C B Baker

K G Hubbard

K Robbins

R Fontenot

X Lin

CiteSeerX

14.2           Preliminary Results from a Field Comparison of Relative Humidity Sensors
K. G. Hubbard1, X. Lin1, K. Robbins2, C. B. Baker3, and R. Fontenot2
1University of Nebraska, Lincoln, Nebraska
2 Louisiana State University
3Nationa Climatic Data Center
1. INTRODUCTION
Chilled-mirror hygrometers and capacitive
relative humidity (RH) sensors are two of the most
commonly used sensors for monitoring air vapor
quantities in the atmosphere. Because of the
increasing need for high fidelity climate data and the
recent availability of new hum idity sensors, there is
a need for field  comparison am ong various air
hum idity sensors to determine field performance.
Three RH sensor types are evaluated:  HMP45C
temperature and relative  humidity sensor, HMP233
“Sm art” humidity/dewpoint transmitter (Vaisala Inc),
and MP101A tem perature and re lative humidity
sensor (Rotronic Instrument Co). Two of each type
RH sensor were installed in the aspirated radiation
shield (Met-One Instruments Inc), which has been
selected for use in the U.S. C limate Reference
Network  (USCRN). Another two HMP45C sensors
were installed in the non aspirated Gill shield. Two
chilled-mirror hygrometers : DewTrack 200M
Meteorological Humidity System (EdgeTech Inc) and
ASOS HO-1088 (Technical Services Laboratory Inc)
hygrometer were also employed at the testing site.
This paper explores both relative hum idity
bias and dew point temperature bias of each sensor
in field observations over four m onth intensive
measurem ents in Lincoln, Nebraska. 
2. EXPERIMENTAL MEASUREMENTS
The experimental m easurements in the field
were conducted from March 2002 to September
2002 at the University of Nebraska’s Horticulture
Experiment Site (40o83' N, 96o67' W , elevation
383m ). The ground surface was typical mowed grass
during this study. The experimental measurements
also include ambient air temperature, air pressure,
solar radiation, and am bient wind speed (Fig. 1). 
Corresponding author address: Kenneth G.
Hubbard, High Plains Regional Climate Center
(HPRCC), University of Nebraska-Lincoln, Lincoln,
NE 68583-0728; email: khubbard1@unl.edu
The DewTrack 200M hygrometer with the
NIST traceable calibration was used as a reference
for both RH and dew point sensors in this study. The
accuracy of the DewTrack 200M is ± 0.25oC dew
point temperature and ±1.0 to 1.5% relative humidity.
The HMP45C, MP101A, DewTrack 200M, and
temperature sensor as well as solar radiation and
wind speed were measured by a CR7 datalogger
(Cam pbell Scientific. Inc.) at the height of 1.5
meters, which refers to air uptake height for
aspirated sensors and the sensor height for non
aspirated shields. The HMP233 and ASOS HO-1088
sensors were detected by directly connecting into a
PC via RS232 cables.  
Fig. 1. Instrumentation illustration: the array of RH
and dew point temperature sensors used (two
HMP45C inside the USCRN shields, two HMP45C
inside the Gill shields, HMP233 sensors and
MP101A sensors in the USCRN shields) at the
experimental field.
All measurement sampling rates were 5
seconds with one minute average outputs. The term
RH and dew point temperature bias for each air
hum idity system (HMP45C, HMP233, MP101A, and
ASOS HO-1088) in this paper is defined as the
difference relative to the DewTrack 200M system.
The available data were taken from June 1st to
September 31, 2002 including 166,742 observations
for all RH sensors and chilled mirror hygrometers.
Less than five days during this period were missing
and not inc luded in following data analysis. 
    
3. PRELIMINARY RESULTS AND DISCUSSION
RH bias is shown in Fig. 2. Two of HMP45C
inside the USCRN shields (HMP45-RH1 and
HMP45-RH2) had a wet bias from 1.1% to 2.9% RH
on monthly average.  However, two non aspirated
HMP45C sensors (HMP45-RH3 and HMP45-RH4)
experienced a dry bias from -0.5% to -1.0% RH on
monthly average. Considering the absolute bias, the
difference between them m ight suggest that USCRN
aspirated shield provided for the HMP45C sensor
might not be better than non aspirated shields during
summ er time. Both case had almost the same
standard deviation for all four m onths from 1.3% to
2.0%  RH. 
Two MP101A sensors (MP-RH1 and MP-
RH2) shown in Fig. 2 presented a more biased
performance on average compared to others.
Around 95% observations of MP101A sensors had
a positive bias. The monthly average bias ranged
from 2.2% to 3.5% of RH. The HMP233 sensors
provided the smallest biases. Both average bias and
standard deviation were relatively smaller than
others (Fig. 2). It suggests that the accuracy of
HMP233 RH sensor in field are the m ost c lose to the
accuracy in the laboratory during summer time.
Noting that we re-calibrated all RH sensors (@
varied temperatures from 0 to 49 oC) right before
installation at the site and the accuracy of each
sensor were within the manufacturer’s specifications.
Fig. 2. RH bias for each type RH sensor.  
A dew point temperature comparison was
conducted between the ASOS HO -1088 / HMP233
and DewTrack 200M sensors. Noting that the dew
point temperature in the HMP233 sensor was a
calculated variable in terms of the ambient
temperature, relative humidity directly detected by
sensor, and real-time air pressure value whereas the
RH readings in DewTrack 200M are derived from
chil led mirror dew point temperature, air
temperature, and assigned (fixed) air pressure value.
Fig. 3. shows that dew point temperatures in the
ASOS HO-1088 were monthly unbiased on average.
Results  showed that calculated dew point
temperature from the HMP233 sensors were about
half degree lower than the DewTrack 200M sensor.
It should be mentioned that we observed the ASOS
dew point te m pe ra tur es  we re  ob vio us ly
contaminated by daily adjustment of chilled mirror
optical balance, which we call an ‘operational bias’
and it usually lasted five minutes to ten minutes per
day. Such ‘operational bias’ data in this study were
not inc luded in Fig. 3. 
Fig. 3. The distr ibution of dew point biases in the
ASOS and HMP233 sensors. 
4. SUMMARY AND CONCLUSIONS
The field comparison of RH sensors and
chilled m irror hygrometers revealed that the field
performance of each type of sensor are different.
The HMP233 performed the best during this study
whereas the MP101A sensor  had the largest bias.
 The HMP45C sensor in the aspirated shield was dry
biased but the aspirated HMP45C was wet biased.
The chilled m irror hygrom eter ASOS HO-1088
system could reach ± 1.0  oC dew point temperature
accuracy (95% confidence level) in field after
removing the ‘operational bias’. The calculated
monthly dew point temperature in the HMP233
sensor was 0.3 to 0.6  oC lower on average. During
the experimental period, monthly average, maximum
and minimum bias, and standard deviation of bias
are summarized in Tables  1 and 2.   
Table 1. Monthly RH bias for each RH sensor
Table 2. Monthly dew point temperature bias.
   
REFERENCES
Andreas, E.L., P.S. Guest, P.O.G. Persson, C.W .
Fairall, T.W . Horst, R.E. Moritz, and S.R.
Sem mer, 2002: Near-surface water vapor
over polar sea ice is always near ice-
saturation. J. Geophyical Research-Oceans
(in press).
Fleming, R.J. 1998: A  note on temperature and
relative humidity corrections for humidity
sensors. J. Atmos. Oceanic Tech.
15(6):1511-1515.


Preliminary Results from a Field Comparison of Relative Humidity Sensors

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Preliminary Results from a Field Comparison of Relative Humidity Sensors

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