Interpolation of the current loop in the setup of the flow meter without documentation.

One day, I faced an unusual challenge: configuring a water flowmeter. The device measured flow using a 4–20 mA current loop. Usually, this isn’t a big deal—grab the documentation, look up the correspondence between current and flow, and you’re good to go. But in my case, there was no documentation.

What to do? I had to get creative and find a solution experimentally.

What was the goal?

I needed to determine the flow rates corresponding to the minimum and maximum current values (4 mA and 20 mA) and calculate the intermediate values. The data I had was scarce: only the flowmeter readings and the measuring device. Eventually, I decided to use interpolation.

What is interpolation?

Simply put, interpolation is a way to calculate values between known points. For instance, if 4 mA equals 0 m³/h and 20 mA equals 50 m³/h, interpolation lets you find any flow rate in between. The formula is straightforward:

Where:

• P — the flow rate you’re looking for,
• I — the current output by the device,
• Pmin​ and Pmax​ — the minimum and maximum flow rates,
• Imin​ and Imax​ — the minimum and maximum currents (4 and 20 mA).

How does it work?

Let’s break it down with examples. Here’s a table showing flow rates for different current values:

What was the result?

Of course, the task wasn’t super simple: I had to manually adjust the values, relying on the measuring devices. But interpolation saved the day! With its help, I restored the missing data, calculated intermediate values, and configured the device to work correctly.

This experience reminded me that math isn’t just formulas we memorized in school—it’s a powerful tool for solving real-world problems, even when you feel stuck.

Now, I always know what to do if documentation goes missing: turn on your brain, recall the formulas, and take action!