Hi Grampe,
Pine bark mulch can be made to work in hydroponics when you take into consideration its acidity. I feel that it will play havoc with your pH measurements and you will need to do a lot of adjusting. Eventually, like sawdust, it will degrade and you will have to replace it, but your future supply seems assured.

As to how long to run your nutrient pumps, that depends on your climate and how well the pine bark drains. With sand, I run my nutrient pumps only for 30 minutes per day, in three sessions, but I am using ever so much more nutrient than you are. The best advice I can offer, is to run the pump to fill the system, then turn it off. Do this at least three times per day if exposed to sunlight.
If you are growing in a closet with lights, I can offer no advice as I do not use lights .... we have plenty of sunlight here.

Cheers,
Jimr007.

IMO i like to use no media or the light weight expanded clay pellets.

the bark will degrade,drop pH, and invite pests. as to how often you need to flood, that depends on your environment and the number and type of plants.

how often did you run when you had pea stone? i would assume you would need to run it less with the bark.

The Ph problems alone would be enough to keep me away from pine bark....
not to mention attracting buggies and pathogens....
not to mention plugging up your pumps, pipes and filters.....
not to mention poor capillarity from uneven particle sizes......
and so on.....

I used Pine Bark mulch in my cucumbers, carnival squash, buttercup squash,and nh watermelons and they did equally as well in the Bark Mulch as in the 3/8" pea stone.

I had no problems with the pumps, pipes filters, and I never adjusted the PH.

I ran my pumps for 15 minutes every hour(7am to 8pm) from about the middle of june, until the 1st of september.

I did learn 1 thing, Don't listen to these text book wizards. Keep your nutrients clean, and if you are using good water to mix them, don't try to use all the fancy ingredients these guys tell you too.

The growing season has come to an end here in NH, and am getting ready for next year. I plan on doubling my growing, and hope to try more varieties.

I found the tomatoes were the best yet. cauliflower and broccoli were also great. I had problems at first with aphids on my eggplant,but got them under control, with a good yield. The cabbage grew too good. I had to give alot of it away.

grampe