You can't be sure your tuna in safe unless you know the pressure for sure so you know the temperature reached 240 degrees for the full 100 minutes.  Botulism is odorless, tasteless and deadly.  If you can reprocess them with 24 hours you will be safe and can save your tuna.  Be sure to use new lids and if there is liquid loss you might add a little water or oil so your tuna won't be too dry.  

If you don't have a working canner then freeze your tuna if it is within 24 hours.  

I would suggest you get a weighed 3 part gauge for your canner, then you would not rely on the gauge but the rocking of the weighted gauge.  It is much easier to regulate the pressure with the weighted gauge.  If you order one to go on your canner order from Presto or the canning company that made your canner because some of the generic ones that look like they are made by the canning company are made in China and not working well.  

Hope that helps.  Thanks for using Ask Extension.  Hope you can salvage your tuna.  

Thanks for using Ask Extension

Nellie Oehler