no chlorine in spa after a shock??

[jsimo7]

Hi we have a new HS Envoy 2 weeks old.  We use chlorine. We did our 2nd shock with chlorine (5 caps bio brand) left the pumps on for 30 min with the cover off, then covered it up and went to bed. The next morning I checked with test strips and the strips said the chlorine level was at 0ppm (can this be correct?) , so I added 2 caps before I went to work. I called our dealer and they said maybe the test strip was bad. I tested that evening I checked again and it was at 3ppm on the strip.
After our 1st shock the next morning the chlorine was at 5-10ppm. I thought that was normal and didn't call anyone. Has anyone heard of this or am I losing my mind??

[Vinny]

I would think that if you had a lot of "stuff" that the chlorine needed to burn up, it is possible to use up all the chlorine.  

Do you know what your combined chlorine (CC) was? It takes 7-10x the CC amount of free chlorine to break this down.

I know in a pool, you can get a chlorine demand that you put in chlorine and it gets used up in a short period of time. This usually happens in a pool due to the lack of proper amount of chlorine was being used and the chlorine is being used up to kill the algae or bacteria. I would think the same can be true in spas.