Instrumentation - Introduction 10/9/2013Ohio University - Dr. Cyders1 Engineering 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 Analog 1:19 Discrete

Instrumentation - Introduction 10/9/2013Ohio University - Dr. Cyders2 Usually, 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. Analog 1:19 Discrete Signal Conditioning – altering a signal using tools such as filtering and amplification to prepare it for further processing Data Acquisition System (DAQ) – system used to collect and record analog signals, usually in digital (discrete) form Analog-to-Digital Converter (ADC) – device that converts analog signals to digital (discrete) data

Instrumentation – Analog Device 10/9/2013Ohio University - Dr. Cyders3 Picture: Ashcroft 1008S Analog Pressure Gauge Bourdon tube design Bourdon tube Full Scale Range psi Resolution - smallest measurement change in input variable that can be detected and indicated – 2 psi Accuracy – deviation of the reading from the real value - ±1.6% Full Scale (1.6 psi) Random or Human Error Graduations - ±1 psi Parallax

Instrumentation – Analog Device 10/9/2013Ohio University - Dr. Cyders4 SSI Technologies P Pressure Transducer Strain Gauge Diaphragm design Full Scale Range psig Resolution – dependent on DAQ Accuracy – ±0.5% Full Scale (0.5 psi) Random Error Noise Input Voltage – voltage supplied to power transducer – 8 to 30 VDC Output Voltage – 1 to 5VDC

Instrumentation – A Brief Diversion - Number Systems 10/9/2013Ohio University - Dr. Cyders5 Think 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 10 0, the second, of 10 1, and so on. The numbers are then added to calculate the total value represented. Example: 7*(10 2 ) + 4*(10 1 ) + 2*(10 0 ) =

Instrumentation – A Brief Diversion - Number Systems 10/9/2013Ohio University - Dr. Cyders6 The 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 Example: 1*(2 4 ) + 0*(2 3 ) + 1*(2 2 ) + 0*(2 1 ) + 1*(2 0 ) = 21

Instrumentation – Digital Acquisition System 10/9/2013Ohio University - Dr. Cyders7 Analog-to-digital conversion works essentially the same way whether you’re 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 2 n, where n is the bit depth of the ADC. Time [sec.]Pressure [psig] bits Example: 5 bits gives 32 divisions (00000 through 11111)

Instrumentation – Resolution 10/9/2013Ohio University - Dr. Cyders8 100 psi 0 psi Analog Signal 0 V 5 V bit Digital Signal psi 42.8 psi 56.6 psi 2.25 V 2.88 V2.83 V 1.89 V 2.14V 57.5 psi 45.0 psi 0 V 5 V n : Bit-depth of ADC (3 in this case) V D : Full-scale input voltage on ADC (5 Volts in this case) V S : 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) R FS : Full-scale range of input value (100 psi in this case)

Instrumentation – Resolution 10/9/2013Ohio University - Dr. Cyders9 100 psi 0 psi Analog Signal 5 V n : Bit-depth of ADC (3 in this case) V D : Full-scale input voltage on ADC (5 Volts in this case) V S : 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) R FS : Full-scale range of input value (100 psi in this case) 1 V 0 V bit Digital Signal V Amp 100 psi 0 psi Amplified Analog Signal 5 V 0 V

Instrumentation – Digital Acquisition System 10/9/2013Ohio University - Dr. Cyders10 Time [sec.]Pressure [psig]

For more information, go to 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