Start Date | Start Time | End Date | End Time | Data Quality Metric |
---|---|---|---|---|
04/03/1996 | 1700 | 05/21/1996 | 1600 | Does not affect quality |
Subject: | SGP/TWR/C1 - 25m and 60m T/RH Calibration |
DataStreams: | sgp1440twr25mC1.a0, sgp30twr25mC1.a1, sgp30twr60mC1.a1, sgp1twr60mC1.a0, sgp1440twr60mC1.a0, sgp1twr25mC1.a0 |
Description: | The T and RH measurements at the 60 m tower level may need to be adjusted for times between the period listed above; this is concluded from calibrations/checks of the T/RH data during 21-24 May 1996. On May 21-23, the 25 and 60 meter level west carriages were located at the surface for installation of the ECOR C1 system and calibrations/checks of the T/RH sensors. ECOR installation took place primarily on 21 May. A table at the bottom of this text gives recommendations concerning use of the data from the period above, based on the data during the period and the condition of the T/RH sensors as found on 22-23 May 1996. I planned recalibration/checks of the 25 m and 60 m level T/RH sensors in conjunction with a trip to the Central Facility to observe ECOR C1 installation and training. The 60 m T data began to show signs of deterioration of accuracy after 20 April 1996. This became most noticeable by 25 April as the occurrence of transitional periods (as determined from comparison of the 25 and 60 m T) became marginal. The 60 m temperature appeared to be too low most of the time. At the same time, the 60 m RH appeared to be too high. It is difficult to determine what the actual difference between sensors is when the carriages are at the 25 and 60 m levels, especially if one temperature sensor is reporting incorrectly. The RH sensor measurements can be compared fairly easily during the day to night transition period (as determined from the temperatures at the two levels) as a difference in temperature of about 0.7 degrees corresponds to an RH difference of about 5%; smaller differences in each are proportional; when the temperatures are within about 0.1 degree, the RH should be within about 1%. Careful study of the data during the transition time (which varies as to time of day during the year) allows me to determine the quality of the measurements. The ECOR C1 installation provided the perfect opportunity to compare the T/RH sensors for a long period of time when the data would have been incorrect anyway. With the carriages and sensors at the tower base, the RH and temperature sensors were compared on 22 May. The 60 m RH probe indicated an average of 62% while the 25 m RH probe and an aspirated psychrometer indicated 51%. The multiplier in the 60 m CR21X program (1.14) accounted for all but 4% of the difference. An interesting, but confusing, sidenote to this is that the multiplier that I entered on 3 April 1996 was 1.19; I did not have time before I left the CF to change the factor in the storage module from 1.14 to 1.19. I submitted a work request to site operations to have the program with 1.19 multiplier downloaded manually from the CR21X to the storage module (a precaution in case power to the logger was lost and the logger would have to restart from the storage module). However, the program download apparently did not work, as at some later time (or perhaps at the time of the download attempt) the program with the 1.14 factor in the storage module was uploaded to the CR21X. No power outages are known to have occurred thereafter that would have caused uploading of the storage module program to the datalogger (which would have resulted in the 1.14 multiplier ending up in the CR21X), except on 21 May 1996 from 2027 to 2127 GMT. At this time, Site Operations had to disconnect the AC charger unit for the CR21X machines to permit the installation of other devices related to the ECOR C1 installation. However, the CR21X battery voltage did not drop low enough for ceasation of the on-board program. Since RH frequently decreases more than 5% between half hour values and sometimes does so between one minute values, it is impossible to tell when the multiplier changed from 1.19 to 1.14. If it appears to the data user that the 25 m RH is low, it could be multiplied by 1.19/1.14=1.044. It may be more appropriate to use the smaller multiplier, since the RH sensor output was drifting upwards anyway. By the end of the period above, the 60 meter RH should be be decreased by a factor of 1.2. Even though the actual difference between the 25 and 60 m RH probes (with a multiplier of 1.0 for both), at the tower base, was only about 4% (within their combined specifications), we removed 60 m RH probe S/N 109 and replaced it with RH probe S/N 234 at 1940 GMT on 22 May. S/N 234 indicated, on average, 55% while the 25 m RH probe and the aspirated psychrometer indicated 51%. These were the same results obtained with the previous RH sensor. Instead of switching back to the original RH sensor, S/N 234 was left on. The instruments were left at the tower base overnight to compare the T and RH from the two levels. Over a range of RH of 50% to 81% the ratio of 25 m to 60 m RH was incredibly consistent at 0.911. Therefore, on 23 May at 2000 GMT, a multiplier of 0.911 was entered in the CR21X program for the 60 m RH. This was justified by comparisons with the psychrometer that indicated that the 25 m RH sensor agreed with the psychrometer. The overnight comparison also showed what manual measurements on May 22 had, that the 60 m temperature sensor was indicating low. An ice bath test of the 25 and 60 m PRTds showed that both were within 0.1 degrees of zero. At ambient T (30 deg. C), the 25 m sensor and the psychrometer agreed, but the 60 m PRTD indicated 1.5 degrees low. Changes in calibration slope of PRTD sensors are not common, but are possible if the actual sensing element is moved inside the stainless steel jacket. Since most of the change in calibration appears to have happened within a week in late April, it is possible that tower vibration may have been the cause. Again, it is not possible to easily quantify how the 60 m T data should be adjusted from 3 April until 21 May. For several consecutive half hours on 22 May, while both levels of sensors were at the tower base, the 60 m T actually indicated a greater temperature than the 25 m level T sensor (and the 60 m RH was lower than the 25 m level). Such erratic behavior precludes being able to make any recommendation for adjustment of the 60 m T beyond 20 April (or for the 60 m RH beyond 8 May). The 60 m temperature sensor was replaced at 1559 GMT on 23 May. An ice bath was used to determine the proper Rs/Ro and an ambient comparison between the 25 m and 60 m PRTDs and the psychrometer was performed. All agreed within 0.15 degrees over a couple of hours. The tower carriages were then elevated to their normal positions at 25 and 60 meters. The next morning the data from the two levels and the SMOS were compared and found to produce reasonable gradients and mean values. A review of the data since that time continues to show that the data quality is excellent. Side by side comparison of the two sets of sensors over an extended period of time (overnight) allows the best evaluation of performance; this technique will continue to be used in the future in the interest of ensuring data quality. One footnote to the T/RH checks/calibrations is that the aspirator threshhold level was reset to 2200 mv for both 25 m and 60 m aspirators. This appears to be the optimum threshhold for both levels at this time; the threshhold cannot be an unchangeable value, as it varies dependent on length of cable on the tower and age and condition of the flow sensor and aspirator fan. I have to continually evaluate the effectiveness of the threshhold and adjust it when appropriate, a result of the sensor being a variable output device, as opposed to the on/off switch type that is used by some manufacturers. Table of 60 m Data Usage Temperature: 3 Apr - 20 Apr no adjustments 20 Apr - 21 May no recommendation Relative Humidity: 3 Apr - 8 May no adjustments 8 May - 21 May no recommendation |
Suggestions: | |
Measurements: | sgp1twr25mC1.a0: more
sgp1twr60mC1.a0: more sgp30twr25mC1.a1: more sgp1440twr60mC1.a0: more sgp1440twr25mC1.a0: more sgp30twr60mC1.a1: more |