Use voltage divider circuit with thermistor to measure temperature of

• ice water (freezing)
• room temperature (check thermometer in room)
• temperature of your hand - 98.6°F
• near boiling (empty crock pot)

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Thermistors (Ref. pg. 160)

Materials Categorized by valence electrons as:

• conductor: (copper) metallic bonding = e- drift freely
• insulator: (polystyrene) strong bonds = few free electrons
•  semiconductor: (Silicon, Germanium) covalent bonding heat?some bonds to break forming holes and free e-

Thermistor = valence controlled semiconductor:

• higher temperatures allow greater numbers of e- to break free, and therefore higher conductivity, lower R

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LabVIEW

• Tool for acquiring, analyzing, and storing data
• Graphical programming language
  • icons represent functions
  • wire icons together to determine data flow
  • works like a flow chart
• Virtual Instruments (VI’s) = LabVIEW programs

Two windows

• Front Panel:
  • Gray screen with buttons/knobs etc.
  • Displays controls (inputs), and indicators (outputs).
• Block Diagram:
  • White screen where icons are wired together.
  • “Guts” of graphical program or flow chart.

 

Palettes: window > show palette / right click on window

Tools Palette (space bar/tab to change tools)

• Labeling tool:  write text

• Positioning tool: move objects

• Operate tool:  move dials

Wiring Tool: connect icons

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To Acquire Data From Circuit:

1.) Connect Circuit to Computer: proto-board

• Terminal Block Pin Outs - page 158

2. ) Differential Inputs (what you will use)

• computer records difference between two measured voltages.
• Computer subtracts values between channel numbers that are separated by 8; so, 0 and 8, 1 and 9, 2and 10...

3.)  use Acquire Samples & Chart.vi

• Channel 0:  Measures difference between 0 and 8
• scaling equations - convert voltage to temperature

To Run Program - click on run arrow

To find out what an icon / object does:

• Left click on an object
• Help > Show context help

A dialog box will appear that gives information on whatever object your cursor is over.

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Errors in Measurements (123-125)
Uncertainty Analysis: Definitions

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• X = True Value
• x_i = measured value
• n = number of measurements taken
• Accuracy = how close x_i is to true value X
• Precision = how good grouping is

Good accuracy poor precision:                                Bad accuracy, good precision:

 

• Mean :

• Median = midpoint in events (½points below, ½ points above)
• Mode = most frequently occurring value

• Error = e_i =  Deviation from true value:  e_i = x_i - X
  • We do not (and cannot) know X - so - Approximate X by the mean µ
  • e_i* = x_i - µ
  • e_i* approximates e_i and is called an estimator of e_i.
  • Random deviations: Human errors, normal distribution
  • Systematic deviations: Calibration errors

• Relative Error = DX/X   -or-  = e_i/m
• Variance

• Standard Deviation = width of distribution, estimate of average error

 

• Standard Deviation of Mean (SDOM)

 
 

• Error Propagation: x is a function of u, v,… & variables are independent

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Resolution

• Input = continuous analog signal
• Output = discrete digital signal
  • digital signal created from bits: 0 or 1

exmaple

• 3 bits can hold 2³ = 8 different numbers:
  • 8 different combinations of 0's and 1's

1.   111
2.   110
3.   101
4.   100
5.   011
6.   010
7.   001
8.   000
 
• 8 different numbers = 8 different voltage values that can be displayed:

for 0 < V < 10

111 =0 V
110 =1.4285
101 =2.85714
100 =4.2857
011 =5.7142
010 =7.1428
001 = 8.5714
000 = 10 V

• Voltages that cannot be represented by bits will be displayed as the closest value:
  • If analog signal is 1.32V, digital signal will record 1.4285V

• Resolution = voltage gap between consequitive numbers that cannot be displayed.

 

• Increase resolution by decreasing (high limit - low limit)

• Warning: If analog voltage signal exceeds limits, output will flat-line at limits
  • if output = 14V and high limit = 10 V, digital display will be 10V
  • if output is -5V and low limit is 0V, digital display will be 0 V.
• Choose high/low limits to encompass entire signal, and maximize resolution