Flow Sensor Maintenance

[ndutton]

A few months ago, forum member Ando had concerns about growth inside his raw water flow sensor. As a result, he provided his sensor to me for a close examination which did indeed show some growth but not enough to affect its operation, at least that was the case on the bench. I went a step further and removed my own sensor for examination as well. It showed evidence of minor growth too but in my case, it DID affect operation. The effect of the growth inside mine caused the movable magnetic spud to stick closed. It wasn't stuck hard but it did not slide like a new one, the way it should.

Flow sensor autopsy

During the development of the sensor for our application, added restriction was anticipated, tested and measured. Bench flow testing of an Oberdorfer M3 with and without a sensor showed a 7% flow reduction imparted by the sensor. It's important to know the test was done with a brand new, pristine sensor.

If you added a deck winch you added its maintenance too. If you added fresh water cooling, you’d have another pump and heat exchanger to maintain. Oil filtration system? Same. And so it is with the flow sensor to preserve its original performance. That is, to keep the restriction it imparts as close to the original and expected 7% as possible.

What measures could you take to maintain the sensor? You want to keep debris out of it so a decent quality raw water strainer would be important and I’m not talking about those horrible external thru-hull grates. To prevent growth inside the sensor you’ll want to flush it with fresh water as a regular part of boat clean-up following an excursion. With a flushing Tee installed it is very easy to do, takes only a few minutes. Regardless of whether you’re RWC or FWC, a regular fresh water flush of the raw water system is beneficial to the entire system too.

Pump intake flushing Tee

Here are a couple of other things to consider:

• Placing the sensor in the discharge side of the raw water pump is preferable to the suction side. It will have less of an effect in that position.
• If you have a Sherwood or Oberdorfer M3 raw water pump, their performance when they were new is less than the Oberdorfer M7 or MMI 502 pumps, worse yet after 45+ years of service so the 7% flow reduction with the sensor installed will have a greater effect with the original equipment pumps. (Hint-hint)

[Wrsteinesq]

I’d like to raise a related problem. I have the EWDS, which includes the flow sensor. I also have a PSS instead of a stuffing box; and because I can’t leave well enough alone, that PSS is fed cooling/lubricating water from the same exhaust from the (FWC) heat exchanger. In other words, the cooling (Bay) water exhausted from the HE is split, with part going to the standpipe and part (what I thought was a small part) going to the PSS/cutlass bearing. But when we test-ran it, a WHOLE lot of water was being emitted from the PSS/cutlass, and relatively little was making it thru the standpipe and out the transom. This didn’t seem prudent, so we removed the sensor, and Presto!—-MUCH more water came thru the transom, and a lot less, but still a pretty decent flow, thru the cutlass bearing. This made me happier than the alternative; but I’m now without the benefit of the flow sensor. I should note that we have the Moyer, not the Oberdorfer, raw water pump. Has anybody else had this problem? Is there a bigger flow sensor I could use, so it doesn’t so much restrict flow? Thanks!

[Peter]

I am curious to know where in your cooling system the sensor was located? If it was in the raw water pump discharge I am having difficulty imagining why it was causing the asymmetry in water flow so far downstream

Peter

[ndutton]

Did you have any elevated engine operating temp problem? How long have you had the sensor installed? Has it ever been flushed as described earlier? I'm also interested in your reply to Peter's question.

I'd be happy to pay round trip shipping if you would send me your sensor for careful examination and testing.

As for the question about a larger sensor, most if not all of the flow sensors I've run across have their switching the opposite of ours. That is, normally open contacts (NO) whereas we need normally closed (NC).

[Wrsteinesq]

It definitely was in the raw water discharge. Leaving the heat exchanger, I took a tee off it, which stepped down to a smaller line, perhaps 1/4 inch or 3/8 of an inch, which ran to the PSS. The remaining, main, portion of the raw water discharge (5/8”?) ran to the sensor, and then to the standpipe. So my belief is, that because not enough water was making it through the sensor, that it was backing up, upstream, and the only place for it to go was out through that 1/4 inch or 3/8 inch line to the PSS.

[Wrsteinesq]

Just now seeing Neil’s post. This was a brand new installation, i.e. a new to the boat A4, never before run. The test was actually conducted while in the slings, using hose water, emptied into a bucket and pulled by the Moyer pump. Thus marine growth was never an issue. For the same reason, I have no feedback on overheating. The engine simply never had a chance to overheat, because when I saw what I saw, we shut things down. There was simply way too much water coming out of the PSS. It just didn’t look right.

[Wrsteinesq]

I have a short video of water coming from the PSS, but apparently I lack the technological expertise to post it.