Jobo 1520 Lid Question

I recently acquired a Jobo 1520 tank and had a question about the lid that reading the instruction manual didn't completely clarify. The manual says:

"At each production step, push in the pressure-compensating membrane and firmly push on the solution-tight cap"

It sounds to me like the center of the cap should be pressed in fully before the lid is attached to the tank, as shown in this photo:




I'm curious because I've watched many Jobo how-to videos over the last few days and I haven't seen a single one that mentioned pushing in the center of the lid like this when using the tank.

I was taught to put the rubber cap on first, and then push the centre in.

You push in the centre first because the cap makes a seal when you put it on. Pushing the centre in when the cap is already sealed compresses the air in the tank.

The point of the centre of the cap is to compensate for expansion of air in the tank when you put in chemicals warmer than the air in the tank (warmer air occupies greater volume than colder air) or pour in a chemical that, when it contacts what's already in the tank, creates a gas of some sort (I think bleach-fix does that when it contacts residual developer on film in the tank). The expanding gas in the tank pushes the centre of the cap out instead of just popping the cap off - and then pouring out any chemical that is inside, if the volume is enough to fill past the halfway mark.

When developing colour film, when the chemicals are fresh, I will need to stand by and wait for the centre to pop out so I can quickly stand the tank up, unseal the cap, push the centre back in, reseal the cap, and resume. Otherwise, the cap does pop off from the pressure.

The idea behind this "bellows" cap is to yield pressure compensation in case of volume changes.
Such changes can be caused by temperature changes or chemical reactions.

That Jobo designs of the past and such of all their competitors work without such feature, shows that it likely is no necessity.
However, already at a preceeding non-bellows design Jobo advised to push-in the cap, to let the volume expand in case. But to my experience this did not work due to the cap yielding too much suction and coming up prematurely.

I remember faintly that at a certain process venting of caps was advised (not by Jobo).

AgX said:

it likely is no necessity

Click to expand...

I've had the centre pop out and the cap then pop off from pressure while running the bleach/fix stage of colour processing - and had 1/4 of the blix pour out into the water bath. I now watch and listen for the centre popping out so I can relieve pressure before it happens again.

This would be a reaction of Blix with residual Developer.

But if this would be a standard occurence more so none of the plain-cap tanks would have ever worked. But still fellows here hail them.

AgX said:

if this would be a standard occurence more so none of the plain-cap tanks would have ever worked

Click to expand...

It is practically never an issue with black and white film developing. It only comes up at higher temperatures or colour developing. I never bother to depress the centre of the cap when developing black and white film.

At any rate, if you're going to depress the centre of the cap, you should do it before putting the cap on or the purpose is defeated.

Interesting that I may have been doing it wrong all these years. From what I recall, my teacher at school who ran the dark room taught me to push the "membrane" after putting the cap on....I was told that compressing the air in the tank was a good thing, helped prevent inconsistencies in case of bubbling of chemicals.

In reality it is probably not important either way. If I've been doing it wrong for 36 years and haven't encountered any issues, it's not vital.

Agulliver said:

Interesting that I may have been doing it wrong all these years. From what I recall, my teacher at school who ran the dark room taught me to push the "membrane" after putting the cap on....I was told that compressing the air in the tank was a good thing, helped prevent inconsistencies in case of bubbling of chemicals.

In reality it is probably not important either way. If I've been doing it wrong for 36 years and haven't encountered any issues, it's not vital.

Click to expand...

Yes I just haven't bothered either way. I just place the cap on after fitting the lid and do nothing more. Seems to work OK that way

pentaxuser