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.
Measurement Uncertainty CALIBRATION Normal Distribution True Value 1s = 68.27% 2s = 95.45% 3s = 99.73% 1s 2s 3s
Calibration Traceability Hierarchy
UKAS Calibration Hierarchy
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
Calibration All Michell hygrometers go through a calibration regime in a purpose built laboratories in the UK, Holland and Japan. DP sensors- Dry down for 5 days Calibration run using high accuracy dew point generators and chilled mirror references Traceable to NPL, Nmi, or NIST UKAS accredited
Calibration Systems: general overview Our systems can be configured to calibrate various types of hygrometer - 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
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 range-one size does NOT fit all.
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
Dew-Point Depression of Cooled Mirror Sensors 1 Stage Minimum Measureable Dew point ( C) 30 20 10 0-10 -20-30 -40-50 -60 90 80 70 60 50 40 30 20 10 0-10 -20-30 Sensor Temperature ( C) 2 Stage Cooled Mirror Hygrometers that use Peltier cooling have a finite ability to cool Multiple Peltier stages may be stacked to achieve low temperatures Additional cooling methods such as refrigeration, fluid jackets may be used to augment the thermoelectric cooling The amount of depression is not equal to the minimum dew point reading attainable. A cooled mirror typically has to cool 5-10 C below the dew/frost point to collect a dew/frost layer
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 CALIBRATION 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
Cooled Mirror Problems Instruments have always perceived to be :- Expensive instrumentation Maintenance intensive Perception of being too good for application But there are lower cost solutions if restriction on range is accepted Today these are low maintenance, stable instruments
Contamination Correction CALIBRATION Any optical system can be affected by contamination of the light path. Cooled mirror dew-point hygrometers are no exception. Contamination of the mirror surface or the detection window will reduce the detection of the light reflected by the mirror. During normal operation mirror contamination will have no measurable effect on the performance of the instrument. However, gross contamination of the sensor can result in inaccurate measurements. The mirror condition is measured and a percentage value should be displayed on the instruments For this reason, all Michell cooled mirror instruments incorporate an automatic compensation system.
Contamination Correction (2) CALIBRATION This works by heating the mirror to a temperature above that of the dewpoint temperature, which burns off any condensation and contaminants. The mirror surface is now clean and the sensor optics are rebalanced to compensate for any reduction in light intensity caused by the contamination. With heavy contamination the automatic system may not be able to compensate and so manual cleaning or the mirror surface may be periodically required. This should be performed with a Q tip bud and distilled water. The sensor window and the mirror should be cleaned.
CALIBRATION
NPL Calibration Certificate
Dewpoint Calibration Systems Various designs from -100 C
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
Basic Rh generator using mixed flow can work with or without a chilled mirror reference
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 CALIBRATION
Simple Rh calibration at ambient conditions
Principle S503
Inside the S503
Rh calibration S904: Allows Rh and temperature control
Schematic
Inside the chamber: Mixing fan and control sensor
Outside of the chamber; 4 peltier ensure temperature uniformity
Chamber with chilled mirror reference
Questions?