Taking a Dot measurement and it is coming back as a negative or minus value.
In the 500 series instrument if Patch Smarts (the instruments logic to automatically know if the patch being measured is a Solid, Tint, or Paper) is turned off under the configuration menu (as is generally recommended) it is possible for the instrument to display negative readings for a dot or tint value measurement. This can occur for several reasons:
In Dot Area or Dot Gain mode, the original patch measured as solid is not the darkest patch being measured. With patch smarts turned off no error checking is done and any value is acceptable as the solid reading
In Dot Area or Dot Gain mode, the substrate and solid are of different hues. Thus when measuring in the tone scale, a switch from one filter to another occurs and unexpected numbers are the result. In this case, measure the solid in the all or auto mode under Density to determine the dominant filter and in Dot Area or Dot Gain mode switch to the single color mode to prevent the crossover while reading the tone scale.
In Dot Gain mode only, you may get a negative reading if you are near a threshold of one of the built in internal references. For example if your internal stored references are 25%, 50% and 75% and you were to measure a tint with an actual value of 72%, the instrument could display this as 22% gain over reference 50% or as -3% loss over 75%.