Post SCMS95 50Y Report
Introduction:
This report looks at the performance of the PAM III hygrothermometer
and BandPass hygro thermometer during the SCMS95 project. Overall the
sensor performance was good. Out of the 6 sensors deployed 2 failed which
were replaced with backup sensors. During the project intercomparison
tests were run using a standard Assman psychrometer.
The sensors are typically calibrated in the Thunder Scientific
humidity chamber over a humidity range of 15% to 95% and a temperature
range similar to the project deployment. A transfer function is applied
to the raw data to compute coefficients. For temperature the transfer
function is a 2nd order polynomial. A 2nd order polynomial in RH
with a cross-product term, RH*T, is used for humidity.
Calibrations prior to SCMS95:
Eight sensors were made available for project SCMS95. Four of
these were interfaced to the hygrothermometer electronics and calibrated
as integrated sensors. The other four were calibrated using the Keathley
2001 DVM. All calibrations were done in the Thunder Scientific humidity
chamber in April 95. A spot check was preformed on each sensor over
a selected humidity and temperature range between the Marshall test
and SCMS95. The one exception was hygrothermometer #4 which was a new
sensor. The Vaportron chamber was used during the spot check test.
Results of the calibration and the spot check are presented below in table 1.
sensor calibration in April 95 spot check
(standard dev. of fit)
T RH T RH
(mean) (mean/sd)
TRH #1 (S/N 15) --- 0.79 0.042 -0.67 / 0.98
TRH #2 (S/N 14) --- 0.85 0.000 1.01 / 0.47
TRH #3 (S/N 13) --- 0.94 -0.058 -0.42 / 0.86
TRH #4 (S/N 12) --- --- 0.000 0.00 / 0.52
BPH #1 (S/N 23) 0.02 0.45 -0.037 -2.51 / 1.13
BPH #2 (S/N 24) 0.02 0.40 -0.054 1.95 / 0.59
BPH #3 (S/N 25) 0.02 0.46 0.020 -0.61 / 0.78
BPH #4 (S/N 27) 0.02 0.42 0.008 -0.83 / 0.62
NOTE:
(1) During the spot check sensor number 25 was cleaned with
distilled water.
(2) S/N12 was calibrated as a spare during the spot check test.
(3) The pre-calibration data for temperature on TRH #1 thru
#3 could not be found.
(4) "sd" is the standard deviation of residuals.
Post SCMS95 calibration check:
Six of the eight sensors were run thru a post project check.
The other two were not tested due to sensor failure. A BandPass unit,
50Y #25, failed during the project. A lighting strike has been given
credit for the failure. The electronics board for the spare
hygrothermometer, TRH #4, did not want to communicate. This sensor was
never used during the project.
Humidity tests covered the range of 20% to 95% at temperatures
15, 25, and 35 C. The temperature test in the oil bath was from 10
to 40 C. Table 2 shows the results of the post project check. Refer
to the figures at the end of this report for more detail information
on the sensor(s) response to humidity.
sensor T RH
(mean / sd) (mean / sd)
TRH #1 (S/N 15) 0.095 / 0.02 -0.23 / 1.2
TRH #2 (S/N 14) -0.014 / 0.01 0.30 / 1.1
TRH #3 (S/N 13) -0.054 / 0.01 -0.17 / 1.4
BPH #1 (S/N 23) -0.06 / 0.007 -2.31 / 1.1
BPH #2 (S/N 24) -0.08 / 0.004 -2.70 / 0.6
BPH #4 (S/N 27) 0.00 / 0.004 -1.12 / 1.1
Intercomparison Tests:
During the project there was the opportunity to conduct
intercomparison tests using a standard Assman psychrometer. A limited
data set was collected; however, it gave us an opportunity to monitor
the sensors' performance in the field. These tests were done only
with respect to the hygrothermometers. Results are shown below in table 3.
sensor # of tests T RH
(mean / sd) (mean / sd)
THR #1 4 0.00 / 0.2 -1.40 / 1.4
THR #2 4 -0.02 / 0.1 -6.83 / 0.9 (before clean)
-2.55 / 1.0 (after clean)
THR #3 3 -0.26 / 0.1 -0.04 / 2.0
Conclusions:
The temperature sensors performed as I would expect. The
platinum temperature device is well known in its curve fit and stability.
TRH #1 appeared noisy during the oil bath test; however, it looked
stable during the tests in the Thunder Scientific. Plans are to retest
this sensor to determine if there is a problem.
The humidity sensor performance was also acceptable. It can
been seen in the results that the integrated units perform better
than the simple transducer. I believe this is due to the hygrothermometers
having a dedicated A/D and processor. A second possibility is the
transducers in the BandPass housing have greater exposure to
contaminates which It is also worth noting the response of TRH #2 before
and after cleaning the transducer during the intercomparison tests.
The improved response argrees with a similar test done before deployment
in SCMS95.
Future Tests:
The next step will be to clean the humidity transducers
with distill water then repeat the humidity check. It is our hope that
the sensors will return to the original response when last calibrated.