Reading pressure /vacuum gauges

[Simi 60]

I have two 1000fg factors with one of these gauges on it and was told rule of thumb was when it reads -20 to change filter.
Have never seen any - or + reading appart from when I clicked lever over to check if gauge worked and it went -

Yesterday we did a 9 hour run, last few hours due to weather I was running a bit harder than usual and when I did an ER check when anchored the gauge was reading +10.
Have never seen a + reading before.

Drained a litre out of the crud sump and spotless
Checked filters and spotless.
Tanks are on same level, being near full there could be slight + head but it doesn't register sitting.

Have googled and found no instructions for this gauge
Anyone got an idea as to why + pressure?

 

[High Wire]

Plus 10 on the inner or outer scale? A better question is why would there be a positive pressure at the gauge location? Could be the gauge is set up backwards internally. It surely can’t be plumbed wrong.

 

[STB]

Where is the gauge plumbed in? Also, just to avoid any misunderstanding, what are the units for the scalar number 20?

Consider the tank, the line from the tank to the primary filter, the line from the primary filter to the lifter pump, the line from the lifter pump to the 2ndary filter, and if you have an electric lifter pump, the lines in and out and where they fit along the path.

Also, is the gauge above or below the fuel level and by 7ow much?

Thanks!

 

[Simi 60]

It is this exact setup

Bottom of tank and pickup is about 2 ft below
Top of tank and fuel level now about 4 ft above

kPa in blue
Psi in black

And it was change at 20psi
And the 10+ reading yesterday was psi

 

[Simi 60]

Just found this article...

Typically, vacuum gauges are installed at or near the primary fuel filter. All Racor tandem series filter assemblies include a compound pressure/vacuum gauge. The pressure portion of the gauge is typically not relevant—unless the filter is installed well below tall fuel tanks that feed from the bottom; in that case, it’s possible that the fuel supplying the filters will be under pressure and the gauge may indicate this. While this is not harmful and while Racor turbine series filters can work under as much as 15 psi (0.1 N/mm²) pressure, the manufacturer recommends vacuum or suction applications.


https://www.proboat.com/2012/04/the-vacuum-gauge-tool/

 

[kchace]

Is this a gauge that has a vent plug in the top? On those that do, the vent plug is supposed to be removed after installation. Readings can be quite erroneous if those type are not set up correctly.

+10 psi is pretty much impossible in a boat style fuel setup so something is wrong. Oh and I’ve seen numbers like 5-7 inches of vacuum is time to change filter.

Ken

 

[Lepke]

What could cause the positive reading is after shutdown, the diesel in the lines in or near the engine expands from the heat. If there's a check valve in the filter housing, it would stop the fuel from going back to the tank. And depending on the engine type, it may not allow the excess fuel in the return line when stopped.

The other cause is the fuel tank level above the gauge.

The is no magic vacuum number. It depends on the quality of the fuel you buy and the micron size of the filter. By inspecting the filter you can pick a number that's good for your boat.

 

[STB]

It is this exact setup

Bottom of tank and pickup is about 2 ft below
Top of tank and fuel level now about 4 ft above

kPa in blue
Psi in black

And it was change at 20psi
And the 10+ reading yesterday was psi

Something seems off in units. 20psi is something like 40 inches Hg or about 140 kPa.

I think most people who use gauges consider -20kPa to be about when to change...about -3psi.

I'm not really super at this type of math, but I think you'd need to get a 25ft tall column of diesel to get 10psi.

If we have a number like +10, I have to believe the units are "Hg, inches of mercury -10 "Hg is about -20KPa, which is a reasonable point to change.

Is there an electric primer or lifter pump anywhere in the path from the tank to the engine? If so, on which side of the gauge? In other words, is it pushing fuel into the gauge or sucking it out?

If there is an electric pump upstream from the gauge, it could put the gauge under positive pressure. If that were to be the case, I couldn't hazard a guess as to what normal would be
It would probably depend upon what the lifter pump and engine were doing as well as the filter.

 

[Simi 60]

The only pump in the system is the on engine cummins PT pump

 

[Simi 60]

What could cause the positive reading is after shutdown, the diesel in the lines in or near the engine expands from the heat. If there's a check valve in the filter housing, it would stop the fuel from going back to the tank. And depending on the engine type, it may not allow the excess fuel in the return line when stopped.

The other cause is the fuel tank level above the gauge.

The is no magic vacuum number. It depends on the quality of the fuel you buy and the micron size of the filter. By inspecting the filter you can pick a number that's good for your boat.

Might be it though In 5 years of operation I have not seen a + before
The only difference was running at around 1250 vs 1150 rpm for a few hours at days end
Engine and ER temp was not noticeably higher.

The fuel tanks are the fullest they have been since our ownership.

 

[AusCan]

Simi,
You might be better off with a gauge with a range of about -20 to +20 psi for reading pressure at the filter. Maximum vacuum for most filters is only about 3-4 psi (7" Hg) so it might be a bit hard to see accurately on your high range gauge.

btw - Head pressure for diesel is 0.37 psi per foot so the 10 psi wouldn't be head pressure unless your tank depth is 27 feet, as noted above. Most likely the valves were shut and you had a bit of thermal expansion. Thermal expansion of liquids can cause very high pressures in isolated lines.

 

[High Wire]

The correct time to read this gauge is when the engine is running at cruising speed. Reading with engine off is meaningless because it doesn’t have a telltale pointer. What you were reading was trapped pressure.

 

[Solly]

If you really have 10 psi to the filters the fuel would shoot out of the drain like a firehose. More than just normal 'head' pressure.

Shut off the fuel to the Racors, unscrew the gauge and see if the pressure drops.

More than likely it's the gauge has gone bad. If you have access to an air compressor drop the regulator and pressure to 0, hook up the gauge and slowly increase the pressure using the regulator to check the accuracy of the gauge. Don't over do it.

 

[Simi 60]

The correct time to read this gauge is when the engine is running at cruising speed. Reading with engine off is meaningless because it doesn’t have a telltale pointer. What you were reading was trapped pressure.

And about 2 hour prior to shutdown I was in the ER doing a regular walk around and it was in its usual zero position.

Sounding like it is as you and others say, trapped pressure.
Odd that I've never seen or noticed it before, but I rarely go into ER immediately after shutdown due to heat and, in the morning when I pulled the filter the gauge was reading zero.

Mystery solved I think
Thanks all.