Basic Information About DC and Low Frequency Measurements
A very big part of electrical calibration procedures consists of DC and Low Frequency (LF) measurements. Multimeters, resistors, power supplies and electrical testers are commonly used instruments that require periodical calibration. Voltage, current, resistance, inductance, capacitance, time and frequency are the principal quantities regarding DC and LF measurements. The Volt (V), the Ohm (Ω) and the Ampere (A) are examples of the basic units used for electrical measurements.
There is a close relationship between mathematics and measurements. All the quantities measured have two parts – the name of the unit and their numerical value. Moreover, the electrical units can be related to each other by using various mathematical equations. For example, the most commonly used equation for DC and LF measurements, Ohm’s Law (V = I x R), shows how the voltage, the current and the resistance in a DC circuit are strictly related to each other. If the values of any two of the above units are known, the value of the third can be calculated.
The SI Volt is a key unit in DC and LF metrology and is defined as power divided by current. Its definition is realized by experiments that compare electrical power against mechanical power via a force-balance. The results of these experiments are used to assign values to the voltage produced by inexpensive, stable, easily reproduced and maintained devices, such as electromechanical standard cells or electronic zener-stabilized voltage standards. These devices represent the SI Volt because their principles of operation do not involve a continuous comparison of electrical power with the power produced by realizations of the SI mechanical units.
The best available representation of the SI Volt is obtained from a Josephson Array. The voltage produced by this device is a function of the frequency of microwaves that irradiate it and the Josephson constant, a universal quantity independent of experimental variables.
The values of the national representations of SI units must be transferred to the representations of the units used by local calibration laboratories. The devices or artefacts which store or maintain all such representations, national or local, are conventionally called standards. The best local standards are characterized as primary standards. Their values are transferred to other local standards, usually called working or secondary standards. The values of the working or secondary standards are then transferred to devices, usually called calibrators, used to extend the value of the SI unit to a wide range of additional values.
In order to perform the best possible measurement, the metrologist must select the appropriate equipment and use it correctly in effective test configurations. The selection of the suitable equipment requires an extensive knowledge of at least the following issues:
- Types of measurement equipment
- Their principles of operation
- Imperfections of equipment and test configurations
- Types of measurements that can be performed
- Purposes of calibration measurements
Some of the generic types of calibration equipment used for DC and Low Frequency measurements are presented below:
These standards usually include the devices that SI units from primary standards are transferred to. Secondary standards are used to calibrate calibrators or to increase the performance of calibrators when extremely accurate measurements are required (i.e. calibration of a laboratory DMM).
Null Detectors are meters which measure voltage differences. When the difference is zero Volts, the meter’s reading is at null. A typical application of a null detector is the measurement of the difference in the output of two resistive voltage dividers whose inputs are in parallel across a voltage source. There are digital and analog null detectors. Typical analog null detectors are actually galvanometers and contain a pointer resting at the middle of a deflection scale when there is zero input voltage. The pointer can move in both directions, indicating in this way the polarity of the difference as well. Digital Null detectors are much more sensitive due to their ability to amplify the voltage difference. They can measure accurately a null between voltage amplitudes from < 1μV up to 1000V.
Calibrators are the most common equipment found in an electrical calibration laboratory. They are widely used to calibrate general purpose test equipment such as DMMs and oscilloscopes. Most calibrators are multifunctional. These means that they can provide a wide range of values for a number of different SI units such as DC voltage and current, resistance, low frequency (and sometimes even RF) AC voltages and AC current. Multifunction calibrators can be used to calibrate even 5 1/2 digit multimeters. Nowadays calibrators can be operated either manually or controlled by a computer by using the appropriate software. These automated measurements help in reducing the time needed for calibration and are more reliable since there is no user interference.
Test leads comprise of the wires, cables and connectors that are used to connect the measurement equipment and the instrument under test. Test leads must be compatible with the measurement that is being performed. For example, when measuring a very low level DC voltage, the low thermal emf error can be avoided by using connectors manufactured by the same material as the instrument’s connectors.
Types of Measurements
When performing DC and LF measurements, several types of measurements can be applied:
Direct measurements are performed when placing an instrument in direct contact with the phenomenon that needs to be measured. For example, when setting a calibrator to provide a DC voltage and then connect a handheld DMM to its output, then the DMM is conducting a direct measurement. The DMM’s display will indicate directly the DC voltage provided by the calibrator.
Differential measurements are performed when using an instrument, such as a null detector, to measure the difference between a known and an unknown quantity of the same value. This method is sometimes more accurate and provides higher resolution than performing a direct measurement.
By conducting a transfer measurement, we actually transfer the value of a known quantity to the quantity under test. An example of a transfer measurement is when providing a voltage across two in series connected resistors. The one is the standard resistor (with a known value) and the other is the resistor under test. The value of the standard resistor is transferred to the resistor under test, by applying Ohm’s Law:
Vstd x Rstd = Vtest x Rtest =>
Ratio measurements are commonly used in DC and LF metrology. The transfer measurement described above is also a ratio measurement since the value of the standard resistor is transferred to the resistor under test via a voltage ratio.
By using indirect measurements, someone can find the value of interest from other values. For example DC current can be calculated (by using Ohm’s Law) when measuring the voltage drop across a known resistance, when the current is passing through that resistance.
Types of Calibration Measurements
By the term Calibration, any one of the following types of calibration may be suggested:
- Verify the performance of the instrument under test
- Adjust the response of the instrument under test
- Provide correction factors for the instrument under test
Depending on the customer’s requirements, any or all of the aforementioned methods can be applied. For example when a DMM is being calibrated for DC voltage, we are setting the calibrator to provide 10 V DC to its input. We observe the reading of the DMM’s display which is 9.95 V. We have just verified the performance of the DMM. If we find it to be out of the specifications, we can perform adjustment (either by hardware i.e. a potentiometer, or by software for newer models). This is the adjustment of the response of the DMM, which now reads 10.00 V. If an adjustment is not possible or the customer has requested to provide only the correction factors, we can state that the reading of the DMM is 9.95 V and the correction factor is 0.05 V.
Of course, all the above information must be properly mentioned and documented in a Calibration Report, providing the measurement uncertainty for each measured value.
Written by Sofia