Note: This TIL only applies to Go!Link, LabQuest 3, original LabQuest, and LabQuest Mini. It does not apply to LabPro, LabQuest 2, or LabQuest Stream.

The 10 V Voltage Probe (VP-BTA) does not read zero when the leads are not connected to anything, which is expected behavior and not a defect.

Potential difference is defined between two points. When the clips of the voltage probe are connected together, they are at the same point in a circuit and the reading will be zero. When one or both clips are disconnected, the leads will float to a non-zero internal reference potential that depends on the interface. The input will usually read 1.4 V (Go!Link), 3.0 V (LabQuest 3), 3.3 V (original LabQuest and LabQuest mini) when the leads are disconnected and floating. If you connect the two probe leads together, or connect them both to a wire, you will see that the signal is 0 V.

In practice, when using the Voltage Probe (VP-BTA), make sure you avoid taking a reading when the voltage probe is not connected to anything. This is not a meaningful reading; ignore it. If you connect the two voltage leads together and you do not get a reading of zero, or very close to zero, then something else is wrong. If you set up the VP-BTA with the leads not connected to anything, note the non-zero reading, and then zero the sensor within the software, this will cause the readings to have a constant offset. You can see this by connecting the two leads of the voltage probe and checking the reading. Only zero the voltage probe when the leads are clipped together.

If you want to avoid this behavior, use our Differential Voltage Probe (DVP-BTA). The output from this sensor will be very close to zero, even when nothing is connected. 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 simple 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)?