Webinar: Diving Deep into Advanced Humidity Measurement and Specifications

Transcription

1 Webinar: Diving Deep into Advanced Humidity Measurement and Specifications Product Manager Jarkko Ruonala Note: The on-demand recording is available at

2 Introduction Jarkko Ruonala Product Manager with 10+ years of experience from international companies, having worked in various tasks the field of process analyzers and industrial instrumentation. MSc in Industrial Engineering and Management. About Vaisala Vaisala is a global leader in environmental and industrial measurement. Building on 80 years of experience, Vaisala provides observations for a better world. We are a reliable partner for customers around the world, offering a comprehensive range of innovative observation and measurement products and services. Headquartered in Finland, Vaisala employs approximately 1,600 professionals worldwide.

3 Agenda Section 1: Humidity measurement technology Section 2: Understanding accuracy Section 3: Traceability and calibration Section 4: The effect of accuracy on business results

4 Understanding accuracy Section 1/4

5 Humidity Measurement Technology What principles are used in humidity measurement? What is the current state-of-the art in humidity sensing? How does a the Vaisala HUMICAP sensor work? What is the achievable accuracy with modern inline sensors? How to measure water in oil or water activity?

6 Measurement by Observing a Change in Materials A change in weight: The Da Vinci hygrometer Change in length: Hair hygrometer Change in resistance: Salt crystal sensor

7 Measurement by Observing Evaporation Temperature Evaporation Thermometer in a wet wick: Wet-bulb thermometer Normal thermometer: Dry-bulb thermometer

8 Measurement by Determining the Saturation Temperature 43 hpa Thermometer for reading the temperature 23 hpa This sphere is cooled by pouring ether on it John Frederic Daniell ( ) Ether starts evaporating and cools the sphere Dew is formed on the sphere. This temperature is recorded from the thermometer

9 ..A Bit More Modern Approach of the Same Principle Photodetector Light source Electrically cooled element

10 Vaisala Innovation: Capacitive Thin-Film Polymer Sensor Vaisala founded by prof. Vilho Väisälä in the 1940 s HUMICAP invented in 1973 For radiosonde application Current de facto technology Later on productized for industrial applications [Name]

11 Vaisala HUMICAP Protective coating The responds directly to changes in Relative Humidity with change in capacitance Operating range: Relative humidity %RH Temperature C Withstands chemicals Withstands wetting Fast response time Low hysteresis

12 What Accuracy to Expect Chilled mirror hygrometer Typical use: Laboratory reference Requires maintenance HMP7 Humidity and Temperature probe Typical use: Field instrument No regular maintenance need Accuracy: +/ %RH Accuracy: +/- 0.8 %RH at %RH

13 Equilibrium Relative Humidity, Water Activity, Relative Saturation By laws of thermodynamics, the humidity tends to equilibritate inside the closed system After the equilibrium is established, the system is relatively in the same humidity Equilibrium Relative Humidity at headspace 50 % The absolute amount of humidity depends on the adsorption of the humidity in the material Relative saturation of liquid 50 %: Water activity 0.50

14 HUMICAP Measurements in Oil HUMICAP sensor withstands immersion to oils, hydraulic fluids and various other liquids The measurement can be made in directly in the liquid medium Faster response time compared to headspace method Humicap180L2 sensor is optimized for this application Very high sensitivity in dry end of the range Relative humidity at headspace 50 % Relative saturation of liquid 50 % Relative saturation of the sensor 50 %

15 Water in Oil: Calculated ppm -value The ability of the oil to dissolve water is called solubility. This is exponentially dependent on temperature. When the solubility of the water in the oil is known, the relative saturation of the oil can be converted to ppm Several factors affect the solubility characteristic of the oil. Mineral oils that have no additives have stable solubility characteristics

16 Understanding accuracy Section 2/4

17 Understanding Accuracy How is the accuracy of the instrument characterized? What kind of inaccuracies are there? What kind of accuracy do I need? What is required for accurate humidity instrument? What other factors do I need to consider when evaluating accuracy?

18 What Kind of Accuracy Do I Need? Repeatability is the ability of the instrument to provide consistent results when measurement is repeated under similar conditions When the absolute level of measurement is not so important, but the ability of the instrument to agree with itself is in interest When the application involves a static or controlled environment

19 What Kind of Accuracy Do I Need? Hysteresis is the variation in the measurement cause by the direction of the change in the measurand Non-linearity is the change in the accuracy related to the level of measurand and the environment, e.g. temperature When application involves changing environment, non-linearity and hysteresis become important factors

20 What Kind of Accuracy Do I Need? Long-term stability is the ability of the instrument to maintain consistent results over time Important in cases where measurement is used over time Drifting or creeping measurements may cause hard to diagnose problems in the application lso related to maintenance and calibration requirement

21 What Kind of Accuracy Do I Need? Calibration reference uncertainty is the level of uncertainty in the calibration of the instrument with respect to international system of units Even the most accurate instrument may be wrong if calibrated to a false reference When measurements are compared to results obtained from other instruments, the calibration reference uncertainty becomes important

22 Questions to Ask when Choosing the Instrument Does the specification cover the different types of inaccuracy? Does the specification cover the full operating range and temperature range? Does the instrument have specified stability or a recommended calibration interval? Does the specification include the uncertainty of the calibration reference? Does the instrument come with a traceable calibration certificate?

23 Example: Poor Linearity over Temperature Range Manufacturer s specification: Accuracy including hysteresis, nonlinearity and repeatability at C and 0..90%RH: ± ( %*mv) % RH Spec is OK for full temperature range, but missing %RH The device under test is out of specification Poor linearity over full working range Deviation from reference [%RH] Reference %RH at 70 C

24 Example: Poor Linearity over the Temperature Range and Incomplete Specification Deviation from reference [%RH] 3.5 Manufacturer s specification: Accuracy at C : ± 0.8 % RH Specification only given for a narrow temperature range The RH accuracy at +70 C is not specified Reference %RH at 70 C

25 Example: Vaisala HUMICAP Deviation from reference [%RH] Vaisala specification: Full operating range specified Reference %RH at 70 C

26 What is the Cause for Humidity Sensor Non-Linearity with Varying Temperature? All solid-state humidity sensors have certain degree of cross-sensitivity to temperature This can be avoided by two methods: Systematic compensation Special calibration and adjustment in nonambient temperatures Systematic compensation is only valid when the manufacturing process is able to produce sensors that are uniform in quality and obey the same characteristics All Vaisala sensors are manufactured in-house in Helsinki clean room facility. A HUMICAP sensor undergoes over 100 statistical quality control checks over the processing time, ensuring uniformity of sensor production.

27 Environmental Factors Affecting Accuracy Sensor purge recovered the original sensitivity of the sensor. In normal operation sensor purge is performed once per day

28 Operating Environment Affects the Calibration Interval The humidity and temperature conditions have an impact on the sensor s recalibration interval Higher humidity levels and elevated temperatures require more frequent calibration for maintaining the accuracy For humid high-temperature applications the warmed probe technology is recommended as this not only helps in avoiding down time caused by condensation on sensor but also reduces RH seen by the sensor

29 Accuracy of Derived Variables All solid-state sensors measure Relative Humidity as the primary parameter +/- 5 g/kg T = 70 C The accuracy of the variables derived from Relative Humidity depends on the operating point, e.g. temperature and humidity level at the sensor by the laws of nature Mixing ratio [g/kg] +/- 0.5 g/kg T = 20 C When using parameters other than relative humidity, it is worth verifying the accuracy in the variable and the actual operating environment +/- 3 %RH Relative humidity [%] Example of converting +/- 3 %RH measurement accuracy to the same in different temperatures. In both cases the RH accuracy remains the same, but the spread in the derived value is ten fold in the elevated temperature case.

30 Section 3/4

31 Calibration and Traceability What is meant by calibration and adjustment? What is measurement uncertainty? What is traceability chain and why is it important? What is accredited calibration? What is calibration uncertainty and why is it important?

32 What is Calibration? Current time is 12:02:12 Time reference Indicated time is 12:04:15 Calibration means a comparison between a measurement and a reference

33 What is Adjustment Current time is 12:23:33 Time reference Indicated time is 12:23:33 Adjustment means adjusting the measurement instrument to meet the reference value In spoken language, calibration and adjustment together are referred to as calibration

34 What is Measurement Uncertainty All measurements include some degree of uncertainty Knowing this uncertainty is important for various reasons: SI system prototype 1 kg = 1kg +/ To be able to compare readings To prove the measurement To develop measurement practices To save money To establish traceability Calibration weight 1 kg = 1kg +/ kg Another calibration weight 1 kg = 1kg +/ kg

35 What is Traceability Traceability is the documented chain of calibrations and references from top level of SI system to the instrument If uncertainty for the measurement is not known, the traceability chain is broken SI Primary reference Secondary reference Reference standards Working standards Measurement equipment BIPM National laboratories Accredited laboratories End users Degree of uncertainty in the measurement

36 Accredited Calibration? Accredited calibration laboratory is running a Quality Measurement System in compliance to ISO/IEC 17025, ANSI/NCSL Z540 or other standard Management system Technical requirements Professionally made and validated uncertainty calculations Verified traceability to international standards Use of accepted and agreed methods Competence of personnel Independence of the organization Privacy of data and records [Name]

37 Calibration Uncertainty The most accurate instruments have error if calibrated against a poor reference Cal reference 1 Ideal measurement Cal reference 2 If the uncertainty in calibration is not known, the traceability chain in the measurement is broken Traceability provides the link between the two measurements that allows comparison of the values When determining uncertainty of comparing two instruments, also the uncertainty of the calibration has to be taken into account SI reference

38 What Do I Have to Know about Calibration? What does my quality policy require? The use and maintenance of instruments is governed by laws and regulations or the quality policy of the instrument How was my instrument calibrated? Is the calibration traceable? Does it establish chain of traceability in my measurement? What is the uncertainty of the calibration of the instrument? How should I calibrate? Establishing calibration interval Is traceable calibration needed?

39 Section 4/4

40 The Effect of Accuracy on Business Results How do accurate instruments contribute to savings? Example: Spray dryer feedstock control by output humidity measurement Example: Cooling tower control by wetbulb temperature

41 Cost-Efficiency Possible in Several Areas Process control and optimization Process validation Quality control and assurance Process safety Condition monitoring

42 Process Control and Optimization Many processes such as drying processes involve safety margins E.g. when material is sold by weight, there is a specified moisture content in the final product Being able to optimize the dryness of the final product allows optimizing drying energy while avoiding quality costs Quality costs from insufficient drying Total cost of production Optimum drying level helps avoiding quality costs while avoiding overdrying Energy costs from excessive drying Amount of drying

43 Example: Spray Dryer Feedstock Fluctuation in the feedstock consistency Hot drying air Fluctuation in the final product moisture Output

44 Increasing Feedstock Rate with Inaccurate Instruments or No Instrumentation at All Feedstock Hot drying air Exceeding the quality limit causes quality losses and increased total productio cost Output

45 Increasing Feedstock Rate Hot drying air Feedstock Accurate instruments help in minimizing variation in final product moisture Smaller safety margin in drying allows increasing the feedstock rate and allows savings in the total production cost without increasing the quality cost Output

46 Example: Cooling Towers and Wet-Bulb Temperature

47 Example: Cooling Tower Control strategy for a cooling tower Output temperature set-point Generally optimal 2..4 C above the wet-bulb temperature, depending on tower design and thermal conditions Too small approach leads to excessive evaporation and windage loss Overdriving the cooling tower may cause problems with pumps and fans

48 How to Measure the Wet-Bulb Temperature Wet-bulb temperature is temperaturesensitive variable Installation position should be well ventilated It is a good practice to avoid radiated heat such as hot walls to minimize impact of temperature Stable RH measurement that maintains it s accuracy in outdoor conditions is required

49 Unshielded Installation with Low-Quality Sensor Radiated heat +3 C 75 %RH calibration after outdoor testing Wet-bulb temperature +/- 2 C Drift of the sensor +/- 5 %RH Impossible to control accurately for optimizing approach (2..4 C)

50 An Optimal Installation with a High-Quality Sensor Radiated heat +0.2 C This is sufficient accuracy for optimizing the cooling tower approach temperature based on the wet-bulb temperature Drift of the RH sensor +/- 2 %RH Avoiding overrunning the fans Avoiding windage losses Avoding excessive vibration and wear Wet-bulb temperature +/- 0.3 C White top surface on each plate reflects heat Black bottom surface on each plate effectively emits any absorbed heat

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