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daletournier 14-04-2018 21:46

Racor vacuum gauge
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Hi all, I've just fitted two racor vacuum gauges to my filters. I've had gauges before on previous boats but these racors are different, they have little blue vent plugs at the top where my previous ones did not.

The instructions say "after install, pull up blue cap on fill/vent plug to full open position. Opening the vent plug will relieve internal pressure changes in the gauge cases that have been induced by environmental changes in pressure /temperature. Without being vented the change in internal pressure could adversely affect the accuracy of the gauge ".

It says nothing about closing it again. Do you leave this plug open all the time?

It seems strange to me that you have a little vent in the top of a vacuum gauge won't this effect its operation?

What am I missing /not understanding? As mentioned previous gauges on my earlier boats didn't have it and they seemed to work fine.

Cheers Dale.

Alan Mighty 14-04-2018 22:25

Re: Racor vacuum gauge
Most interesting, Dale.

I've a dual Racor with a vacuum gauge on Led Myne. And I've had single Racor filters on previous boats.

I note that Racor has given me, both by e-mail and by phone, excellent customer service. So an e-mail (to should work.

My reading of what you've written suggests the vent lets you zero the gauge to suit your conditions (you could be on a high altitude lake, a low altitude lake, or at sea level in between). If that's the case, then seems to me that you open the vent to zero the gauge and then close it because you've established zero for your atmospheric conditions.

RacorTech should answer you fast and more reliably than me.

I will note another issue: Racor vacuum gauges (without a vent) are unreliable if they're in a situation where fuel is supplied to them under pressure (by a pump) and especially if the pump pressure varies. The gauges are vacuum gauges, measuring any "vacuum" created by clogging of the filter element. If the fuel is supplied under pressure, especially varying pressure, that defeats a lot of the thinking behind a vacuum gauge. But I have trouble thinking that the vented gauge was designed for that situation.

daletournier 14-04-2018 22:41

Re: Racor vacuum gauge
Hi Alan, this is my thoughts but the instructions say absolutely nothing about pushing the vent closed again, surely they don't assume all that fit them no to push them shut one gauge is equalised?
As you suggested, I'll send them a email.

Dsanduril 14-04-2018 22:58

Re: Racor vacuum gauge
This is from their marketing fluff:


Stainless Steel case and face increased from 2.0 inches to 2.5 inches diameter for easier reading

The improved cases feature a pressure relief valve to adjust to ambient barometric pressure changes

Acrylic lenses upgraded to polycarbonate

Needle stops replaced with internal locking mechanism to prevent damage due to an accidental overpressure

Internal sealing at the fitting, lens, and pressure relief valve reduces the likelihood of internal contamination by splashing fluids
Sounds like they've gone to a sealed case to prevent contamination inside the case, which in turn means the case needs a vent. Since their fluff says the vent is to 'adjust to ambient barometric pressure changes' I'd suspect it has to stay open.

Funny that, 20 years ago Racor advertised getting rid of liquid filled gauges and moving to silicone damped so they could get away from the liquid/vent etc.

Pelagic 15-04-2018 01:02

Re: Racor vacuum gauge
I've had 4 of the older cardio Racor Vacumn gauges leak and fail, so I just gave up on them .

Seems they have tried to improve, but with a Duplex system, I don't really need them

Eigenvector 15-04-2018 08:46

Re: Racor vacuum gauge
Sounds like you have diaphram pressure gauges which isolates the diesel from enterning the gauge. These yours are also liquid filled. By burping the little plug on top you eliminate the small atmosperic delta across the diaphram. This is important for low pressure measurements where the change in atmospheric pressure is significant to the measured reading. Burp and close. Closing prevents contamination.

daletournier 15-04-2018 09:12

Re: Racor vacuum gauge
These are the ones I purchased

My feeling is they should be closed, no mention of burping in instructions , just open once insta . I've sent of an email to Racor for clarity.

Eigenvector 15-04-2018 09:29

Re: Racor vacuum gauge
Ok, looked up what you have. Those don't appear to be liquid filled like I'm used to.

So a more technical explination of the funtionallity: You have two types of pressure/vacuum measurements: Absolute & Gauge Pressure.

Gauge pressure measures the difference between atmospheric pressure and the line pressure. An abolute pressure gauge on the other hand would read 14.696 psi (at STP for the sticklers) with no line pressure.

You are measureing gauge pressure of the fuel relative to the ambient atmospheric pressure. Because the case of the gauge is sealed well, the atmosperic pressure on your fuel system may drop with a front passing through, but the gauge is sealed and has a resulting internal pressure higher than atmosphere during this event. By burping the vent, the gauge returns to Zero, where it should be because you're not running the engine. You could leave it open, but you run the risk of contamination. Just burp it before you read it. See if the needle moves.

daletournier 16-04-2018 10:00

Re: Racor vacuum gauge
Recieved a reply from Racor "Hello Dale,

Once you have equalized pressure at sea level, close the blue plug. There is usually no further need to open it. "

I think their instructions that come with the filters need to ne clearer.

Bleemus 16-04-2018 12:18

Re: Racor vacuum gauge
You close them after a few seconds.

davil 06-06-2018 04:39

Re: Racor vacuum gauge
any experience with the "New" filter


CamanoTye 06-03-2020 15:08

Re: Racor vacuum gauge
Thanks for posting this. I just did a web search to see what the deal was with the Racor gauges and the blue plastic cap and found this thread as I was about to call Racor. They should have a note in the instructions saying to close the blue cap on the fill/vent plug.... it is a little confusing....

Thanks for your post!!

sailah 06-03-2020 16:59

Re: Racor vacuum gauge
1 Attachment(s)
My racor gauge failed so I built a new one myself on my lathe. Well actually I did the brass machining and used a SPAN usa made gauge. Works awesome and very sensitive. I believe it should be a direct swap and SPAN makes stuff for the Navy. I got it free as as a sample since they were a distributor.

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