PHOENIX VAISALA PRESSURE SENSOR
Mars Pathfinder (MPF) and Phoenix Pressure Issues, Transducers Use, & Issues Raised by the Finnish Meteorological Institute (Updated 9/24/2015)
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Segment from Section 2.2 on Mars Pathfinder (MPF) and Phoenix Pressure Issues.
The range of sensitivity and accuracy of the Vaisala Barocap® and Tavis sensors are crucial. With Mars Phoenix, three Barocap sensors [LL(B1), and RSP1 (B2, B3)] were used. They had problems associated with a nearby heat source. Problems were particularly noted when temperatures rose above 0ºC. According to Taylor et al. (2009) calibration coefficients were also withheld from the Finnish Meteorological Institute (FMI) due to International Traffic in Arms Regulations (ITAR). The 5-12 mbar range of Barocaps was probably due to the data from the Tavis sensors before, but Tavis sensors were limited due to radio occultation pressure experiments (not as accurate as in situ measurements) by the Mariners. Radio occultation results are discussed further in Section 5.
An issue with respect to how fast the dust filters for transducers on landers could have clogged relates to when the air tube was initially exposed to ambient conditions. If open to space all the way down, then air might not rush in so fast; while if the tube were suddenly opened on the surface, more dust might be expected to rush in, even at supersonic speeds. Alvin Seiff, et al. (1997) indicates that for Pathfinder the plan was for atmospheric pressure (and temperature) to be measured during parachute descent from ~8 km to the surface. The air inlet was connected to the flared tube fitting shown in Figure 10B by one meter of 2 mm inside diameter tubing. Dr. Robert Sulliavan (Cornell University) told us (on July 27, 2011) that while 1µ particles on the surface of Mars clump together quickly, larger particles that were easier to move would be lifted on landing. He was not sure about whether they would clog a dust filter as fast. But if MPF suddenly ingested 1µ particles suspended in the air below 8 km right after parachute deployment, the hot air associated with the entry-related heat might cause a problem for the tiny filter.
Figure 1 above – The top transducer is for Phoenix. Note the tiny dust filter shown under Praw (adapted from Doc. No: FMI_S-PHX-BAR-TN-00 FM-00 Revision 1.0 dated 2009-02-26). The report is entitled The Time Response of the PHOENIX Pressure Sensor). An area of concern for clogging by dust is highlighted. The photo on the right is adapted from http://www.space.fmi.fi/phoenix/?sivu=instrument. The bottom pictures are for MSL
The following is an extract from my Mars Report:
5.1 PRESSURE RANGES OF VAISALA BAROCAPS AND TAVIS SENSORS.
The range of sensitivity and accuracy of the Vaisala Barocap® and Tavis sensors are crucial. With Mars Phoenix, three Barocaps sensors [LL(B1), and RSP1 (B2, B3)] were used. Again, they could only detect up to 12 mbar, and there were problems associated with a nearby heat source. Problems were particularly noted when temperatures rose above 0o C. Calibration coefficients were also withheld from the Finish Meteorological Institute (FMI) due to International Traffic in Arms Regulations (ITAR). 29 Obviously, the limited range of Vaisala sensors was due to the data from the Tavis sensors before. But what range of sensitivity did Tavis employ? The Tavis transducers were too limited due to findings by the Mariners. Remember, Mariner 4 daytime temperatures were estimated at -100 degrees Celsius, when in fact they can get as high as +27o Celsius.12 Gas pressure varies with absolute temperature.
The Alvin Seiff Papers9 indicate that the Viking 1 was restricted to only 25 mbar capacity, and CAD drawings of the Viking parts provided by Tavis backed the 25 mbar range for Viking (see Figure 7); but the report by Michael Mitchell8 in 1977 put it at18 mbar (see Figure 6). Pathfinder, the Tavis spokesman thought, used Part 10484 (Tavis Dash No. 2) which a CAD drawing listed as having a 0.174 psia limit (12 mbar, the same exact limit was later imposed by Vaisala on Phoenix). Apparently NASA also ordered the 110 to 150 gram Tavis transducer (Part 10484, Tavis Dash No. 1 – see Figure 8) that supposedly remained on Earth (probably for a calibration check). This second sensor could measure from 0 to 15 psia. The Tavis transducers sent to Mars were apparently designed based on data obtained in the Mariner probes that never got closer than 1,500 km from Mars. Those estimates, from 2.8 mbar to 10.3 mbar (Mariner 9), became enshrined as fact, and were built into every probe that ever landed on Mars. The issue of pressure sensors is clouded by restrictions on information related to ITAR (International Traffic in Arms Regulations) that handicapped the Finish Meteorological Institute (and Vaisala) with respect to the calibration coefficients needed for analysis of raw pressure data on Phoenix (Peter A. Taylor, et. al, 2009).29 Apparently missile reentry issues and related security clearances interfere with data analysis. This was hinted at in a second reference to ITAR by Taylor et al. that indicates a number of problems associated with pressure analysis for Phoenix. Barocap® pressure sensors used on Phoenix depend on Vaisala Thermocap® temperature sensors. But, “After Phoenix landed it appeared that the actual thermal environment was worse than the expected worse case. The temperature was not only changing rapidly, but there were also fast changes in the temperature gradient due to a nearby heat source. Information on a re-location of the heat source had not been provided initially due to ITAR restrictions.” The Mitchell report8 on Tavis Viking sensors show that they were designed for a temperature range of -28.89o C to +71.11o But it gets down to -100o C at night, and never above +27o C during the day.
5.1.1 Issues Raised by the Finnish Meteorological Institute (FMI) (updated 12/16/09). The report by Kahanpää and Polkko of the FMI33 published on 2009-02-26, states of the Vaisala pressure sensor used on Phoenix that,
"We should find out how the pressure tube is mounted in the spacecraft and if there are additional filters etc."
This is an enormously important statement. Four landers were supposed to measure pressure on Mars. FMI designed the Vaisala pressure sensor. I challenged the above statement on 11/14/09, and published a stinging criticism of it in an earlier version of my report on 11/17/09. On 12/15/09 I received the following partial answer/admission from Henrik Kahanpää:
“Your nose smelled also a real issue. The fact that we at FMI did not know how our sensor was mounted in the spacecraft and how many filters there were shows that the exchange of information between NASA and the foreign subcontractors did not work optimally in this mission!”
In his e-mail of 12/15/09, Kahanpää made clear that there was no extra filter. But the confusion in his report raises another possibility. There is supposed to be only one thin tube that connects the transducer to the outside atmosphere. The filter is very small (probably under 10 square millimeters). The transducer was exposed to a vacuum on the way from Earth to Mars. When the Phoenix landed, a lot of dust was raised by the retrorocket. The air pressure outside was supposed to be low, almost as low as outer space. The flow of air into the transducer therefore should not have been too fast. But, if the pressure outside was higher than expected, the rate of flow and dust into the Phoenix would be faster than planned for, with the result that dust would be rapidly sucked in just like a vacuum cleaner would suck it in. So, a tiny filter might well quickly clog with dust, perhaps so fast that it would prevent much more air from reaching the pressure transducer. With a clogged filter, the pressure sensor would stay pegged at a low pressure reading. It would look like there were more filters present, and thus Kahanpää and Polkko would pose the question that they did about them in their report.
Kahanpää repeatedly mentioned funding problems, although the meteorology package for Phoenix cost $37,000,000. However, not only was an anemometer unfunded, but a way to change the dust filter was also left off the shopping list! Everyone who has ever dealt with a vacuum cleaner with a clogged filter knows that it simply don't work until the filter is changed. The filter of Phoenix might well have clogged within seconds or minutes of landing. It is possible that the same applies to the Tavis transducers too.
With respect to the Vaisala sensor, and possibly the Tavis transducers too, these issues are now central. (1) How much testing was conducted in reference to amounts of dust required to clog the filter? If any such tests were conducted, what size dust particles, and what density of dust particles were involved? (2) How was the tube to the ambient air mounted? If somebody added extra turns, that’s more places for dust to clog. Answers to these questions are critical because the Mars Science Laboratory (MSL) is slated for launch in 2011.
Kahanpää stated that MSL is a 2 billion dollar cornerstone mission and is therefore handled in different way than the $454 million dollar "scout mission" Phoenix.” However, MSL’s pressure sensors are slated to be in the 0-11.5 mbar (1,150 Pa) range – still far too low. If so, and with the same inadequately designed sensor that does not account for dust filter clogging, the results can be expected to be similar to those refuted throughout this report. We need a new approach.
For more about Vaisala on the MSL, see http://finance.yahoo.com/news/mars-rover-curiosity-equipped-vaisalas-111002257.html.