Diaphragm Type Metering Pumps

This blog will go over the simplest ways to change the flow rate on a Madden metering pump. To start, all Madden chemical metering pumps are designed with a manual stroke length adjustment built right into the pump. This allows for a very linear turndown of the pump’s output or flowrate and it is quite simple. A knob and dial scale on top of the pump allow you to turn down the ouput at a 10:1 ratio of your pump model’s max capacity.

But if you’re looking for something to easily control the output of your chemical injection pump, that also has automatic control capababilities, a simple Variable Frequency Drive could be just the thing you need.

So how do they work?

Variable Frequency Drive (VFD), a Quick Explanation

VFDs are able to increase and decrease the frequency (or hertz) that passes through them. In our case with metering pumps, this would be the power supplied to the motor. The frequency supplied to the motor is directly related to RPM’s. For a standard Madden pump motor, either single phase or 3 phase power is supplied to the motor at 60 Hz. This will run the motors at 1,725 rpm’s, giving you the rated outputs you see in our literature.

One of the special features of a Madden positive displacement metering pump, is that changing the flow rate via motor speed is very linear. For example, if you have an MF136A Madden metering pump which has a max output of 36 GPH, turning down the VFD to 30 Hz will result in a flow rate very close to 18 GPH.  This can’t be said for many other types of pumps. And if you’re looking to get an exact output via motor speed change, that’s easy too! Simply calibrate your pump using a VFD rather than the manual stroke length adjustment knob. This will allow you to make your own pump curve specific to your application’s conditions, which in turn will show you what you can expect your flow rate to be at any setting on the VFD.

What all can you use a VFD to do with a Madden metering pump?

Manually Turning the Pump’s Flow Rate Up or Down

The most obvious and simplest thing you can use a VFD  to do has already been mentioned and that is to manually speed up and slow down the motor from the digital display.

But why is this beneficial? Why would you use it?

Though it would be unlikely and unnecessary to buy a VFD just to manually speed up and slow down your pump’s motor, if you had one available here’s an example of when you would use it. Many metering pumps are used in green house applications to dose small amounts of Oxcide into the irrigation system. Oxcide helps reduce scale build up in the green house. Sometimes workers are the best judges of how much more or less oxcide is needed. For example, when scale build up is clearly visible in a green house, they may go over and turn the pump’s VFD up which increases the motor rpm’s and therefore increases the pump’s output. It’s as simple as hitting the up arrow on a keypad.

Now, having a VFD with a digital display that allows you to speed up and slow down your pump’s flow rate with the push of a button is nice, very convenient and easy to do. But that only scrapes the surface of ways the VFD can help you with in your metering application.

Remotely Turning the Pump’s Flow Rate Up or Down

With a VFD wired to the motor and back to a control system, an operator could turn up or slow down the pump’s flow rate from their desk! In an ever changing world leaning towards more and more automation, we realize our pumps are no longer always serviced by a physical operator every day. The convenience of adjusting your pump from a desk could be just what you need to efficiently run your application.

Automatically Controlling the Pump’s Flow Rate Up or Down

And finally, with the right programmable logic controller (PLC), and a programmer, you could use your VFD wired Madden chemical metering pump to adjust its flow rate automatically. For example, back to the green house, you could program your PLC to read the flow rate of the main city water lines being used for irrigation, and tell it to send a signal to the VFD to slow down or speed up the motor to keep the metering pump’s flow rate at a consistent proportion to the irrigation water.

Whether you’re dosing oxide into the irrigation, dosing sulfuric acid into water to treat it, dosing caustic soda, or anything else into another process, a VFD and PLC working together to control a Madden pump will ensure you are dosing the exact amount of chemical the process needs. It will do this for you, automatically, and you’ll know you aren’t wasting money in excess chemical dosing.


If you are in the market for a heavy duty chemical metering pumped designed to run 24/7/365 that can also be set up to be a very flexible and adaptive pump, give a Madden metering pump a try. Our product’s quality along with our company’s responsive customer service help us compete in the massive metering pump industry.

Contact your local Madden Rep (link), or give us a call or email directly to give our smaller, family owned company a chance to prove our product is one of the best on the market! Thanks for reading, and have a great day!