Hi, Where does your service sit in the service order in relation to StdWXCalculate in weewx.conf under [Engine] [[Services]]? One possible reason is that if your station does not provide field ET (not many (any?) provide a numeric value in every archive record) and the StdWXCalculate service then calculate ET if it can. If your service that references ET is executed before StdWXCalculate then field ET will not (yet) exist in the archive record. In this case the solution is put your service after StdWXCalculate.
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