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17-Jan-14 www.ppminc.com Cleveland OH 44122 1 THE NULLMETER A 21 st Century Instrument Or A 19 th Century Instrument? PPM, Inc. October 2007.

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Presentation on theme: "17-Jan-14 www.ppminc.com Cleveland OH 44122 1 THE NULLMETER A 21 st Century Instrument Or A 19 th Century Instrument? PPM, Inc. October 2007."— Presentation transcript:

1 17-Jan-14 www.ppminc.com Cleveland OH 44122 1 THE NULLMETER A 21 st Century Instrument Or A 19 th Century Instrument? PPM, Inc. October 2007

2 17-Jan-14 www.ppminc.com Cleveland OH 44122 2 INTRODUCTIONS Presentation: Chas Gilmore Managing Director, Strategy PPM, Inc. Sponsor: Mark Reid techniCAL Copies of Presentation Available at techniCAL

3 17-Jan-14 www.ppminc.com Cleveland OH 44122 3 AGENDA What Is The Nullmeter Why Is It Used How Is It Used Null Meter Basics (Principles & Use) Alternative Techniques Comparison Of Techniques Common Measurement Problems Summary/Questions

4 17-Jan-14 www.ppminc.com Cleveland OH 44122 4 TAKE AWAY What The Nullmeter Is What The Nullmeter Is Not Where A Nullmeter Is Used Nullmeter Vs. Digital Instruments In Classic Use Practical Issues With Nullmeter Measurements When To Use A Null MeterAnd When Not To The Future Of Nullmeters

5 17-Jan-14 www.ppminc.com Cleveland OH 44122 5 WHO ARE YOU? Involvement With DC Calibration Techniques?

6 17-Jan-14 www.ppminc.com Cleveland OH 44122 6 WHAT/WHY/WHO? An Instrument To Compare Two Voltages ( ) –Typically Moderate Voltages (Few to Few 10sHas a much wider dynamic range if needed) –Compared Within Micro-Volts or few 100s of Nano-Volts –Analog Display (Meter) For Ease Of Use –Not A Precision/High-Resolution Voltmeter Compare An Unknown To A Known –Standard Voltage, Current or Resistance To Unknown Calibration Laboratories Manufacturers Product Design Organizations

7 17-Jan-14 www.ppminc.com Cleveland OH 44122 7 HOW? High Gain, Low-Drift, DC Amplifier Unipolar (Amplifies Positive & Negative Equally) Amplifier Output Drives A Zero Center Meter Input Can Be Attenuated Or Amplifier Gain Changed

8 17-Jan-14 www.ppminc.com Cleveland OH 44122 8 NULLMETER CHARACTERISTICS DC Voltmeter High Sensitivity (Few µV Minimum) Wide Range Of Sensitivities (Manually Selected) Low Drift (Holds Zero) Relatively High Input Impedance (1, 10 Or > M) Analog, BiPolar Display (Meter With ± Scales) High Common Mode (Often Via Battery Operation) Filtered For 2 – 5 Second Response (Or >) Ability To Zero The Meter

9 17-Jan-14 www.ppminc.com Cleveland OH 44122 9 THE NULLMETER ADVANTAGE NULL Means Just That Null/Nada/Nothing OHMs Law I = E/R At Null E = 0 so I = 0 The Meter Draws No Current From The Circuit Under Test At Null Null Very Easy To See (Zero Center Meter) The Higher The Input Impedance, The Less The Off-Null Disturbance

10 17-Jan-14 www.ppminc.com Cleveland OH 44122 10 A BIT OF HISTORY The Tangent Galvanometer First Described In 1837 By Claude-Servais-Mathias Pouillet (1790-1868) Used Galvanometer To Verify Ohm's law Galvanometer Is On A Level Surface The Coil Aligned With Magnetic North-South Current Flow/Direction Observed As Compass Needle Movements

11 17-Jan-14 www.ppminc.com Cleveland OH 44122 11 MORE EARLY METERS C 1890 C 1930

12 17-Jan-14 www.ppminc.com Cleveland OH 44122 12 NULLMETERS YOU MIGHT OWN FLUKE 845 AB Mid-1960s PPM AVM-100 Mid 1990s KEITHLEY 155 The 1970s HP 419A 1960s HP 413A 1950s

13 17-Jan-14 www.ppminc.com Cleveland OH 44122 13 A 21 ST CENTURY NULLMETER Traditional Features MIRROR BACKED ZERO-CENTER METER WITH -3/+3 & -10/+10 SCALES ZERO/OPERATE MODE ZERO ADJUSTMENT ISOLATED OUTPUT LO-THERMAL EMF INPUT

14 17-Jan-14 www.ppminc.com Cleveland OH 44122 14 A 21 ST CENTURY NULLMETER 21 ST CENTURY FEATURES INPUT IMPEDANCE SELECTION FILTER SELECTION 0.1 S – 100 S SELECTABLE OFFSET VOLTAGE 0 – 30 mV STATUS LCD INPUT OFFSET (I & V) ADJUSTMENT SCALABLE ISOLATED OUTPUT RANGE 100 nV – 1 KV

15 17-Jan-14 www.ppminc.com Cleveland OH 44122 15 SIMPLE COMPARISON An Un-Calibrated Source To A Standard Source Standard Is 2 µV < Un-Calibrated Source; 2 Parts In 10 7 Errors: Nullmeter ± 100 nV, Noise/Thermals 500 nV Measurement Uncertainty : 0.51 Parts In 10 6 Major Uncertainty: The Standard (A few ppm?)

16 17-Jan-14 www.ppminc.com Cleveland OH 44122 16 EXTENDING THE DYNAMIC RANGE Bridge Circuits (Wheatstone/Kelvin) –General Ratio Measurements –Ultra-Low Resistance Decade/Reference Divider –High Precision Decade Division Ratio Divider –High Precision 7-Digit Ratio Divide Another Kelvin Bridge Scotland

17 17-Jan-14 www.ppminc.com Cleveland OH 44122 17 BASIC BRIDGE Compare Resistance Ratios Null Achieved When R 1 / R 3 = R 2 / R4 A - B Voltage = Zero Extensions Compare: –Voltage – Temperature –Current – Pressure –Resistance – Force AB

18 17-Jan-14 www.ppminc.com Cleveland OH 44122 18 DECADE DIVIDER Also Called Reference Divider Decade Input / Output Voltage Ratios 0.01:1, 0.1:1, 1:1, 10:1, 100:1 Accuracy > 0.5 ppm Self Calibrating (With Nullmeter) Limited Comparisons

19 17-Jan-14 www.ppminc.com Cleveland OH 44122 19 A NULL METER & A RATIO DIVIDER Kelvin-Varley Ratio Divider Ratios: From 0 To 1.1 Ratios: 7 Decimal Places (Linearity 0.1 ppm or >) Example 90.62441 V To 10 V ± 906 nV (~1 ppm) Null Error 100 nV Thermals 500 nV Uncertainty 1.04 ppm

20 17-Jan-14 www.ppminc.com Cleveland OH 44122 20 WHAT ARE THE ALTERNATIVES? Direct Measurement With Long-Scale DMM –DMM On 200 V Range (For 90 V Example) –Resolution: 8 Digits 90.624,410 (1 µV) –Uncertainty 3 PPM (271 µV) –Very Fast Ratio DMM & 10 Volt Reference Standard –Uncertainty 2 PPM –SlowerBut Only Uses Two Pieces Of Equipment

21 17-Jan-14 www.ppminc.com Cleveland OH 44122 21 USE A DMM AS A NULLMETER? High End DMMs Have The Resolution High Sensitivity DMMs Have The Resolution Difficulties: –Identifying The Null –Lack Of Filtering And Digits Flicker/Blur –Sufficient Filtering And Changes Jump –Common Mode (Usually Not Battery Operated) Generally Difficult To Accomplish

22 17-Jan-14 www.ppminc.com Cleveland OH 44122 22 OBSERVING A 1µV NULL ANALOGDIGITAL Range ± 3µV Range ± 200.00 mV

23 17-Jan-14 www.ppminc.com Cleveland OH 44122 23 NULLMETER Vs. DMM Nullmeter –Easily 1 Part in 10 6 –Typically 5 Parts in 10 7 –1 – 2 Parts in 10 7 With Care –Preserves Existing Procedures –Measurement Time: 10 – 20 Minutes (experienced metrologist) –Complex Setup (Lots Of Equipment) –Lack Of Automated Data Recording DMM Direct –Typically 3-5 Parts in 10 6 –Measurement Time 1 – 2 Minutes (minimal experience) –Automated Data Capture DMM Ratio-meter –Typically 2-3 Parts in 10 6 –Measurement Time 3 – 5 Minutes (moderate experience) –Automated Data Capture How Good Do You Need To Be? Is The Change Justified?

24 17-Jan-14 www.ppminc.com Cleveland OH 44122 24 ALL IS NOT ROSY Noise (Man Made & Natural [Johnson]) Thermal Voltages Electro-Chemically Generated Voltages Input Bias/Offset Currents Pump Out Currents Impact Of Induced Voltages (Line Frequency) Impact Of Differing Input Impedances

25 17-Jan-14 www.ppminc.com Cleveland OH 44122 25 MR. JOHNSONS NOISE RESISTANCE OHMS Noise (nV/Hz) Johnson Noise nV Hz Vs. Resistance

26 17-Jan-14 www.ppminc.com Cleveland OH 44122 26 WHEN A SHORT IS NOT A SHORT Links Straps Plugs Copper-to-Copper: 200 nV/°C Copper-to-Gold: 300 nV /°C Problems: Heat From Your Hands Heat From Inserting A Banana Plug Handling Contaminants Make Batteries Keithley Instruments

27 17-Jan-14 www.ppminc.com Cleveland OH 44122 27 OFFSET VOLTAGES Voltages From Multiple Sources

28 17-Jan-14 www.ppminc.com Cleveland OH 44122 28 COMMON MODE VOLTAGES Another Source Of A DC Signal

29 17-Jan-14 www.ppminc.com Cleveland OH 44122 29 COMMON MODE COUPLING Capacitive Inductive

30 17-Jan-14 www.ppminc.com Cleveland OH 44122 30 INPUT CURRENTS Current Generated By The Instrument Itself Current Flows Thru Resistances In MMT Circuit Creates Additional (Unknown) Voltage Drop Example: 50 pA @ 100 k = 5 µV

31 17-Jan-14 www.ppminc.com Cleveland OH 44122 31 PUMP OUT CURRENT DC Amplifier Stabilized By Chopper Chopper Converts DC Signal To Square Wave Square Wave Amplified Without Amplifier Drift Chopping Generates Small (pA) Currents Charging Input Capacitor

32 17-Jan-14 www.ppminc.com Cleveland OH 44122 32 SUMMARY The Nullmeter Is A 21 st Century Instrument (What else would we say!) Where You Need It: –Trend Indications –Comparison Measurements From 1 To 0.1 ppm –High Common Mode/Isolation Measurements –When It Doesnt Make Sense To Change Procedures –When Nearly Zero Isnt Good Enough Where You Dont Need It: –When Direct Measurements Will Do –When Multi-Digit Measurements Are Required

33 17-Jan-14 www.ppminc.com Cleveland OH 44122 33 ITS INEVITABLE If You Havent Faced ItYou Will The Old Instruments Are Dying The New High-Accuracy, High-Sensitivity Instruments Require High-Precision Calibration Sub-Part Per Million Is Here Do I Totally Change Procedures/Processes? There Are Just Some Measurements You Cant Do Without A Nullmeter

34 17-Jan-14 www.ppminc.com Cleveland OH 44122 34 QUESTIONS???

35 17-Jan-14 www.ppminc.com Cleveland OH 44122 35 QUESTIONS???

36 17-Jan-14 www.ppminc.com Cleveland OH 44122 36

37 17-Jan-14 www.ppminc.com Cleveland OH 44122 37


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