The problem is that the ULI, or SBI, can only read positive voltages. Listed below are several options to read both positive and negative voltages:
A) Use our Instrumentation Amplifier (takes +/- voltages and amplifies them so that they can be read by the ULI (voltage ranges include +/- 20 mv, +/- 200 mv, +/- 1000 mv.
B) Use our Current and Voltage Probe System. The voltage range using this is +/- 6V.
C) Put a battery in series with the voltage probe of the ULI to offset the voltage you are measuring by 1.5 volts.
D) Build a simple op-amp circuit to handle the input and feed it to the ULI voltage leads.
E) Use our Go! Link or LabPro interface with a voltage probe. Both of the newer interfaces have a range of +/- 10V.