Is it possible to reset numeric entry fields to their initial blank states?

Hi, I've built an inventorying interaction in which learners are asked to "inventory" a set of compressed gas cylinders (this is for a gas safety course). For each cylinder, they have to identify its contents (that is, what gas does it contain) and pressure (at what pressure is the gas stored in the cylinder). I provide a text entry field for them to enter the name of the gas, and a numeric entry field for them to enter the pressure.

When they've worked through all the cylinders in the set, they click Submit for feedback about how they did. At that point I also provide a Try Again button in case they didn't do as well as they'd like and want to start over and do the inventory again.

The Try Again button has triggers attached to it which attempt to reset the various text entry and numeric entry fields by resetting the variables associated with them. This works fine for the text entry fields, but the only values assignable to numeric entry fields are numbers. Yet, when learners first encounter these fields, they have no numbers in them--they are just blank fields. How can I reset these fields to their initial blank states? I tried setting their associated variables to zero, but then they show up with the zero in the field and I want the field to be blank. Is there a way to accomplish this? The last trigger on the Try Again button is a jump to the current slide, which is set to "Reset to initial state" when revisiting. 

Thanks!

    -Ray