True RMS Measurements

Appendix G

Introduction

The instrument measures the true value of ac voltages. In physical terms, the rms (root- means-square) value of a waveform is the equivalent dc value that causes the same amount of hear to be dissipated in a resistor. True rms measurement greatly simplifies the analysis of complex ac signals. Since the rms value is the dc equivalent of the original waveform, it provides a reliable basis for comparing dissimilar waveforms.

Effect of Internal Noise in AC Measurements

With the input shorted and the channel set for ac volts (VAC) measurement, internal amplifier noise causes a typical display reading of approximately 0.50 mv ac. Since the instrument is a true rms responding measurement device, this noise contributes minimally to the readings at the specified floor of each range. When the rms value of the two signals (internal noise and range floor) is calculated, the effect of the noise is shown as:

Total rms digits = Square Root of (0.502 + 15.002) = 15.008

The display will read 15.01. At the 28.00 mV input level on the 300.00 mV range in the slow rate, the display will read 28.00 with no observable error.

Waveform Comparison (True RMS VS Average Responding)

Figure H-1 illustrates the relationship between ac and dc components for common waveforms and compares readings for true rms measurements (Hydra) and average- responding measurements. For example, consider the first waveform, a 1.41421V (zero- to-peak) sine wave. Both the instrument and rms-calibrated average-responding measurement devices display the correct rms reading of 1.0000V ac (the dc component equals ). However, consider the 2V (peak-to peak) square wave.

Both types of measurement correctly display the dc component (0V), but the instrument also correctly measures the ac component (1.0000V). The average-responding device measures 1.11V, which amounts to an 11% error.

Average-responding measurement devices have been in use for a long time; you may have accumulated test or reference data based on such instruments. The conversion factors in Figure G-1 can help in converting between the two measurement methods.

G-1

2635A specifications

The Fluke 2635A is a highly advanced multichannel data acquisition system designed for engineers and technicians requiring precision and versatility in their measurement tasks. This instrument excels in monitoring and capturing data from various sources, making it a valuable tool in laboratories, production environments, and field applications.

One of the standout features of the Fluke 2635A is its ability to support up to 20 measurement channels. Users can simultaneously monitor multiple signals, which is crucial for complex testing scenarios where numerous variables need to be tracked. The device is equipped with a 6.5-digit precision measurement capability that ensures high accuracy and resolution in readings. This level of precision is particularly beneficial in applications such as environmental monitoring, research and development, and industrial process control.

The unit incorporates a variety of measurement modes, including voltage, current, temperature, and resistance, making it exceptionally versatile. The Fluke 2635A not only handles standard signals but also integrates with various transducers, providing extensive compatibility for users engaged in diverse testing applications.

To enhance data management, the Fluke 2635A offers advanced data logging capabilities. Data can be stored internally or transferred to external devices for further analysis. The internal memory can accommodate extensive datasets, which can be crucial in long-term monitoring tasks. Integration with external software enables users to analyze and visualize data effectively, allowing for informed decision-making based on comprehensive results.

The user interface is designed for ease of use, featuring a large, bright LCD display that clearly presents measurements and status information. The device is also equipped with a simple navigation system, allowing users to switch between channels and measurement types without hassle.

In terms of construction, the Fluke 2635A is built to withstand rigorous environments. It features a robust design that ensures reliability and durability, making it suitable for both laboratory settings and field operations. Furthermore, the device supports a wide temperature range, thus enabling usage in diverse conditions.

Overall, the Fluke 2635A is a powerful and flexible instrument that provides precise measurements across multiple channels, supporting a wide range of applications. Its advanced technology, combined with user-friendly features, makes it an essential tool for professionals in various fields. The combination of accuracy, flexibility, and durability solidifies its standing as a trusted choice for data acquisition and monitoring tasks.

Fluke 2635A user manual True RMS Measurements, Effect of Internal Noise in AC Measurements

Models: 2635A