Multimeters operate differently than our sensors. Multimeters are designed to average the data and update their display only once a second or so. Our sensors, on the other hand, sample data much faster, updating several times a second. Because some power supplies are "noisy" or because of grounding loops, fast updates may make the reading appear to change erratically.
Noisy Power Supplies
In many power supplies, there is a 120 Hz ripple in the signal from the wall outlet to which the power supply is connected. Sometimes this noise can be measured on the multimeter by using the AC mV setting, but a multimeter usually masks the noise by averaging several readings before displaying a value.
When collecting data from a noisy signal like this with a Vernier sensor, the noise typically does not look like a sine wave because the sampling rate is set much lower than the frequency of the noise in the signal. Faster sampling rates can alias the signal and produce a nice sinusoid.
To verify that your Voltage Probe is working correctly, the best test is to measure the voltage across a battery. Because the battery is a simple DC voltage, there are no problems from signal noise.
Another simple test is to keep your circuit the same and set the software sampling rate to 1000 points/second for 0.1 seconds. If the ripple is the problem, you will see a drastic variation in the voltage with a period of 60 or 120 Hz (in North America), or 50 or 100 Hz (outside North America).
If you are seeing a lot of noise with your Vernier voltage sensor, you can correct it by trying the following:
1. Get a different power supply, use batteries instead of a power supply, or add a capacitor to the output of your power supply to filter its output.
2. Or to minimize this problem in software, you have two options:
A) When available, turn on Oversampling in dialog box where you control the data collection rate.
B) You can also set the sample rate to a number not a factor of 60 [or 50 outside North America] to improve things.
The voltage reading noise may also be a result of grounding loops, when the power supply ground reference does not match the ground reference of the interface/computer recording the signal.
Additional grounding issues can occur when using both a Current Probe, order code DCP-BTA, $39 and Voltage Probe, order code VP-BTA, $12. For example in an Ohm's law experiment, the negative terminal of the current probe needs to be connected to the negative terminal of the voltage probe. This will introduce a typically small error in the voltage readings, but should eliminate many grounding issues. Alternatively, these issues will be eliminated if a Differential Voltage Probe, order code DVP-BTA, $39 or Go Direct® Voltage Probe, order code GDX-VOLT, $69 is used to measure the voltage.