Thanks for these comments. Yes, definitely not linear... in any case and more especially as open ball valves can permit syphoning or simple gravitational flows.Just to be clear (for others who come across this), this may be working fine for you, but is not a configuration that many have had success with. Being that the pumps are a centrifugal type head and somewhat slow speed, your range of adjustment is going to be very small and definitely not linear. Additionally, since the pumps are magnetically coupled you will run a much higher risk of decoupling at a slower speed. Pump lift variations, flow restrictions, wort viscosity, etc. are all going to become variables in your success to throttle the pump.
Tim - have you recorded your flow rates at various duty cycles? What is the effective range of duty cycles that you've been successful with? Knowing your operating parameters could give others a starting point if they want to experiment with this.
I haven’t done any controlled flow tests. My approach has been somewhat trial and error to avoid what would inevitably be complex mathematics. Initially I worked up pump d/c schemes for relatively slow fly sparging that wouldn’t require any manual restriction of non-automated valves (say to regulate flow rate of sparge water into the MLT or outflow from MLT to BK), and that that would work with my elec ball valves under On or Off direct control. I did a few trials with plain water to begin with and was able to develop a repeatable and effective process. My pump duty cycles range considerably from just 3% pulses, upto 75/80% and largely depend on the relative levels of liquids in HLT, MLT and BK. The latter depends on whether I’m doing a 5 gallon or 10 gallon batch. For example, with a 10 gallon batch toward the end of the lauter I’m effectively draining “uphill" from my mash tun to the kettle, so have to use a 15-20% d/c to maintain a slow enough flow rate. Overall, a surprise finding was that even small changes (say between 3% and 10%) significantly affect the flow. Also, I’ve been successfully using 75% d/c of both HLT and MLT pumps during the HERMS mash cycle (to get a decent heat transfer) and 5-10% d/c in transferring chilled wort through my plate heat exchanger to the fermenter to keep the transfer speed down and ensure a perfect and rapid temp drop from BK temp to ca. 25C.
I had indeed realized that factors such as wort viscosity, sediment and particles (also temperature and pump location) could easily create differences in outcomes, but further tests including tweaks to my all-water trials during several live brewing processes, have yielded excellent and repeatable results. I screen out any particles large enough to restrict pumps and viscosity seems to have little effect. My pumps are all in fixed locations approx 24 inches below my kegs.
A final FYI, although I began using and achieving a fully balanced fly sparge, in the end I’ve opted for a batch-continous lauter process. This begins with a fly sparge with given water volume, and ends with [almost] draining my MLT to attain my desired boil volume (last runnings around 1.010). Am getting high (>80%) mash efficiencies.
Overall, therefore, I've found that pump duty cycling is a really useful functionality that overcomes random fiddling with manual valves and also takes account of the On or OFF only position allowed by electric values.
