How to measure % Dissolved Oxygen in water?

Is there anyone out there who has a kind heart to help me?

For an arduino to read the value constantly or for a human user?

How about using a DO (dissolved oxygen) sensor?

zwieblum: For an arduino to read the value constantly or for a human user?

For an Arduino

As wvmarle wrote google for "Dissolved Oxygen Sensor" ... but it's not cheap. There are some handheld warter quality meters from PCE that come with rs232

I had a similar requirement few months ago. I ended up using this sensor

https://www.ebay.com/itm/254255019357

Instead of providing data via a serial port, it connects via Analog Pin on the arduino.

It does have some UART commands for calibration I suppose, there is arduino code at the bottom of the description which explains the same.

The UART commands will go to the Arduino, not to the sensor. There's a big chunk of code on that page, I didn't analyse it.

Calibration is as usual: take a few samples with known DO, see what value the sensor gives. If linear, two points are enough (or if the slope is known a single point can do). That gives you the calibration value and allows you to calculate the DO of an unknown sample.