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Instrumentation - Introduction10/9/2013Ohio University - Dr. Cyders1Engineering measurements are usually taken by some form of transducer. A transducer transforms one form of energy into another an example would be a thermocouple, which converts a temperature differential to an analog voltage.

Analog continuously variable, as opposed to discrete, which is quantified in finite units of data examples would be a dimmer vs. a switch, or the audio signal from a vinyl record vs. that from a compact disc

Analog1:19Discrete

Instrumentation - Introduction10/9/2013Ohio University - Dr. Cyders2Usually, engineering transducers are set up to either be read visually or to produce an electrical signal that can then be conditioned and converted to a discrete digital signal by a data acquisition system with an analog-to-digital converter (ADC). Digital signals can then be manipulated for analysis on a computer.

Analog1:19DiscreteSignal Conditioning altering a signal using tools such as filtering and amplification to prepare it for further processingData Acquisition System (DAQ) system used to collect and record analog signals, usually in digital (discrete) formAnalog-to-Digital Converter (ADC) device that converts analog signals to digital (discrete) data

Instrumentation Analog Device10/9/2013Ohio University - Dr. Cyders3Picture: http://www.instrumart.com/assets/1008A1_200.jpg

Ashcroft 1008S Analog Pressure GaugeBourdon tube designFull Scale Range - 100 psiResolution - smallest measurement change in input variable that can be detected and indicated 2 psiAccuracy deviation of the reading from the real value - 1.6% Full Scale (1.6 psi)Random or Human ErrorGraduations - 1 psiParallax

3Instrumentation Analog Device10/9/2013Ohio University - Dr. Cyders4SSI Technologies P51-100 Pressure TransducerStrain Gauge Diaphragm designFull Scale Range - 100 psigResolution dependent on DAQAccuracy 0.5% Full Scale (0.5 psi)Random ErrorNoiseInput Voltage voltage supplied to power transducer 8 to 30 VDCOutput Voltage 1 to 5VDC

4Instrumentation A Brief Diversion - Number Systems10/9/2013Ohio University - Dr. Cyders5Think of the decimal number system (what we commonly use) as a series of switches. Each switch has ten settings, zero through 9. The first switch denotes the multiplier of 100, the second, of 101, and so on. The numbers are then added to calculate the total value represented.

Example: 7*(102) + 4*(101) + 2*(100) = 742102101100999888777666555444333222111000

5Instrumentation A Brief Diversion - Number Systems10/9/2013Ohio University - Dr. Cyders6The binary system works the same way, but with switches having only two settings, 0 (off) and 1 (on). Because the switches have only two values, they are multiplied by successive powers of two.

24232221201111100000Example: 1*(24) + 0*(23) + 1*(22) + 0*(21) + 1*(20) = 21

6Instrumentation Digital Acquisition System10/9/2013Ohio University - Dr. Cyders7Analog-to-digital conversion works essentially the same way whether youre reading a gauge visually or using an analog to digital converter. Since analog devices usually have theoretically continuous behavior, resolution is usually limited by the bit depth of the ADC.

Bits are the most basic unit of memory, representing a binary number (0 or 1, transistor switch)

The number of divisions over the full scale reading is equal to 2n, where n is the bit depth of the ADC.

Time [sec.]Pressure [psig]0.0049.550.1047.960.2049.160.3049.270.4048.125 bits24232221201111100000Example: 5 bits gives 32 divisions (00000 through 11111)

7Instrumentation Resolution10/9/2013Ohio University - Dr. Cyders8100 psi0 psiAnalog Signal0 V5 V1110003-bit Digital Signal00101001110010111037.8 psi42.8 psi56.6 psi2.25 V2.88 V2.83 V1.89 V2.14V57.5 psi45.0 psi0 V5 Vn : Bit-depth of ADC (3 in this case)

VD : Full-scale input voltage on ADC (5 Volts in this case)

VS : Full-scale output voltage on analog device (5 Volts in this case)

B : Range type (0 for unipolar, 1 for bipolar; 0 in this case)

RFS : Full-scale range of input value (100 psi in this case)

8Instrumentation Resolution10/9/2013Ohio University - Dr. Cyders9100 psi0 psiAnalog Signal5 Vn : Bit-depth of ADC (3 in this case)

VD : Full-scale input voltage on ADC (5 Volts in this case)

VS : Full-scale output voltage on analog device (4 Volts in this case)

B : Range type (0 for unipolar, 1 for bipolar; 0 in this case)

RFS : Full-scale range of input value (100 psi in this case)1 V0 V1110003-bit Digital Signal0010100111001011105 VAmp100 psi0 psiAmplified Analog Signal5 V0 V

9Instrumentation Digital Acquisition System10/9/2013Ohio University - Dr. Cyders10

Time [sec.]Pressure [psig]0.0049.550.1047.960.2049.160.3049.270.4048.12

10For more information, go to http://www.library.ohiou.edu and search the ALICE catalog for The Uncertainty in Physical Measurements by Fornasini it is an electronic resource you can access anywhere with your student login