Rotameter

A vertical tube, wider at the top, and the taper is the calibration.

1. Take a clear acrylic tube about 300 mm long and 25 mm bore.
2. Bore or ream it to a slight taper — about 1 mm larger at the top than the bottom.
3. Mount it truly vertical, checked in two planes.
4. Fit inlet at the bottom and outlet at the top.
5. Add stops so the float cannot escape at either end.

Vertical is not optional. The float's weight is the reference force the flow works against, and tilting the tube changes the component of gravity acting along it. A rotameter mounted even slightly off vertical reads high, and one mounted horizontally does not work at all — which is why they always appear on a plumb-vertical section of pipe.

The float's weight sets the range; its shape decides how well it behaves.

1. Turn one float from aluminium and one from brass, both to the same outside diameter.
2. Give each a conical lower face so the flow centres it.
3. Cut fine angled slots around the rim of one float so it spins as fluid passes.
4. Weigh both and record.

The spinning is where the name comes from, and it is functional. Angled slots make the float rotate, which keeps it centred in the tube and stops it rubbing against the wall or sticking. A float that touches the glass reads erratically and wears a mark. Rotation is a bearing made of nothing.

Because the answer is read directly, the scale must be marked in real units.

1. Set a flow, let the float settle, and mark its height on a strip beside the tube.
2. Collect and weigh the output over a timed interval to get the true flow.
3. Label that mark with the measured flow rate.
4. Repeat at eight or ten points across the range.
5. Examine the spacing of your marks.

The float balances weight against drag, so anything changing either invalidates the scale.

1. Calibrate with cold water and record several points.
2. Repeat with hot water at the same flows and compare.
3. Swap to the heavier float without changing the scale and note the error.
4. Tilt the tube five degrees and read again.

Karl Küppers patented the variable-area flow meter in 1908 in Aachen, and Felix Meyer commercialised it — the firm Rota gave the instrument its common name. It spread because it needs no power, no secondary instrument and no computation, which in a plant full of pipes is worth a great deal.

The three flow meters in this catalogue answer the same question three ways. The Venturi and the orifice plate both hold the AREA fixed and let the pressure drop vary with flow — so they produce a differential that must be square-rooted to become a reading. The rotameter holds the pressure drop across the float essentially CONSTANT and lets the area vary instead. That inversion is why its scale is linear and readable, and it is a genuinely different idea rather than a refinement of the other two.

Where each belongs: orifice plate for cheap, standardised, high-volume metering where a control system will do the arithmetic. Venturi for large continuous lines where pumping energy matters. Rotameter for local indication where a human needs to see the flow — laboratory gas lines, medical oxygen, chemical dosing. The rotameter is the one you fit when the reader is a person rather than a computer.

Its honest limits: vertical mounting only; calibration tied to one fluid at one density; the tube must be transparent, so it is unsuited to very high pressures or opaque service; and it cannot easily produce an electrical output for a control loop without added parts. For local reading it is close to perfect and for remote control it is the wrong choice.