Need help creating a while loop

I have a script that is being used to shut down a UCS blade server - here is a code snip:

try { 
    if ( $ProfileFull.Operstate -eq "ok" ) {
        Get-UcsServiceProfile $ProfileId | Set-UcsServerPower -State soft-shut-down -force
        start-sleep -s 120
        wait 150
    }

#########

I want to change the code to add in a while loop that waits for the blade to power off... what is the best way to do that? Would this work?

try { 
    if ( $ProfileFull.Operstate -eq "ok" ) {
        Get-UcsServiceProfile $ProfileId | Set-UcsServerPower -State soft-shut-down -force
            $bladeDn = $ProfileFull.PnDn
            while ( Get-ucsblade | ?{$_.Dn -eq $bladeDn } ?{$_.OperPower -eq "on" ) {            
                  start-sleep -s 120        
            }
    }