The Trials and Tribulations of Metering the Washing Machines

The initial idea was to use threaded fittings to interface the flowmeter with the washing machines. Fittings make sense, they are usually secure and won’t leak, provided the threads (A) fit together properly, and (B) are designed to seal. If that fails, there are ways to make poorly fitting threaded connectors seal, usually PTFE thread tape or anaerobic sealants.

Brass fitting assembly with residue of Loctite 574 on ¼ in. threads

The Koolance FM-17N flowmeter is threaded for ¼” female BSPP (British Straight thread Pipe Parallel, also known as DIN ISO 228-1). Being familiar with NPT (National Pipe Thread) which is a tapered or conical (self-sealing) design, I didn’t know how BSPP would work, until I tried it. I should have looked at the Koolance fittings more closely before committing to a large parts order of expensive ISO conversion (ISO/NPT) fittings from Swagelok, but I had perhaps too high expectations of Loctite thread sealant. The way ISO straight/BSPP fittings work is that they tighten against a gasket, rather than sealing the threads. The Koolance hose barbs and fittings are designed to work this way, but I didn’t quite understand how this worked, and therefore went with a faulty assumption that I needed to use a thread sealant/threadlocker. Alas, the tolerances of the Koolance fitttings are too wide, and the ISO fittings from Swagelok don’t fit—and of course never seal, even with the assistance of PTFE tape or Loctite 565 thread sealant. Note: the Koolance FM-17N has nominally 1/4 in. ISO straight threads.

Bosch Automotive Handbook: (Krickau & Nöcker, p. 318)

The way Loctite anaerobic sealants/threadlockers work is that they cure in the absence of oxygen, under pressure (such as in the tiny space between threads. A glob of it will stay a slightly sticky glob, but won’t really solidify the same way it will under pressure. This is especially important in applications where it is necessary to wash away the excess that gets squeezed out from between mated surfaces, such as in an engine (use Loctite 574 (the famous Loctite Orange) to put your Porsche case halves together—using RTV will cause your engine to destruct when bits of solidified RTV (silicone) clog the bearing oil passages). With rather large gaps between the threads of the fittings and ends of the flowmeter, the Loctite doesn’t quite cure, and therefore any disturbance causes the adhesive to shear from the surfaces and thus it starts to leak. The idea with Loctite thread lockers is that the shear strength keeps the threads mated.

To add to the slow leaks, the overall length of the assembly is rather long, and with an end rigidly attached to the hose bib or back of the washing machine, any torque causes the acrylic case of the flowmeter to crack—a catastrophic failure in my case. Flooding a dorm basement is not an option.

Ultimately the question is what cycle did the user select, or slightly more generally, what wash temperature did the user select. Without splicing into the water system, it may have been possible (and far cheaper) to use a set of thermocouples or thermistors on the intake hoses, but this relies on there being a substantial change in temperature to indicate what’s going on—and it may not be the case that the cold hose/pipe gets appreciably colder when the water flows through it, and the hot hose may stay warm (above threshold) in two immediate consecutive washes. It would take a bit of tuning to set thresholds, and this approach would have required a positive signal of when the machine is running (cycle start and end), which has been a little more of a challenge, using either an electret microphone to detect vibration or a Hall effect sensor on the power cable. Neither of those are fully foolproof, but fortunately the most positive system for measuring the cycles has been made workable and safe.

Installing the flowmeter in a segment of flexible hose solves the problem of putting stress on the acrylic meter body and cracking the meter, and using the Koolance gasketed BSPP fittings solves the problem of fittings not quite threading together and sealing. The flexible hose effectively prevents transmission of torque to the meter, provided the force is not excessive, and by using a hose clamp and slightly undersized inside-diameter hose, the possibility for leaks is minimized. To further reduce the possibility of leaks, the threads can be sealed with Loctite 574 thread sealant, providing a measure of resistance against the fittings unscrewing and sealing the threads ahead of the gasket integrated into the base of the hose barb.

Koolance hose barb with integrated polymer gasket

FM-17N installed in pressure-rated hose with hose clamps


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