I've just bought myself a £20 ORP meter off ebay.

Unfortunately it comes with no calibration solution.

The seller tells me it is precalibrated and will not need recalibrated for several months.

So I dunked the end into my newest planted tank (only 3 weeks old) and it reads 165mV after 2 mins.

Some de-ionised water reads 126mV after 5 mins.

My scottish soft tap water is only 99mv.

I was of the opinion that a good redox value was 300mv!

As this is my 1st ORP meter can some wise person guide me?

Is it broken?