Calibration General Principles & Theory, Equipment Considerations
Metrological Principles Accurate but not Precise Calibration A A+ Correction = Precise but not Accurate Correction B Ref Std (Radio Link to Atomic Clock) True Value: The best estimate of the true value. To the Metrologist the true value is never known Accuracy: The deviation from the estimated true value. Ex: the true value was 0.01 C and the instrument read 0.05 C (error of 0.04 C) Precision: How close measurements can be made Resolution: The smallest increment of the measurement Repeatability: The ability to reproduce the same reading Uncertainty: A statistical probability of a measurement being within a specified tolerance: Ex. ±0.5% with 95% confidence Calibration: The process of recording an instrument s reading against a reference standard Correction: The process of adjusting an instrument s reading based on the measurements of a reference standard.
Statement of Accuracy True value = The best estimate of the actual value Confidence Level: The probability that the true value lies in the given range Uncertainty is is defined as the tolerance band in which (at k=2) a measured value will be in agreement with the true value for 95% of the measurements made.
Calibration Traceability Hierarchy MICHELL TECHNOLOGIES FOR MOISTURE
UKAS Calibration Hierarchy MICHELL TECHNOLOGIES FOR MOISTURE
Factors Limiting Performance of a Calibration Methodology Physical facilities of the laboratory used temperature control air quality stability of electrical power supply Calibration equipment Accuracy Repeatability Traceability Quality of staff training
Calibration schedule MICHELL TECHNOLOGIES FOR MOISTURE
Calibration All Michell hygrometers go through a calibration regime in a purpose built laboratories in the UK, Holland and Japan. MICHELL TECHNOLOGIES FOR MOISTURE Traceable to NPL or NIST UKAS accredited We use the systems to calibrate: Capacitive dewpoint sensors Cooled mirror dewpoint meters Relative humidity probes Electrolytic hygrometers
Considerations What devices do you want to calibrate What parameters are the devices measuring? What are the %RH & T, or DP measurement ranges of the devices? What are the stated accuracies and resolutions of the devices? Does the calibration need to be performed over the entire operating range of the device? Do you want to calibrate at set points, or do they want to be able to make fine adjustments? Do you want a system which has manual, semi, or fully automated adjustment of set points? Do you want a system which has manual, semi, or fully automated logging of devices under test and the reference? Consider the dryer and generator Consider the reference instrument Are these devices sensors or instruments? What size and shape are they? Do the devices require a flow past/through, or are they making an environmental measurement? Consider an appropriate calibration manifold or chamber
Calibration Systems: general overview Traceability to National Standards Air Source Dryer Reference Dewpointmeter Generator Test Chamber Unit under test
Pressure Swing Dryers Provides dried air to moisture & dew point generators in absence of suitable site air Two column timed re-generation Oil-free compressed air feed Self-contained unit Two ranges available - -80 C dewpoint -100 C dewpoint
Moisture Generation to -75 C Enclosure is T controlled at 35 C Two-stage mixing of full dry and saturated gas flows allows high precision adjustment of generated dew-points Generation of dew-points from - 75 Cdp up to ambient T Designed to be fed by a pressure swing dryer or dry instrument air
Manual Dewpoint Generator Model DG-2 Range -75 to +20 o C Dewpoint Two Stage Flow Mixing Type MICHELL TECHNOLOGIES FOR MOISTURE
Automatic Dewpoint Generator Model DG-4 Range -75 to +20 o C Dewpoint Two Stage Flow Mixing up to 11 Points MICHELL TECHNOLOGIES FOR MOISTURE
Moisture generation to -100 C Temperature controlled enclosure Three-stage mass flow controlled flow mixing Generation of dew-points from -90 Cdp up to ambient T ±0.25 Cdp output stability
Chilled mirrors come in different types with different ranges.
Cooled Mirror Advantages fundamental measurement principle high accuracy drift free operation traceability to national standards
Basic Cooled mirror technology The cooled mirror sensor principle : cooling of a metal mirror by a thermoelectric cooler. At dew point, the mirror temperature is measured by a precision thermometer. Fundamental, high accuracy and long-term stability. Accuracy is limited by the PRT Range is limited by cooling methods and number of peltier used
Frost Formation on a cooled Mirror and ensuring frost is formed is key to accuracy Clean Mirror Mirror with even formation of frost Super-cooled water can form between 0 and -40 C leading to a 1 C error for every 10 C depression
Condensation Hygrometer Stabilization A cooled mirror modulates the power applied to a thermoelectric cooling module based on optical feedback. Stabilization typically requires oscillations around the actual dew point. PID control algorithms are employed to provide this function
FAST ensure ice not water is formed Super-cooled water causes 1 C /10 C depression error
NPL Calibration Certificate MICHELL TECHNOLOGIES FOR MOISTURE
Dewpoint Calibration Systems Various designs from -100 C MICHELL TECHNOLOGIES FOR MOISTURE
Combinations in Dew-Point Calibration Systems -60Cdp Capability DG2 S8000 Integrale PSD2-80Cdp Capability DG4 S8000 RS Pre-Dryer -100Cdp Capability PSD4 VDS3 S8000 TRS
Rh Calibration Still requires the basics - Air source - Controlled moisture generation Can use chambers - Bigger the chamber the more potential errors Reference can be CM or a suitable probe - What traceability - Accuracy Trained staff and methodology
Dew point v %Rh accuracies MICHELL TECHNOLOGIES FOR MOISTURE
Full chamber system. This complete, automatic calibration system is computer controlled to provide a calibration environment for various types of humidity sensor from -20 to +50 C and 1 to 95% relative humidity (-50 to +50 C) Systems can provide profiling of both parameters as well as a comprehensive data logging capability.
Full chamber schematic MICHELL TECHNOLOGIES FOR MOISTURE
HG-1 The HG-1 is an easy to use, bench-top generator designed to give a complete solution in a single box. Comprises: pump, dryer, divided flow generator, calibration test chamber and reference dewpointmeter, with direct display of relative humidity and chamber temperature.
Basic Rh generator using mixed flow can work with or without a chilled mirror reference
Simple Rh calibration at ambient conditions
Principle S503 MICHELL TECHNOLOGIES FOR MOISTURE
Inside the S503 MICHELL TECHNOLOGIES FOR MOISTURE
Rh calibration S904: Allows Rh and temperature control
Schematic MICHELL TECHNOLOGIES FOR MOISTURE
Inside the chamber: Mixing fan and control sensor
Outside of the chamber; 4 peltier ensure temperature uniformity
Chamber with chilled mirror reference MICHELL TECHNOLOGIES FOR MOISTURE
Questions? MICHELL TECHNOLOGIES FOR MOISTURE