Sorry to bring up an old thread, but I just can’t get this card to work. 
I have the file dual-gauge-card.js in www, and in ressources I have /www/dual-gauge-card.js and javascript module. Still I get “Custom element doesn’t excist: dual-gauge-card”. Can somebody please give me a pointer to what I’m doing wrong?
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Did you get an answer to this? Short version is; in settings->dashboards click the three dots in the upper right. Then “add resource” for the path to the .js file so ui-lovelace can find the dual-gauge-card.js. For instance, my path was “/local/community/dual-gauge-card/dual-gauge-card.js” and be sure to select JS module in the Add Resource dialog.
Thanks, but I managed to finally get it working by using HACS, I never could fix it manually.
That’s because the HACS installation automatically adds itself to that dashboard resource list. 
Typically the way you ended up doing it is a lot smoother and the ‘right’ way. It just doesn’t work 100% of the time 
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