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DC Metrology
11 questions By Tony R. Kuphaldt
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Question 4 of 11
If a voltmeter is to be used to directly measure the voltage of an unknown source, it must first be calibrated so as to ensure an accurate measurement:
What is the minimum number of points along the meter’s range that it needs to be calibrated at, given the assumption of perfectly linear response?
If a voltmeter is to be used to measure the voltage of an unknown source, as a differential indicator only, what is the minimum number of points along its range that it needs to be calibrated at?
Reveal answerA perfectly linear voltmeter, used to measure voltage directly, needs to be calibrated at two points along its measurement range in order to ensure measurement accuracy.
A voltmeter used for differential measurement need only be calibrated at a single point along its range, and that single point is zero.
Notes:Ask your students if the two calibration points on the meter’s range should be close together, or far apart, for the best (most comprehensive) calibration possible.
Discuss the easier calibration requirements of the differential meter. Challenge your students with this question: “Does the measurement linearity of a voltmeter matter as much if it is used to make a differential measurement, as compared to if it is used to make a direct measurement?” Why or why not?
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Question 5 of 11
Explain the simplest way to perform a single-point calibration on a highly sensitive, precision voltmeter. How do you ensure the voltmeter is receiving a fixed input of known quantity, especially without having expensive calibration equipment available?
Reveal answerShort the voltmeter’s test leads together. This creates an input condition of zero volts.
Notes:Challenge your students with this question: “If zero is an appropriate signal to use for a single-point calibration, then why not just leave the two test leads disconnected? Why should you short them together?”
If you happen to have a sensitive voltmeter available in the classroom, the answer may be demonstrated with ease. This is especially true if you place the voltmeter in the “AC millivolt” range so it picks up stray power-line voltages from nearby electrical devices and utilities.
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Question 6 of 11
This voltage divider should output half the battery’s voltage between points A and B:
However, if you perform this same experiment using a real voltmeter, the measurement obtained with the meter will be substantially different from what should be there, based on a prediction of 1/2Ebattery. Explain why the voltmeter registers as it does. What is it about this circuit that causes the measurement to be so far off from the prediction, when we know full well that other voltage divider circuits we’ve constructed do not exhibit any significant error?
Reveal answerThis circuit is a demonstration of voltmeter “loading” on the circuit, by causing a falsely low measurement.
Notes:Meter “loading” is a serious problem in electrical metrology. It is a basic principle of measurement that a measuring instrument always impacts the quantity being measured, to some extent. In cases like this, the extent of impact is severe.
For those instructors with some background in quantum physics, please refrain from perpetuating the myth that meter loading is an example of Heisenberg’s Uncertainty Principle. The Uncertainty Principle has nothing to do with the impact of a measuring instrument on something we measure. Rather it describes an uncertainty inherent to the quantity itself. If you want to share a true electrical example of this uncertainty principle with your students, wait until they study harmonics and spectrum analyzers, where you can tell them it is impossible to measure both the instantaneous amplitude of a signal and the frequency of a signal with unlimited certainty.
