I am a firm believer there is no such thing a a "dip test" for ORP especially more so in salt water. Depending on the quality of the probe it can take anything from 2 days to 2 weeks for the ions to attach themselves to the probe to read the ORP value correctly.
My point here is, if reading ORP do it as a monitored measurement over time, and not with a hand held dip tester.
Avoid cheap probes (often found on fleebay), the platinum can be thin or not pure and the ORP readings can vary hugely from probe to probe.
It is common to see a difference in readings from monitor to monitor anything up to 50 points, as Shawn stated, ORP really is not an exact science. It should also be seen as a reference point rather than a specific reading, this is why suggest an Ozone controlled reading not above 400mV.
Is ORP any good for non Ozone injected systems? Yes once the probe has settled keep note of this reading, expect the reading to rise also, this does not always mean the water quality is getting better, what is more important is looking out for a drop in a reading, this indicates a potential water quality issue (overfeeding, dirty filter sock, poor skimming, or someone just put their hands in your sump
Calibrate once a month some probes have a habit of over reading if left.
Calibration - use the fluid the manufacturer recommends, 400mV works with Pinpoint and many others but does not with ProfiLux and others just as an example. Profi uses 220mV as it is the expected mid point.
