So from this I presume that the di vessel you are talking about is the small di attached to some small r/o where you can see the inside as the housing is clear.
DI vessels work by distributing the incoming water into the resin at the top of the housing. The water finds its way down to the bottom of the housing through the resin where it exits via a tube up the inside of the housing and out of the top. Air rises so the flow of the water doesn't allow it to 'bleed' away normally. Unscrewing the housing slightly will allow the air to escape. Once the air has escaped then tightening it back up will mean the housing will remain full of water.
As PF have said, its how they all work and over time the di will slowly fill with water.
The only way to check if the di is working is to check the end product water after di. If the tds of the output is the same as the tds of the input, then the resin is spent.
Hi M & C, that's it you must have a similar setup.
I had always, wrongly, assumed that it filled the housing first and that was how it went through the DI. So when I saw it not filling I assumed the water was bypassing the DI. It turns out they all do similar and sometimes fill and sometimes don't, but it always goes through the DI so really doesn't matter.
All a panic for nothing
If the di worked with the flow the other way, ie. water in at the bottom and exit at the top, then the di would fill with water straight away as the air bled off. But the designers must have seen that the di doesn't work well this way.
This flow of water through the di is different to other applications. For example, a diesel filter on the fuel line of your van must have all the air bled out of it before it will service the injectors properly with diesel. Having air in the line causes issues. So its only logical to take this filter method and apply it to every other filter; well I think so anyway.
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