Separ SWK-2000-18 with vacuum gauge: what to expect?

I am going to replace the covers of some Separ filters with the one below, that includes a vacuum gauge for early spotting of progressive clogging in the filter cartridge.

But those vacuum gauges are very rare to in European boats (not only for Separs BTW, but also Racors), while I can remember to have seen them in several installations in the US.

So, I was wondering if someone can give me an idea of what I should expect to see as "normal" reading.
I mean, in principle I would expect a reading very close to zero with clean new filters, and progressively increasing as the filter gets dirty.
But I don't know what sort of PSI (if any) should be expected with clean new filters, and at which vacuum level it would be recommendable to replace the filters, so any suggestions/experiences would be appreciated.

Just in case it matters, the engines burn 45 GPH @ WOT each, so the filters are way oversized, considering their rating of 285 GPH max.
I suppose that could contribute to make any reading differences less noticeable, but again, no idea about what numbers to expect...

Thanks in advance!

Doh! Thanks for the link, I actually bought the gauges from the Separ distributor in the US, and didn't think to check their UK website.

But there is a statement in those instructions which I'm not sure to understand:
"There is a small cap on the top of the gauge. When supplied a certain amount of air will be present inside the gauge; this should be expelled by the temporary removal of this cap. With the air bubble gone, the normally damped operation of this gauge can commence."

Now, here's how the top of the gauge looks like:

So, I assume that by "temporary removal of the cap" what they actually mean is a rotation of the yellow clip towards the open position.
But should I keep it open with the engine running, till the air bubble is gone, and then close it again?

@d_meister:
the following pic shows you the scale better, but why do you think the position at rest should be relevant?
I mean, I understand that the static position can be affected by the tank position, and also by how full they are, but for the intended purpose of the gauge, its reading is only relevant while fuel is circulating, "sucked" by the engine fuel pump and possibly restricted by a clogged filter.
So, the position at rest should be irrelevant - or am I missing something?


@Capt J:
you are correct ref. the fuel circulating being well above the max fuel burn, but according to the MAN manual the pump moves 198 GPH at the max rpm (2300), so the 285 GPH the filter can handle still gives a decent margin.
The 5-10 PSI vacuum at cruise speed that you mention, is it referred to the same brand and size of filters?And with which engines?
Ideally, it would be great to hear from anyone who experienced the same filters/gauges with MAN engines...

Very interesting to hear that, also because you seem to confirm that these gauges are frequently installed in boats at your side of the Atlantic, right?
Don't ask me why, but in Europe they are as rare as hens' teeth.
I can only remember to have ever seen them on a very old Akhir - and only statically.

Besides, I wouldn't have expected the baseline to be so similar between different boats, with different fuel lines/tanks/primary filters.
That's also a useful reference: if by chance in my installation they should show say 20 psi right away, with fresh cartridges, maybe there would be something else to be concerned about.

I will post my findings in due course, but thanks a lot for the moment.

All understood, thanks.
In fact, I was also wondering how/why the air bubble could disappear.
I mean, other than thermal dilatation of the fluid inside, that could temporarily reduce the air bubble, what else could fill the air space and leave the gauge interior permanently filled?!?

I am not sure of what the EU regulation restrictions are on fuel filters (if any), but while metal bowls are indeed common, actually also sight glass bowls are not rare at all, also in OEM installations.
My boat is just one example, but I can think of several others.
They did fit the version with metal heat deflector though, which as I understand is also USCG accepted, but frankly I don't know if that was a requirement from RINA (the certification authority) or not.
Anyway yes, indeed I've only seen vacuum gauges on one old boat (and they were probably retrofitted), while I've seen them in several boats in the US.

Ref. the 20 PSI with fresh filters, that's not what I'm expecting, because the engines never had any fuel problem so far, and I fitted the gauges just for the convenience of having an early warning.
It was a number I used as a hypothetical example of something obviously strange.
20 PSI would be even off-scale on the gauge...

Lastly, the reference red needle just happened to be there when I took the pic, I will reset it to zero before running the engines.
But are you sure it's supposed to be "pushed" by the working black needle, in order to stay at the maximum vacuum reached after running?
It seems rather stiff, when rotating it with the knob, so I doubt that the working black needle can move it.
And both the new gauges that I bought behave exactly in the same way (and have the same air bubble), so it seems unlikely that both are defective in the same way...

Thanks for the detailed explanation on the yellow little valve.
I finished installing the new covers & gauges yesterday (yes, perfectly up-rite).
So, even if I had not read your post yet, by coincidence they did sit overnight unmolested.
Now, I just tried putting the yellow valve on "open", and nothing happened, and not even the smallest drop of glicerin came out, so after a while I put it back on "close"

Statically, the black needle actually isn't exactly on zero, but just very slightly on the pressure side - barely noticeable, actually: less than one psi.
I guess this is because the tanks are almost full, and their max level is somewhat higher than the gauges.
Both gauges are consistent anyway, so I don't think they are defective.
I also tried pushing the glass faces a bit, but they feel very solid, and I could only see the air bubble veeery slightly reacting to pressure.
The needles didn't move.

Another thing I checked is how the tell tale red needle works, and I can confirm that both you and Capt J were correct:
if I rotate it clockwise, it drags the black needle together with it.
And when rotated CCW, the black needles follows, up to its zero position (well, just above as I explained, but let's call it zero).
So, obviously when the opposite will happen (i.e. it's the black needle rotates CCW due to vacuum), the red tell tale must be dragged up to the max swing reached by the black needle while the engines are running, which is indeed very convenient.

I can't yet make a full live test now with the engines running, but as soon as I will be able to make it, I'll report back.
In the meantime, thanks a lot everybody.
Amazing how after decades fiddling with boats there is always some bit of kit on which to learn something new!