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:
| Current (мА) | Volume water (м³/h) |
Formula |
| 4.0 | 0.0 |
$$ P = \frac{50 \cdot (4 - 4)}{16} = 0.0 $$ |
| 6.0 | 12.5 |
$$ P = \frac{50 \cdot (6 - 4)}{16} = 12.5 $$ |
| 9.0 | 28.1 |
$$ P = \frac{50 \cdot (9 - 4)}{16} = 28.1 $$ |
| 12.0 | 37.5 |
$$ P = \frac{50 \cdot (12 - 4)}{16} = 37.5 $$ |
| 16.0 | 46.9 |
$$ P = \frac{50 \cdot (16 - 4)}{16} = 46.9 $$ |
| 20.0 | 50.0 |
$$ P = \frac{50 \cdot (20 - 4)}{16} = 50.0 $$ |
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!