string.find help

Want to find a variable in a string. The variable changes.

volume = GetFieldValue("sqltable", "sqlfield")

if string.find(header.InnerText, "vol.(volume) Barcode:", "") ~= nil then
local bcode = string.gsub(header.InnerText, "vol.(volume) Barcode:", "");
SetFieldValue("sqltable", "othersqlfield", bcode);
end

I am not doing something right here. How do I put the variable 'volume' in the pattern?

instead of try

Thanks! Just started trying to teach myself Lua for work apps. Much appreciated.