PH meter reading fluctuating

[ciscube]

Hi,

I just bought a Taiwanese ph meter with Dymax probe. But I found that the meter reading was fluctuating too much .. from 6.5 to 7.5.

I did a few experiments as below:

1. When I switched off my T5 lights, the reading will be stabilised at +/-0.01.
2. With the light switched on, I did a PH measurement on a cup of water at the side of the tank, away from the light. The reading is also stable.
3. By raising up the lightset by 6in with the probe inside the tank, the reading is also stable.

It seems that the light is affecting the reading when the probe is inside the tank.

Any advise on how to solve the problem? I wish to have ph reading when the light is switched on.

Thanks,

[BFG]

Could be a current leakage from the light set. Is the light set and the ph meter plugged from the same multi plug?

[Shadow]

EMI (Electro Magnetic Interference) issue?

[ciscube]

No. They are not using the same plug.
I'm suspecting that the probe is picking up interference. I've moved the probe and meter around the tank, but not a single position worked
It's only when I raised up the lightset, then the reading is stabilised.