Insane Hydraulics

Site theme image

Tear-down of Webtec FI750 Variable Orifice Flow Indicator

I have loved taking things apart ever since I was a little boy. There wasn't a toy in our house that I didn't "hack" just like there will never be a gadget in my possession that I don't "break into" at a certain point just to see what's inside. So... Want to see what's inside a variable orifice flow meter?

This is a Webtec FI750 flow indicator, model 120-ABOT, which means that it has a flow rate range of 120 lpm (32 USgpm), an aluminium body with 3/4'' BSPP threaded ports, it is calibrated for 28 cSt oil and has a built in temperature indicator. According to Webtec the meter can withstand 420 bar, and has an accuracy of 4% FS (in other words ±4.8 lpm or 1.3 USgpm) - which would be "not stellar" for a digital flow meter, but is more than OK for a purely mechanical instrument with a magnetically-coupled needle dial.

This particular meter was completely stuck due to "severe contamination abuse" and, eventually, got binned, and, of course, when I saw this I couldn't resist the "urge to revive". I did manage to get it unstuck and I will be testing it soon to see how it fares after a "sanding session", but for now we can appreciate its design.

The acrylic cover of the dial is held in place by a snap ring, and there's an o-ring underneath to prevent the ingress of contamination. The machining of the body is pretty tight and clean-looking. I like it.

The needle and the dial itself are also very easy to remove. The needle is made of bronze, and the dial is made of aluminum. There's another o-ring seal underneath:

Pulling on the needle shaft reveals the rest of the needle-moving mechanism. It has a bronze body, that houses a tiny 1/8'' x 5/16'' ball bearing, the shaft is made of stainless steel, and the shiny round "puck" in the middle is a diametrically-magnetized magnet.

There's not much else to see from the outside, so let's have a look at what's inside now. One can see the sharp-edged orifice and nose of the moving tapered metering piston in the inlet port and the spring in the outlet port:

But we want to see all the parts, don't we? Luckily, the internals can easily be pulled out after removing the single snap ring. The oil channel is 22 mm wide, and the orifice has a diameter of approximately 15 mm. The sharp-edged orifice itself was the only thing that I didn't manage to remove, possibly because it's assembled with a thread-locking glue, and I didn't want to massacre it trying to torque it out with a pair of pliers. There's no need for this anyway because one can easily see what it is:

The tapered metering piston does not have a straight taper shape but is rather bullet-shaped, and I suppose they had to come up with this profile to make the digits on the dial more or less evenly spread. You can also see the shavings on the base of the nose where the piston was "catching" the orifice edge. There's an axially magnetized magnet in the center of the metering piston, held in place by what looks like some kind of epoxy glue. I think that this arrangement is smart - having two constrained (one rotationally, and one linearly) magnets moving perpendicular to each other makes sure that the center line of the rotationally constrained one always points to the center of the linearly constrained one, independently from their strength (which can vary with temperature). And then, there's also a short u-shaped bronze pin located at the bottom of the pressure-reading port, which extends into the fluid channel and prevents the metering piston from rotating:

So, as you can see, it doesn't take too many parts to make a simple mechanical flow indicator:

Once my "frenetic schedule" allows it (a couple of weeks from now, hopefully), I'll make sure to hook this baby up to our test bench and then report on the results.