CHAPTER 6

Indicating, Recording & Display Elements

Introduction

• The final stage in a measurement system comprises an indicating and/or recording element.

• Digital voltmeters (DVMs) are commonly used as these are convenient.

• Cathode ray oscilloscopes (CROs) are also widely used.• In large-scale systems, data loggers incorporating data

acquisition systems (DAQ) are extensively used for data recording.

6.1 Digital Voltmeters (DVMs)• Convert analog signals into digital presentations which

may give an electrical digital signal.• Measures DC voltage signals, but other variables can also

be measured with appropriate elements.• Classified into two types:

1. Non-integrating

2. Integrating• A voltage-to-frequency converter, which converts DC

signal to a periodic signal of frequency proportional to the magnitude of the signal.

Fig. 6.2 Voltage to frequency integrating DCM

6.2 Cathode Ray Oscilloscopes (CROs)

• High input impedance voltage measuring device, capable of indicating voltage signals from the intermediate elements as a function of time.

• Electrons are released from the cathode, accelerated towards the screen by the positively charged anode.

• The position of the spot on the phosphorescent screen is controlled by voltages applied to the vertical and horizontal plates.

• The impingement of the electron beam on the screen results in emission of light, thus the signal is visible.

• Essential components in a CRO: Tube, Vertical amplifier, Horizontal amplifier, Time base, Trigger, Power supply.

6.2 Cathode Ray Oscilloscopes (cont.)• Digital CRO can store, analyse and display a signal.• It uses digital acquisition system including an analog to digital

converter (ADC), digital memory, digital to analog converter (DAC) and a display system.

• The frequency response is limited by the sampling frequency.

Fig. 6.3 Block diagram of a CRO

6.7 Data Acquisition Systems (DAQ)• Used for large-scale data recording, eg. Power plant,

where input signals like temperatures, pressures, flow rates from a number of locations, need to be recorded continuously.

• Fig. 6.15 shows sensors being of analog types. • After signal conditioning, including amplification, a

multiplexer is used to enable each input to be sampled in turn.

• A sample and hold (S and H) device is used where A-D converter is employed.

• Computer controls the addressing and data input and processes signals for display, printing and storage.

6.7 DAQ (cont.)

6.8 Data Display & Storage• The data may be analog or digital form. The display

device may be any of the following types:

1. Analog indicators – motion of needle on metre scale

2. Pen trace/light trace

3. Screen display – CRO, TV

4. Digital counter – counter wheel

5. Digital printer – data in printed form

6. Punches – punched cards/tapes

7. Electronic displays – LED, LCD, etc

8. Storage may be on cards, magnetic tapes, disks.

9. Permanent record of data from a computer