Most engineers have had the importance of units drilled into their head starting in Middle School math class. This has led many engineers to adopt a more-is-more philosophy when it comes to displaying units. This is contradictory to what is best for machine operators. Operators need the relevant information to run their equipment, but nothing more.
When deciding what units to display to operators, it is imperative to understand how they are going to interact with the equipment. Let's look at how operators from 3 different facilities may interact with a pair of paint tanks - 2500 and 1000 gallons.
Operators at plant #1 have no paint responsibilities. Paint is managed by a third party, operators must only know to shut the line down before paint runs out.
Solution: Operators at plant #1 do not need to know tank capacity or gallons of paint remaining. A simple percentage graphic that alarms when the Low threshold is met will suffice.
Operators at plant #2 are responsible to keep their paint tanks full. Operators are supposed to call for a 500 gallon delivery when their tanks have capacity.
Solution: Percentage is not the right unit for these operators, especially considering that Tank A and Tank B have different capacities. Showing tank available capacity is what these operators need.
Operators at plant #3 are responsible for minimizing paint waste and are tasked with running their paint levels down as low as possible during runs.
Solution: In a perfect world, the display would tell the operators how much product the remaining paint can produce or how many minutes of runtime the paint can supply.
This requires a known and consistent application rate, which may or may not be possible. At a minimum, a trend showing the tank level allows an operator to guesstimate how much paint they need in order to finish the run.
A valid argument could be made that operators at all 3 facilities would benefit from a trend like this. Depending on the other operator responsibilities and screen real estate, this may be true.
Key takeaway - It is important to understand how an operator is going to utilize a value when deciding what units to display it in.