Across Ocean Systems NMEA 2000 pressure sensor, smart & useful

Ben Ellison

Ben Ellison

Panbo editor, publisher & chief bottlewasher from 4/2005 until 8/2018, and now pleased to have Ben Stein as a very able publisher, webmaster, and editing colleague. Please don't regard him as an "expert"; he's getting quite old and thinks that "fadiddling fumble-putz" is a more accurate description.

17 Responses

  1. Ben Ellison Ben Ellison says:

    Belt and suspenders: when I do get the old vacuum gauge off the Racor, I may use a T fitting so Gizmo can have both the AOS sensor and the newer model Racor RK19669 gauge:

    Also, I think that Ben Stein is going to test an AOS pressure sensor with his boat’s air conditioning, so second opinion coming.

  2. Donald Joyce says:

    Thanks Ben,
    This is very interesting. My Racor filters precede a transfer pump on the way from the main tanks to the day tanks on Cats Meow. The transfer pumps pull hard enough to peg the racor vacuum gauges even without a filter. I’ve been thinking of installing a differential gauge measuring both Racor input and output vacuums but haven’t found anything that has the sensitivity I’d like. I’ll look at the specs to see whether using two AOS sensors and subtracting on measurement from the other provides a meaningful result.

    • Ben Ellison Ben Ellison says:

      Don, why not a single sensor that can measure higher suck levels and then see if you can distinguish normal transfer pumping versus what happens as the filters collect dirt?

    • Donald,
      if I understand your setup correct, when you operate your pump, you are probably getting the Racor gauge pegged towards the pressure side, not towards the vacuum side. I’m not sure what exact vacuum gauge you use, but some of them have no ability to measure positive pressure, they are just setup to measure negative pressure, thus easy to peg in the opposite direction. If the pressure that your pump generates, is less than 30PSI, it will fit easily in the acceptable measuring range of our -14.5PSI to +30PSI pressure transducer range. In order to get proper result for the state of your filter you will still have to install the pressure transducer between the filter and your engine. You may be able to get away with installing single pressure transducer and monitor the positive pressure that the pump will create. Theoretically speaking, once your pressure at the transducer drops by about 5PSI it will be probably time to change the filter, but it will not be so critical in your case because you might still have positive pressure at the engine, e. g. from +15PSI drop to +10PSI will indicate restriction of the filter, but still allow the engine to operate properly as it is being fed with 10PSI of fuel pressure. If the pump wasn’t not there, we will have different situation. in this case with restricted filter, the pressure will drop from 0PSI to -5PSI, at which point or soon thereafter the engine might start starving for fuel. I can’t testify if using pump before the filter is a good practice or not, as I’m not sure what forcing fuel through filter does to the quality of the filtration. It is a fact that restricted filter will cause pressure drop regardless if in the positive or in the negative pressure range. All you need to know is what is the normal pressure of your engine running at cruising speed with new filter and use this as a baseline when monitoring your filter condition.

  3. Ron Micjan says:

    I have the maretron system with racor vacuum displayed on the n2k View monitor on RV Zephyr. My analog gauges we’re welded into place on the racor manifold as well. I used a heat gun to warm it up and then it finally budged. I believe they used red loctite. It’s quite useful to have racor data up in the pilothouse when it’s too rough for the crew to be doing the hourly engine room checks, as long as one has ER cameras too.

  4. Lee Enyeart says:

    I am considering installing an AOS pressure sensor into my existing Racor fuel filtration system. The original Racor vacuum gauge was replaced with a SMX drag pointer vacuum gauge with standard bottom mount from Seaboard Marine. This gauge has a yellow valve on the top which prevents the fluid from leaking out during shipment. The first photo shows the AOS pressure sensor attached to the top of the Racor vacuum gauge. Did that fitting exist or did you have to drill and tap a new one? In addition, will you need to drill and tap a fitting hole on the top of the drag needle vacuum gauge from Defender Marine?

    • Ben Ellison Ben Ellison says:

      Sorry, Lee, that photo shows both my first and second install mistakes. There was a rubber cap in the top of my old Racor gauge and threads that took the AOS sensor, but the filter vacuum was not present there. I have to remove the gauge and then use a Tee fitting if I want both AOS and analog gauges.

  5. JIm Hebert says:

    On a tangential topic: can this pressure-sensing device be used with gasoline fuel systems?

    • Ben Ellison Ben Ellison says:

      Hi Jim, This from AOS: “Yes the pressure transducer can be used for gasoline. Of course, it will require proper installation to insure that there are not leaks when the sensor is mounted…. just make sure you mention that :)”

  6. Lee Enyeart says:

    Ben E:
    Am I correct to assume the required T-fitting is 1/4″ x 1/4″ x 1/4″ NPT fitting with a male fitting on one end to screw into the manifold base, a female fitting on the other end to accept the vacuum gauge and a female fitting on the side branch to accept the AOS pressure sensor plug?

  7. Bill Coleman says:

    I cannot tell the difference between the Racor RK19669 Vacuum Gauge and the RK19671, that is half price, and comes with a SS handle.

  8. Jordan says:

    Now AOS Ltd, NMEA 2000 pressure sensors are available with T-handle and can be ordered here:
    The proper picture with the T—handle is not posted on the website yet, but I presume they will do that shortly.

  9. steve says:

    All I know about boats I have read online. This sensor looks like the answer to the question how to eliminate condensate from your fuel tanks. Rig it up to a solenoid that controls the flow of a off the shelf available in any port cannister of argon gas.
    As the fuel goes down the argon fills the tank. When the tank is refueled the valve is turned to atmosphere and the argon dissipates and the cycle begins again.

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