moontree - if you will read through the BER FAQs here as well as just a few of the many discussions about it on the forum that the search will pull up for you, you'll discover that the primary causative factor in it is unstable, inconsistent soil moisture levels. Assuming the potting mix contains adequate Ca, and most do - that soil moisture problem prevents adequate/even distribution of the available Ca to the fruit tissues.

You are apparently trying to grow plants in containers and indoors. That creates a whole different set of problems and we'd need much more info to narrow down the contributing factors.

From the info provided, you are using a far too small container for most tomato plants - even for a determinate - and using a coir mix which may or may not be compounding both the Ca and the soil moisture issues. And doing it indoors so it lacks sufficient sun. Plus your plant food is lacking in some vital nutrients.

So given all that it is very unlikely that the low humidity is contributing to the problem. Rather the growing conditions that you have describe are far, far more likely to be causing the problems.

Dave

PS: it would be helpful to know your zone/location as well.

BER is one of the widely talked about tomato problems. As mentioned there has been numerous threads.

Just search this forum : Key word : "BER"