ORP Sensor Problem

You don't say what the water sample is that you're testing - waste water, drinking water, ground water, pool water, etc. so it's not possible to assume what reading you should expect, you also don't mention how negative the value is..... An ORP/redox meter is simply a mV meter, it measures the voltage across a pair of Platinum or gold probes ,compares that to a reference cell and gives a reading, it is basically measuring the amount of dissolved oxygen in the water...... Generally less dissolved Oxygen indicates higher contaminant levels and a lower mV reading..... Assuming that the calibration has been done correctly and the probes and meter are in working order, then the state of the test sample has to be considered as the cause of the negative reading...... Positive readings always indicate oxidation potential - ie. higher dissolved oxygen and better ability to disinfect. The solution is drawing electrons from the probes....... Negative readings indicate reduction or anti-oxidizing potential - ie high contaminant levels or a highly alkaline solution. The solution is supplying electrons to the probes. High turbidity can also result in negative readings...... It's unusual for any two meters to have identical readings, often due to differing calibration, contaminated probes, differing reference cell concentrations etc., so go with the one you trust most.