TAVIS PRESSURE SENSOR SUSPICIONS
ANNEX G To MARS CORRECT: CRITIQUE OF ALL NASA MARS WEATHER DATA: Tavis Transducer Specifications and Test Results (Updated 1/12/2016)
This Annex presents data from the NASA Ames Historical Archives and other sources in an attempt to clarify the question of what transducers were available to go to Mars during the Viking1 and 2 plus Pathfinder missions. The initial operating assumption was that Professor James Tillman is correct about 18 mbar Tavis transducers used for Vikings 1 and 2, with a 12 mbar Tavis sensor sent on Pathfinder, but all of them suffered from problems related to dust-jammed air intake tubes and clogged dust filter. However, exactly which sensors were sent to Mars is still an issue. The first entering argument against the 25 mbar sensor it is based on the Alvin Seiff Collection as summed up in Figure G1 below:
Figure G1 to Annex G – Tavis pressure sensors tested according to the Alvin Seiff papers. Data compiled by Adrian, S.P., (n.d.). Guide to the Alvin Seiff papers. Retrieved from http://www.oac.cdlib.org/data/13030/08/kt738nd508/files/kt738nd508.pdf
The records on Figure G1 cover the period between 1969 and 1975. Viking 1 launched on August 20, 1975. Viking 2 was launched on September 9, 1975. Note that no sensor listed was for 18 mbar. There are four references to 0-25 mbar sensors, and one reference to a P-4A rated at 0.1 Absolute Pressure per Square Inch (PSIA). By 25 mbar, it is apparent that this rating is actually a rounded figure that pertains to the Tavis sensor rated at 0.36 PSIA. The 0.36 PSIA figure equals 24.82 mbar. The Tavis CAD for that sensor was shown earlier as Figure 9A in the Basic Report, but for convenience it is shown again below in this Annex as Figure G2.
The records on Figure 1 cover the period between 1969 and 1975. Viking 1 launched on August 20, 1975. Viking 2 was launched on September 9, 1975. Note that no sensor listed was for 18 mbar. There are four references to 0-25 mbar sensors, and one reference to a P-4A rated at 0.1 Absolute Pressure per Square Inch (PSIA).
By 25 mbar, it is apparent that this rating is actually a rounded figure that pertains to the Tavis sensor rated at 0.36 PSIA. The 0.36 PSIA figure equals 24.82 mbar. The Tavis CAD for that sensor was shown earlier as Figure 10A in the Basic Report, but for convenience it is shown again above in this Annex as Figure G2.
Figures G3 to G5. Figure 3G: Adapted from NASA Report No. TM X-74020 (the Mitchell Report) published in March 1977. Page 4 of the report specifies that the two sensors tested were P-4 sensors having serial numbers S/N 1583 and S/N 1591. Figure 4G: Temperature Malfunction During (Viking) Cruise Environment. Adapted from Figure 20 in NASA Report TM X-74020 (the Mitchell Report). Figure 5G: Tavis P-4 Transducers (S/N 1583 and S/N 1591) used for test of Viking pressures sensors after the launch of the two Vikings. NASA Report TM X-74020 (the Mitchell Report).
So, the question must be asked, does any NASA document back the 18 mbar figure given the Professor Tillman, the Director of the Viking Computer Facility? The answer is yes. His numbers are supported by the NASA Report TM X-74020, Evaluation of Viking Lander Barometric Pressure Sensor (dated March 19877) by Michael Mitchell (hereafter referred to as the Mitchell Report). Its abstract in block 16 is of particular interest. See Figure G3 above.
Figure G6 - Photo of the Tavis P-4 pressure sensor, and written indication that a P-4A was ordered. The date of this particular order is not clear. Figure G7 - Transducer Selection Slide by Professor James E. Tillman
Page 4 of the Mitchell Report under Test Results states the following about what sensors it examined: “Two Tavis Corp. Model P-4 sensors, having serial numbers S/N 1583 and S/N 1591, were chosen to be evaluated using the Viking Mini-Mission format. On September 23, 1975, the sensors were connected to the vacuum system and pumped to less than 10-1 N/M2 (10-3 mb).” The full report is 110 pages, but what immediately catches the eye is the sensor tested (the P-4) and the date of the tests (starting on September 23, 1975. This testing was thus begun after both Vikings had already been launched (Viking 1 launched on August 20, 1975, Viking 2 on September 9, 1975). A picture of the P-4 was supplied to me by April Gage, the NASA Ames historian. The photo clearly indicates that the P-4 was rated at 0.2 PSID – see Figure G6. However, the writing on red ink on the document provided by NASA indicates that Model P-4A was purchased!
Does the figure above, or its writing in red, support the 18 mbar (or 17.9 mbar) figure offered earlier in the Mitchell Report? No. The P-4 shown in Figure G6 is clearly labeled as having a range of 0 to 0.2 psid (not psai). What does that mean? Differential pressure measurement is the difference between two unknown pressures. Output is zero when the two pressures are the same, regardless of magnitude. Differential Pressures are notated as "D" (PSID). The magnitude of the common pressure is called "static" or "base" pressure. Differential transducers are usually "wet/wet" construction. This definition is taken from http://www.iprocessmart.com/techsmart/pressure_help.htm. However, if we assume that one side of the sensor feels less than 0.001 mbar, then essentially the sensor tested was capable measuring a difference of up to 0.2 psi. That amount converts to 13.79 mbar, not 17.9 not 18 mbar.
What about the red writing that indicates a P-4A was purchased? There is nothing on the document that indicates the date it was purchased. But what was the capability of the P-4A? See Figure G1. According to the Guide to the Alvin Seiff papers (Box 2 Folder 2), it is apparent that there was an “Engineering Evaluation Test Report for 0.1 Absolute Pressure per Square Inch Tavis P-4A Transducer, 1973.” That is 0.1 PSIA. This amount equates to 6.9 mbar, still not close to 18 mbar. We pressed Professor Tillman hard on these issues for most of 2010. On November 25, 2010, he finally sent us an e-mail with two attachments. I was surprised to find that the 110-page Mitchell Report was the first of them. We had debated that report back in May 2010 when he first informed us about the radioisotope thermoelectric heaters (RTGs) that were supposed to protect the transducers from external temperatures that were clearly much colder than the -28.89° C tested (see block 16 – the Abstract on the Mitchell Report shown on Figure G3 above) in the very late tests that occurred well after both Vikings were on their way to Mars.
How much colder than -28.89° C was it on Mars? See Appendix 1 to Annex D of our report. It shows that the temperature reported from the surface of the planet on VL-1 Sol in the 0.22 time-bin was -85.76° C (the first temperature recorded at time-bin 0.02 on VL-1 Sol 1 was -78.28° C (in summer at Ls 97.196).
For Viking 2 the first temperature recorded was also at night. It was -72.05° C in the .06 time-bin (VL-2 Sol 1.06), but by Sol 1.18 it was down to -80.26° C (still in the summer). So the obvious question here is, Just how fast did the RTGs kick on and was it fast enough to prevent damage to the transducer? All requests to Professor Tillman for specific information about RTG operations have gone unanswered. It is important to know (1) how fast they began to operate and (2) what triggered their operation – temperature outside, inside, or a simple timer?
The minimum temperature recorded in Viking 1’s first day (-85.76° C, or -122.368 °F) was 54.78°C (98.766° F) colder than what was tested for in the Mitchell Report. And yet the Vikings were both subjected to far colder temperatures as they moved from the summer temperatures felt on landing to the winter lows. For Viking 1 the coldest temperature felt (in its tropical location) was -95.96°C (-140.728°F). For Viking 2 the temperature got as low as -121.01°C (-185.18 °F).
Figure G11 in this article (the Tavis Corporation’s transducer ordering information) yields a -53.89 °C minimum temperature allowed, but that is still not as cold as what was felt by either Viking immediately upon landing.
Now aside from the issue of whether the temperature was too cold for the transducers, there is the issue of the red writing on Figure G6. It is not at all clear as to why NASA would want a transducer that is limited to 0.1 psia/6.9 mbar. As was shown on Table 5 of the Basic Report of my report, Mariner 4 only attempted two pressures readings – and one of them was between 7 and 9 mbar. Mariners 6 and 7 attempted a total of four readings, and two of them ranged from 6.9 to 7.3 mbar. Finally, Mariner 9 saw 10.3 mbar. All of these measurements were in NASA hands well before the Vikings were launched. And yet, the second of two attachments sent to me by Professor Tillman on November 25, 2010 seems to allude to similar sensor with a 0 to 7 mb range as is seen on Figure 5.
Figure G8 - Adapted from Tavis CAD Diagram 10484. For Mars Pathfinder Tavis Dash No -2 had a 0.174 PSIA limit (12 mbar). However, Pathfinder Tavis Dash No -1 had a 15 PSIA limit (1,034 mbar). Source: Personal communication, Tavis Corporation 10/29/2009.
While Professor Tillman has not yet answered questions about the mechanism for RTG operation/timing, the slide shown o Figure G7 was extremely important for three reasons:
(1) It shows that in September 2005, long before my study began in 2009, he was quoting the pressure range of a Sieff (presumably Alvin Seiff mentioned earlier in conjunction with Figure 1) suggested Tavis pressure sensor rated at 0.0 to 18.0 mb (mbar). The 0.26 PSIA figure actually converts to 17.926 mbar.
(2) It provides the resolution of the sensor as 0.088 mbar. That matches what I found and discussed in conjunction with Section 2.4.1 of my Basic Report (The issue of Viking pressure reports and digitization).
(3) It mentions a similar Tavis sensor with 0.0 to 7.0 mbar range with zero shift ≤ 0.02 mbar in 20 years. This is almost certainly the P-4A. It is not clear from the slide as to which project selected vendor was rejected by Professor Tillman, but since the slide dates from before the launch of the Phoenix, it may be a reference to the Vaisala transducer selected for that mission. Since the Vaisala was limited to 12 mbar (11.5 for MSL), and since Viking 2 measured at least 10.72 mbar on its Sol 277.34, it would not make sense to back a sensor that could only see 11.5 or 12 mbar.
For the benefit of those who want to investigate the issue of possible confusion with respect to Tavis sensors and their capabilities, this Annex/artcle also includes the Tavis CAD for the Pathfinder mission. Shown in the Basic Report as my Figure 10B, it is labeled as Figure G8 in this Annex. Finally, the three pages of the Tavis specifications and parts order information received from the NASA Ames historical office are included as Figures G9 to G11. Note that on Figure G9, for the Tavis P-4, the minimum pressure range is 0.1 psi and the maximum is 100 psi. Again, 0.1 psi is 6.8945 mbar, while 100 psi is 6,894.5 mbar! Thus one Tavis transducer with the same model number could apparently be tweaked by the producer to produce results that differed by three orders of magnitude. This is a thousand fold potential source of error. In looking at Figure G8, there were clearly two entirely different pressures given – 0.174 PSIA (12mbar) and 15 PSIA (1,034 mbar. Martian weather simply does not match the lower pressure range offered.
Figure G9 - Table of Characteristics of Tavis transducers (Models P-1, P-2, P-4, P-5, P-6, P-7 & P-8).
In the Mitchell Report under a section entitled Cruise Environment and in conjunction with its Figure 20 there are a number of inconsistencies, typos and problems. The two Tavis Model P-4 pressure sensors tested were S/N 1591 and S/N 1583. The sensors are shown on Figure 8. The Abstract states that these tests were conducted just after the Vikings launched “to determine their performance characteristics related to Viking Mission environment levels.”
The document states that:
On the 9th day, S/N 1591 and S/N 1583 experienced a drop in zero output voltage of 8 mV and 41 mV, respectively, due to a sudden drop and recovery of approximately 67oC. This temperature drop was due to a temporary malfunction in the thermal environment chamber which dropped the temperature to approximately -51o C in one hour. Figure 20 shows a more detailed account of this incident.
The Mitchell report’s Figure 20 is colorized and relabeled as this Annex’s Figure 12. There are numerous issues raised by the above report quotation. First, it seems odd that two sensors, experiencing identical drops in temperature, would have such different voltage drops. Forty-one mV is over 5 times greater than 8. Note that this was during the cruise stage with very low pressure 0.1 N/m2 (0.001 mbar). Next, the -51ºC temperature is lower than the -28.89ºC temperature specified for the test.
Looking at Figure 12, the top graph Y axis is labeled SENSOR OUTPUT (VOLTS). S/N 1591 started with about 0.49 VOLTS. As the temperature drop ensued, the voltage climbed (according to the graph) to about 0.54 VOLTS and then fell to about 0.41 VOLTs. So, overall, it fell from 0.49 to 0.41, a drop of 0.08 – but not mV unless the y Axis is labeled wrong. It probably should read a drop of 0.049 to 0.041. So there is an apparent one order of magnitude in error here someplace. Is the error on the write up, or on the graph?
S/N 1583 started with about 0.53 VOLTS. We'll ignore the decimal place for now as it's already addressed in the previous paragraph. As the temperature drop ensued, the voltage climbed (according to the graph) to about 0.61 VOLTS and then fell to about 0.45 VOLTs. So, overall, it fell from 0.53 to 0.45, a drop of 0.08 volts. This does not line up well with the drop of 41 mV as specified in the write up. It looks like the person generating the graph might have confused the minimum voltage there of 0.41 (or, really, 0.041) for sensor 1591 with the drop in voltage for sensor 1583.
Finally, the difference in voltage AFTER the temperature climbed back up to almost the right temperature was only about one sixth of what it was before the temperature drop. What might this indicate? Perhaps after the Viking Tavis pressure sensors experienced the REAL cold temperature on Mars, they would spit out essentially identical, but meaningless pressure readings. In-other-words, they were ruined. The area in red on Figure 11 represents the difference in mV between the two sensors tested. Figure 12 illustrates why it is important to understand how fast the RTGs started heating and maintaining uniform temperatures after landings occurred. To understand how small the Tavis and Vaisala dust filters were, see Figure 9.
Added to the above question about the Viking Tavis sensors and the affects of low temperature on them is the fact that during Mars Pathfinder pre-launch calibration of its Tavis transducer, both the flight and flight spare pressure sensors were inadvertently exposed to temperatures 30 K below their design limits (see Annex H and http://starbrite.jpl.nasa.gov/pds/viewInstrumentProfile.jsp?INSTRUMENT_ID=ASIMET&INSTRUMENT_HOST_ID=MPFL).