Friday, June 27, 2008

Digital Scales and Accuracy in Baking

If you read my review of Artisan Bread in 5 Minutes a Day, then you know I have been baking.

Since I am a bona fide nerd, that means that I have been reading a lot about baking from all kinds of different sources.

And that my friends, means that I have repeatedly run across the completely ridiculous assertion that weighing things in grams is somehow intrinsically more accurate than weighing things in ounces.

So here is a little primer, let's call it "Balance 101".

  • A balance is what scientists call a scale.

  • Units of weight measure are ways of describing how much of something you are weighing.

  • Units of measure for weight are always convertible to any other unit of measure for weight (1g = 0.03527oz). They have nothing to do with either precision or accuracy.

  • Precision is the number of significant digits used for the representation (1.000g = 0.03527oz), or the degree to which fractional units of measure in different units can be considered to be the same, within the constraints of accuracy.

  • Accuracy is the degree to which the displayed precision reflects reality. (My balance reads 1g, but I know that really means 1g, plus or minus 1g, so there is somewhere between 0g and 2g of stuff being weighed).

So where does this annoying assertion come from? Why do so many sources claim it?

Well, let's look at the specific case of my kitchen balance. It has a precision when weighing in grams of 1g (that is, it displays no fractional grams), but a precision of 0.1oz when weighing in ounces. As long as the accuracy of the balance reflects the least precise of these measures (0.1oz is less precise because it is 2.8g), then it doesn't matter which unit of measure you use to weigh. That is, if the accuracy of the scale is plus or minus 3g, then it doesn't matter at all whether you weigh in grams or ounces.

However if the actual accuracy of the balance is plus or minus 2g (or less) then it actually is better (more accurate) to weigh in grams.

Notice that this has nothing to do with the fact that grams are more precise than ounces. It has to do with the fact that the firmware in the balance fails to represent the values in ounces with sufficient precision to reflect the actual accuracy.


No comments: