Using a field variable with exp10()

[Glen Bojsza]

This must be simple thing but I have tried several variations and was
hoping soeone would know off the top of their head.

I have a field vTXpower that has the number 2 in it

I would like to use the exp10() function

normally if you did 

put the exp10 of (2)

results in 100 which is correct

I can do 
put 2 into tnum
put the exp10 of (tnum)

and this works

but I cannot get 
put the fld "vTXpower" into tnum
put the exp10 of (tnum)

So I guess it is not taking the 2 from the field "vTXpower" as a real number??

Any insight is appreciated.

thanks