Due to the simple nature of the sensor, the ±10 V Voltage Probe (VP-BTA) may not read 0 V when the leads of the sensor are not connected to anything. While not desirable, this is expected behavior for this sensor. The value you get depends on which interface you are using (see the table below). If you connect the two voltage leads of the ±10 V Voltage Probe together and still do not get a reading very close to 0 V, then something else may be wrong.

Potential difference is defined between two points. When the clips of the ±10 V Voltage Probe are connected together, they are at the same point in a circuit and the reading is expected to be 0 V. When one or both clips are disconnected from your circuit, the leads will float to an internal reference potential that depends on the interface. If you connect the two probe leads together, or connect them both to a conducting wire, you will see that the sensor reading is 0 V.

In practice, when using the ±10 V Voltage Probe, avoid taking a reading when the voltage probe is not connected to anything. When one or more of the leads are not connected, the reading is meaningless and should be ignored. A common student mistake happens when they notice a non-zero reading and mistakenly zero the sensor while one or more of the sensor leads are not connected to anything. Doing this applies a constant offset to all future sensor readings. This will make the reading when the leads are connected together be non-zero when otherwise would have been 0 V. Because of this, it is import to remind students to only zero the voltage probe when the sensor leads are clipped together.

If you want to avoid this behavior entirely, use our Differential Voltage Probe (DVP-BTA) or 30-Volt Voltage Probe (30V-BTA). The output from these sensors will be very close to zero, even when the leads are not connected to anything. In addition, the Differential Voltage Probe has the significant advantage of being a true differential measurement and does not ground any part of the circuit. In other words, it behaves like a voltmeter, unlike the ±10 V Voltage Probe.

For related information see:
Is it possible to use the Voltage Probe (VP-BTA) when doing an Ohm's Law experiment, or is the Differential Voltage Probe (DVP-BTA) or Go Direct Voltage Probe (GDX-VOLT) required?
What is the difference between a Differential Voltage Probe (DVP-BTA) or Go Direct Voltage Probe (GDX-VOLT) and the simple Voltage Probe (VP-BTA)?