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?

Redox meters need to sit for awhile prior. Give them 30 min or more.
Also, any stray current will reduce the readings etc.

Regards,
Tom Barr

So best to put a sample of the water into a cup and leave it sitting for 30mins is what you're saying?

Goodness, didn't read that it took so long to do a test.
Thanks.

It takes awhile to get steady readings often.
Stray current will mess with it, same with pH etc.
Turn lights off etc.

See how much drift it has.
Check with a calibration set up.
I use a calomel ref electrode for the Redox meters, that way it's always references when I take a reading, they ain't 20 pounds though

You might want a new probe for good keeping.
Not sure.

Regards,
Tom Barr