Create totalizer

Hello,

i'm a new user of PI procesbook.

i hope that someone can help me with the following issue.

I want to create a totalizer in a flow to a certain tank.

the totalizer must be initiate when the tag 635GO141.pv (valve)is switched to "on" en must be stopped when switched to "off".

i have already filled in in the expression field:

if 'COD:635GO131.PV' = "on" then TagTot('COD:635FY020.PV','t','*') else 0. The totalizer now  runs over 24 hours. Is there a possibilty to make a switch between time 't' and '*' and use "on"and "off" instead?

i hope you can help me

regards

martin