There are times when you want to include a component in your mixture in every run of your DOE, always at the same concentration or volume. This is a constant.
For example, you might be growing a specific cell line that is auxotrophic and requires a specific compound in order to grow. It may be well known that varying the concentration of this compound has little to no effect on the growth characteristics of your cell line, but just that at least a small amount must be present. In this situation you will want to make sure that every mixture you make as part of your DOE gets the same amount of this compound.
In Synthace, you would define this compound as a constant and define the volume or concentration at which it is present in your mixtures. Synthace will do the rest, and make sure it is added to every mixture in your DOE.
To learn how to define constants as a part of your DOE, click here.