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Vaisala Sensor Calibration for Automatic Weather Stations

Vaisala Sensor Calibration for Automatic Weather Stations
Overview of calibration procedures for Vaisala humidity and temperature sensor HMP155, air pressure sensors PTB330 and BARO-1, and wind sensor WMT700. The webinar covers also the calibration fundamentals, traceability chain and accreditation.

​​​​​​​​Data quality is increasingly playing a more important role in surface weather data management, and calibration is the basis for data quality of humidity, temperature, pressure and wind measurements. Regular calibration reduces measurement errors, enables consistency between measurements, and increases the reliability of accurate measurements.

This webinar provides an overview of calibration procedures for Vaisala humidity and temperature sensor HMP155, air pressure sensors PTB330 and BARO-1, and wind sensor WMT700. The webinar covers also the calibration fundamentals, such as traceability chain and accreditation.

The webinar speaker is Miguel Menendez, Calibration and Repair Service Manager at Vaisala

Questions & Answers

Q: Please clarify on non breakable chain for calibration comparison.
A: 1. An unbroken chain of comparisons that go back to stated references acceptable to the parties, usually a national or international standard (in the USA, NIST)
2. Each step in the traceability chain must provide calculated or estimated measurement uncertainty.

Q:Is a sling pychometer or a fan driven psycrometer a valid standard?
A:It can be as long as it is calibrated, has more accurate or of the same accuracy and uncertainty of the product it is being used to test against. 

Q:How many readings do you recommend to do a effective spot checking on field (I work at the airport)? We have a PTB330TS.
A: Whenever possible, we like to capture 10 consectutive readings and record the average reading as the ”actual” recorded value. This also can help calculate the standard deviations of the readings, and will provide a visible sign of whether the readings are stable.

Q: Are Vaisala non-accredited calibrations traceable to international (SI) standards?
A: Non-accredited calibrations performed by Vaisala are traceable to NIST.

Q: What is the best method you recommend to keep the saturation salt solution temperature stable? And temp and RH, which one should do 1st?
A: The best way to keep the salt solution temperature stable is to keep it in a room (preferably a humidity and temperature controlled room) that has very small temperature deviations. It is also important to keep the containers properly sealed when not in use.

Q:When you calibrate temp. in bath, what type of protective plastic( is it special for that or you can use any)
A: At Vaisala we use heat shrink tubing manufactured by 3M. This keeps all the liquids from penetrating into the sensing element. Further, the test results have shown to be very good. 

Q: Are there plans to modify / replace the PTB330TS, our engineers much preferred the PTB220TS as less complex than PTB330TS with its multiple probe format, and different display format.
A: We do thank for the feedback as that is excellent information for the future development. Naturally this will be taken into account when the next generation PTB330TS is being developed, but the time is not right for that yet.

Q: Do vaisala 2 point cals (fcrh,LCP) have a software maximum change per cal, and multiple cal attempts may be needed at times?
A: This depends on the product and on how far out of tolerance the readings are. For the majority of our products there is no limit to how much you can adjust it. For others, however, there are some cases where you can only adjust the unit in increments of ~3%RH. Therefore, multiple attempts may be needed. 

Q: Does Vaisala Worburn give tours, first week in June?
A: We normally provide free of charge calibration seminars here (mostly once per quarter) at that time we provide all of the attendees with a tour of our laboratory. Other than that we do not normally open the labs out to the open public. 

Q: Will you be covering calibration of WMT520 and WXT520 in your future seminars?
A: WMT52 and WXT520 are products which cannot be adjusted by a customer. When there is a reason to believe that instrument is not within specs, it shall be sent to Vaisala for calibration and adjustment. Customer can replace the PTU module by purchasing a new calibrated PTU module as a spare part.

Q: What liquid is used in the TA-calibration when testing range below zero?
A: denatured ethanol 

Q: What is a brand and type of Vaisala factory calibration reference for air pressure instrument PTB330 ? 
A: Factory Reference instrument for PTB330 barometers (A class) in Vaisala factory is Fluke PPC4 Pressure Controller or alternatively DHI Instruments PP3.

Q: At how many point Vaisala WMT700 ultrasonic sensors speed and direction are checked in the manufacturing, what are the points?
A: Wind speed at two points: 0 m/s and 7 m/s. Wind direction at one point: 0 degrees.

Q: When calibrating HMP155 with additional temperature sensor and warmed probe, do I also need to put the additional temperature probe inside the calibrator?
A: When pressing HMP155 adjustment button, it shuts down probe warming. If only relative humidity is calibrated and adjusted, there is no need to take additional T-sensor inside the calibration chamber. Before the calibration and adjustment, allow enough time to let the HMP155 probe to cool down.

Q: How many HMP155 probes can be connected to MI70 during field calibration?
A: The user can connect up to two HMP155 probes to MI70 indicator at the same time.


​The webinar speaker is Miguel Menendez, Calibration and Repair Service Manager at Vaisala

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