The best way to improve this is to choose a scale with better resolution. An ideal choice would be Arlyn Scale’s Ultra Precision Surface Acoustic Wave (SAW) scale. These scales provide accuracy that is 20 times better than the standard strain gage scales. But the price is quite similar to a high quality strain gage scale.
A good comparison is to repeat the example above using Arlyn’s SAW-C scale. It also has a capacity of 100lb. But the resolution is 0.001 lb and the accuracy is 1/100 of 1%. In this situation, the SAW scale will read to 76.630 lb. So the accuracy will be the scale’s accuracy of 1/100 of 1%, which is excellent. The blue can now be measured to 1.185 lb, meaning that the actual weight can be between 1.1845 lb and 1.1855 lb, due to rounding errors. Therefore, the error is limited to .04%. The yellow can also have an error of .04%. The ratio can now be off by .08%, which is thirty times better than the first example. This will result in excellent color matching.
There are a number of other excellent reasons for choosing the SAW scales. Full buckets of paints are placed onto, and lifted off of the scale. In an industrial environment, it is quite possible for the bucket to be handled roughly, and for the scale to experience large shock loads. This is a major cause of damage to standard strain gage scales. The SAW scales have an overload specification of 500% of the full capacity of the scale. They will provide much better service, even with this kind of use. Another important feature of the SAW scales is that they remain much more accurate when there are changes of temperature within the work environment.