Re: Calculating flow rate from Watts & RPM on variable speed
You are right that is theoretically possible to do that although there are some complications in practice. Each pump, VS or otherwise can be characterized by three parameters (Watts, GPM and RPM) which are unique at each operating point on any plumbing system. So theoretically, if you know two, you can determine the third.
The VF pump does not have any more sensors than any of the other VS pumps. It determines flow rate from Watts and RPM both of which are known, watts from a measurement and RPM from the drive setting. So the software simply looks up the appropriate GPM value that corresponds to the Watts and RPM measurements. Then through a hunting loop, they can zero in on a particular GPM setting by adjusting the RPM.
But the problem is that the slope of watts/GPM for a given RPM is quite shallow so when one inverts the equation to determine GPM, if there is a small error in watts, there can be a huge error in GPM. I believe Pentair calibrates the drive in such a way to maintain this accuracy but unfortunately the calibration tables are not available to the public.
Pentair publishes energy data in graph form (i.e. back of the manual) and there is also the California Energy Commission's limited measurements (2 RPMs and 3 plumbing curves), which by the way do not seem to match the published data in all cases so there is variance in the data as well which complicates things.
However, I do have a power equation that I use for the Intelliflo and works well when you know GPM and RPM and are looking for watts. But like I said the error tends to grow when inverting the equation to determine GPM from watts. The problem gets even worse at lower RPM.
So I have come up with several different methods to estimate pump flow rates from RPM and not directly from watts and seem to be more accurate.
The first method is the least accurate but only requires a somewhat vague knowledge of your plumbing setup. I have developed a set of plumbing curves with descriptions and you just need to match the description of your plumbing to that of the standards. This method is described in the Hydraulics sticky below. For each plumbing type there is a constant that when divided into RPM, will yield GPM.
The second method uses a description of your suction side plumbing in conjunction with your filter pressure and pump speed. This is a bit more accurate because it uses the filter pressure for the return side head loss. Only the suction side head loss is estimated from the plumbing description. To estimate flow rate for this, I would need just the filter pressure, gauge height above water level, type of backwash valve and a detailed description of the suction side plumbing (valves, distances, pipe sizes, # runs, etc).
The third method which is most accurate, requires a measurement of both the filter pressure and suction at the pump. Head is then calculated directly and flow rates determined from the head curves. Of course the accuracy is still limited to the accuracy of the gauges and to the head curve.
Also, it is important to do the calculation at high speeds and then use the pump affinity equations for the lower speeds because it is very difficult to make measurements of any sort at low speed.
It turns out that using the Watts method was no better in accuracy than the first method which is why I now offer three alternatives. In fact, I am skeptical that the VF is actually setting the flow rate to any sort of accuracy although Pentair claims it is within a few percent.
But I always have to ask, why do you really need to know your flow rate? Usually a rough estimate is all that is needed.
Here is a table I put together that shows the relationship between Watts, GPM and RPM for both the Intelliflo and EcoStar pumps. However, keep in mind that these values are extrapolated and interpolated from the 6 CEC measurement points so there is likely to be some error:
Watts, GPM & RPM Table
