Viking balance

A home owner from California says:
Please advise which29" series 351 channel balancer I need for a sash approx.29"high and 46"wide.Is it a Viking balance fx2830 or 2840.The internal spring length is 2830 18" and spring of 2840 is only 16,5".
I like to order the correct channel for above sash.
Thank you,
Ulrich
P.S. Have a nice weekend.
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Tom from SWISCO responded:
Thanks for contacting us. The spring length doesn't really indicate anything, I'm afraid. The sash size also isn't all that reliable. What's more important is the sash weight. You will need to weigh your sash on a scale to confirm what will work best.
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