Vernier Tech Info Library TIL #690

Question

Voltage Probe does not read correctly on LabPro.

Answer

You are probably assuming the wrong input line is being used. On LabPro and CBL there are two voltage input lines: Vin (-10 to +10 V) and Vin Low (0 to +5 V) . Most of our sensors use the Vin Low line. The Voltage Probe that comes with LabPro uses the Vin (-10 to+10 V) line.

If you are using Logger Pro and a computer, plug in the Voltage Probe and start Logger Pro, or choose New from the file menu to set things up for using that Voltage Probe. Another option is to choose Setup/Sensors and set the channel for Voltage (-10 to +10 V) manually.

If you are using a calculator and DataMate, the channel will be set automatically when you plug in the Voltage Probe.

The voltage probes that ship with LabPro are all -10 to +10 voltage probes. The ones that we used to ship with ULI and Serial Box Interface were 0 to +5.-

Here is another example of how this can cause confusion. If you are using a -10 to +10 volt probe and the program thinks you are using a 0 to 5 volt probe, then you will get what seems to be meaningful readings, but they are wrong. The reason is that the two different types of voltage probes use different wires for input. If nothing else is hooked up to (for this example) the 0 to 5 volt input line, and you are applying a voltage to the -10 to +10 volt lead, then the 0 to 5 volt input may somewhat follow that voltage and trick you into thinking it is working, but it will not read correctly.

The message of all this is that if you are getting suspicious readings from a voltage probe and Logger Pro, always go to the sensor setup dialog box (by clicking on the LabPro symbol) and check whether the probe is -10 to +10 V or 0 to 5 V.

Created by: dvernier on July 17 2000
Last updated by: dvernier on September 10 2001