How to Measure Physical Quantities

Measurement of Physical Quantities

When a measurement is made (such as with a thermometer, a caliper, or a gauss meter) a numerical quantity is determined from a physical quantity, at a specific time and location. It is important to define exactly what is being measured: is it all three components of a vector, one component of a vector, a magnitude, or a scalar? Also, are you measuring the instantaneous value (DC value), or the range of variation (AC value) of the quantity? If AC is chosen, there are various ways to measure this number (RMS, absolute average variation, peak-to-peak, peak negative or peak positive). With AC, frequency range is also an important consideration, as well as the averaging time.

Most measurements are displayed as a digital quantity (as actual numerals). The number of digits of the display is another way to express the dynamic range of the display. For example, a four-digit display that reads only positive numbers or zero (typical of AC measurement) has a dynamic range of 10 000:1. That is, it reads 10 000 different numbers (0 to 999 counts with a resolution of “1”). A four-digit display that reads both positive and negative numbers (typically of a DC, or “instantaneous” measurement) can read just under 20 000 different numbers (0 to 9999 and –1 to –9999) so its dynamic range is about 20 000:1. A 3 ½ digit meter (the most common type) reads up to +/- 1999, while a 4 ½ digit meter (most AlphaLab meters are 4 ½ digit) reads up to +/- 19 999. Most meters allow you to switch the display to other maximum values, effectively increasing the dynamic range by a factor of 10 X or 100 X. Typically, a meter might have three ranges of +/- 199.99, 1999.9 and 19999 of whatever unit is being measured, such as gauss. A high dynamic range is useful because it allows you to see small changes even if the actual number is big. For example, you can se the difference between 1939.6 and 1939.7 gauss with a gauss meter that has a 4 ½ digit display, but you can’t see that subtle difference with only 3 ½ digits.

While a measurement is being made, there may be some fluctuation or variation in the displayed number. Part of this fluctuation may be due to the fact that the physical quantity is actually changing; the remainder of the fluctuation is due to noise in the meter. This fluctuation can be reduced by changing the meter so that it displays an average value over a long sampling time, rather that displaying the instantaneous value. Although such a change will reduce the variation over time, it will also slow down the response speed of the meter, so that several seconds may be required to make each measurement. In most types of meters, the last digit (least significant or right-hand digit) may fluctuate up and down by one count regardless of how much averaging is done.

Overall accuracy is an important issue, and is indicated by both gain and offset errors. The “offset” is what the display reads when the meter should be reading zero; this is like the tare weight of a scale. This offset is usually expressed as being at maximum a “+/- number”, with units, such as +/- 5 gauss. The offset error can usually be removed by dialing an offset knob so that the display reads zero (while the meter is held in zero field so it is supposed to read zero), or by pushing a button usually called “OFFSET”, “ZERO”, or “REZERO”. While the meter is held in a location where it is supposed to read zero. The gain error is usually expressed as a percent of the reading, so for example, if the meter reads “1000.0 gauss” and the error is at most +/- 2%, then the actual field is between 1000 – 20 gauss and 1000 + 20 gauss (between 980 and 1020 gauss).