DOE/SC-ARM/TR-050

> Tower Temperature and Humidity Sensors (TWR) Instrument Handbook
>
> January 2007
>
> D.R. Cook
>
> Work supported by the U.S. Department of Energy,
>
> Office of Science, Office of Biological and Environmental Research

# Contents {#contents .Heading-Front}

[1 General Overview 1](#general-overview)

[2 Contacts 3](#contacts)

[3 Deployment Locations and History
4](#deployment-locations-and-history)

[4 Near-Real-Time Data Plots 4](#near-real-time-data-plots)

[5 Data Description and Examples 4](#data-description-and-examples)

[6 Data Quality 11](#data-quality)

[7 Instrument Details 14](#instrument-details)

**Figures**

[1 NSA Barrow 40-meter tower 1](#__RefHeading___Toc121187762)

[2 SGP CF 60-meter tower 2](#__RefHeading___Toc121187763)

[3 SGP Okmulgee 21-meter tower 2](#__RefHeading___Toc121187764)

[4 Southeast site temperature 8](#__RefHeading___Toc121187765)

[5 Southeast side relative humidity 8](#__RefHeading___Toc121187766)

[6 Southeast side vapor pressure 9](#__RefHeading___Toc121187767)

**Table**

[1 Deployment locations. 4](#__RefHeading___Toc121187800)

# General Overview

Three tall towers are installed at atmospheric radiation measurement
(ARM) Climate Research Facility (ACRF): a 60-meter triangular tower at
the Southern Great Plains (SGP) Central Facility (CF), a 21‑meter walkup
scaffolding tower at the SGP Okmulgee forest site (E21), and a 40-meter
triangular tower at the North Slope of Alaska (NSA) Barrow site. The
towers are used for meteorological, radiological, and other
measurements. Instrument systems installed on the towers are listed in
Section 7, and instrument details are listed below, although only the
Temperature/Relative Humidity/Vapor Pressure (T/RH/VP) measurements made
on the SGP CF tower are described in detail. Information on measurements
made by the other instrument systems on the towers can be found at
individual instrument systems web pages. Photographs of each tower are
shown below.

![](media/image1.png){width="6.278472222222222in"
height="6.049305555555556in"}

[]{#__RefHeading___Toc121187762 .anchor}Figure 1. NSA Barrow 40-meter
tower.

![](media/image2.png){width="4.608333333333333in"
height="4.243055555555555in"}

[]{#__RefHeading___Toc121187763 .anchor}Figure 2. SGP CF 60-meter tower.

![](media/image3.png){width="4.591666666666667in"
height="3.626388888888889in"}

[]{#__RefHeading___Toc121187764 .anchor}Figure 3. SGP Okmulgee 21-meter
tower.

# Contacts

## Mentor

David R. Cook

Environmental Research Division

Argonne National Laboratory, Bldg. 203

Argonne, Illinois 60439

Phone: (630) 252-5840

Fax: (630) 252-5498

[drcook@anl.gov](mailto:jcliljegren@anl.gov)

## Instrument Developer

Different tower manufacturers and installers provided the three towers.
For more information on the tower manufacturers and installers, please
contact the tower mentor. For information on the vendors of the
individual instrument systems mounted on the NSA and SGP Okmulgee
towers, please contact the mentors for those instrument systems. Each
tower receives a thorough inspection every two years; this inspection is
conducted by Tower Systems, Inc. for the SGP CF and NSA Barrow towers,
and by the mentor for the SGP Okmulgee tower.

The vendors for the T/RH probes, aspirators, and datalogger equipment on
the SGP CF 60-meter tower are as follows:

T/RH: Vaisala, Inc. on the west and southeast elevators, 25-meter and
60-meter levels. Platinum Resistance Temperature Detector (PRTD)
temperature: Inco Products, Inc. on the west elevator 25-meter and
60-meter levels. Aspirators: Qualimetrics, Inc. on the west elevator
25-meter and 60-meter levels; R.M. Young, Inc. on the southeast
elevator, 25-meter and 60-meter levels.

Flow Sensors: Qualimetrics, Inc. on the west elevator 25-meter and
60-meter levels.

Dataloggers: Campbell Scientific CR21X dataloggers were used for the
west side measurements until about 2001; CR10X dataloggers have been
used since then. A CR10X datalogger has been used for the east side
measurements since their inception in 1996.

Note: Qualimetrics, Inc. T/RH probes were used on the west elevator
levels until September 1996. They were inadequate and inaccurate. All
west elevator T/RH data from 1993 through August 1996 is of questionable
quality.

# Deployment Locations and History 

[]{#__RefHeading___Toc121187800 .anchor}Table 1. Deployment locations.

+-----+-------------+---------+-----------------+-----------+--------+
| *   | *           | **      | *               | **In      | **     |
| *AR | *Location** | Tower** | *Manufacturer** | staller** | Date** |
| M** |             |         |                 |           |        |
|     |             | **Stru  |                 |           | *      |
| **  |             | cture** |                 |           | *Insta |
| ACR |             |         |                 |           | lled** |
| F** |             |         |                 |           |        |
+-----+-------------+---------+-----------------+-----------+--------+
| NSA | Barrow, AK  | 44-inch | Sabre           | Tower     | May    |
|     |             | tri     | Communications, | Systems,  |        |
|     |             | angular | Inc.            | Inc.      | 1997   |
+-----+-------------+---------+-----------------+-----------+--------+
| SGP | Central     | 42-inch | Rohn, Inc.      | Tower     | De     |
|     | Facility,   | tri     |                 | Systems,  | cember |
|     |             | angular |                 | Inc.      | 1992   |
|     | (Lamont,    |         |                 |           |        |
|     | OK)         |         |                 |           |        |
+-----+-------------+---------+-----------------+-----------+--------+
| SGP | Okmulgee,   | a       | UpRight, Inc.   | East      | July   |
|     | OK          | luminum |                 | Coast     |        |
|     |             | scaf    | (provided by    | Rigging   | 1997   |
|     |             | folding | Horizon High    | and       |        |
|     |             |         | Reach, Inc.)    | Co        |        |
|     |             |         |                 | ntracting |        |
|     |             |         |                 | Company,  |        |
|     |             |         |                 | Inc.      |        |
+-----+-------------+---------+-----------------+-----------+--------+

# Near-Real-Time Data Plots

To view near real-time plots of data from instrument systems on the
towers, visit the NCV website at <http://dq.arm.gov/ncvweb/ncvweb.cgi>.

# Data Description and Examples

## Data File Contents (SGP CF 60-meter tower only)

Only data from the T/RH/VP measurements made on the SGP CF 60-meter
tower are described below. Although the tower was erected in December
1992, T/RH instruments were not installed until March 1993. For other
measurements made on this tower or on the SGP Okmulgee and NSA 40-meter
towers, see the websites of the individual instrument systems.

### Primary Variables and Expected Uncertainty

**West elevator, 25-meter level (sgp1twr25m, sgp30twr25m datastreams):**

**1 and 30 minute:**

Temperature (temp): +/- 0.2°C

Relative humidity (rh): +/- 2% (0-90% RH), +/- 3% (90-100% RH)

Vapor Pressure (vap_pres)

**West elevator, 60-meter level (sgp1twr60m, sgp30twr60m datastreams):**

**1 and 30 minute:**

Temperature (temp): +/- 0.2°C

Relative humidity (rh): +/- 2% (0-90% RH), +/- 3% (90-100% RH)

Vapor Pressure (vap_pres)

**Southeast elevator, 25-meter and 60-meter levels (sgp1twr10x,
sgp30twr10x datastreams):**

**1 and 30 minute:**

Temperature (temp_25m, temp_60m): +/- 0.2°C

Relative humidity (rh_25m, rh_60m): +/- 2% (0-90% RH), +/- 3% (90-100%
RH)

Vapor Pressure (vap_pres_25m, vap_pres_60m)

Notes: Vapor pressure is calculated from the temperature and relative
humidity measurements. Uncertainties have not been provided by the
sensor manufacturers, therefore T and RH accuracies provided by the
manufacturer have been quoted instead.

#### Definition of Uncertainty

We define uncertainty as the range of probable maximum deviation of a
measured value from the true value within a 95% confidence interval.
Given a bias (mean) error *B* and uncorrelated random errors
characterized by a variance *σ^2^*, the root-mean-square error (RMSE) is
defined as the vector sum of these,

$\text{RMSE} = \left( B^{2} + s^{2} \right)^{\frac{1}{2}}$.

(*B* may be generalized to be the sum of the various contributors to the
bias and *σ^2^* the sum of the variances of the contributors to the
random errors). To determine the 95% confidence interval we use the
Student's *t* distribution: *t*~n;0.025~ ≈ 2, assuming the RMSE was
computed for a reasonably large ensemble. Then the *uncertainty* is
calculated as twice the RMSE.

### Secondary/Underlying Variables

This section is not applicable to this instrument.

### Diagnostic Variables

**West elevator, 25-meter level (sgp1twr25m, sgp30twr25m datastreams):**

**1 minute:**

Aspirator status (aspirator)

Battery voltage (vbat)

**30 minute:**

Standard deviation of temperature (sd_temp)

Standard deviation of relative humidity (sd_rh)

Standard deviation of vapor pressure (sd_vap_pres)

Aspirator status (aspirator)

**West elevator, 60-meter level (sgp1twr60m, sgp30twr60m datastreams):**

**1 minute:**

Aspirator status (aspirator)

Battery voltage (vbat)

**30 minute:**

Standard deviation of temperature (sd_temp)

Standard deviation of relative humidity (sd_rh)

Standard deviation of vapor pressure (sd_vap_pres)

Aspirator status (aspirator)

**Southeast elevator, 25-meter and 60-meter levels (sgp1twr10x,
sgp30twr10x datastreams):**

**1 minute:**

Battery voltage (vbat)

**30 minute:**

Standard deviation of 25-meter temperature (sd_temp_25m)

Standard deviation of 25-meter relative humidity (sd_rh_25m)

Standard deviation of 25-meter vapor pressure (sd_vap_pres_25m)

Standard deviation of 60-meter temperature (sd_temp_60m)

Standard deviation of 60-meter relative humidity (sd_rh_60m)

Standard deviation of 60-meter vapor pressure (sd_vap_pres_60m)

Note: The aspirator status levels for the west elevator are set in the
CR10X datalogger programming to a level that will always indicate that
the aspirators are working. It was found that a reliable indication of
aspirator status was not possible with the long cable lengths on the
tower. Therefore, the aspirator status diagnostic variable is of no
practical use; it is maintained only to preserve the datastream form
used since the T/RH system was installed in 1993.

### Data Quality Flags

**West elevator, 25-meter level (sgp30twr25m datastream):**

**30 minute:**

qc_temp

qc_rh

qc_vap_pres

qc_aspirator

**West elevator, 60-meter level (sgp30twr60m datastream):**

**30 minute:**

qc_temp

qc_rh

qc_vap_pres

qc_aspirator

**Southeast elevator, 25-meter and 60-meter levels (sgp30twr10x
datastream):**

**30 minute:**

qc_temp_25 m

qc_rh_25 m

qc_vap_pres_25 m

qc_temp_60 m

qc_rh_60 m

qc_vap_pres_60 m

### Dimension Variables

Note: lat, lon, and alt refer to the ground surface, not to the
instrument system height

**1 and 30 minute:**

lat

lon

alt

base_time

time_offset

## Annotated Examples

The following plots of 25-meter and 60-meter temperature, relative
humidity, and vapor pressure on August 1 and 2, 2004, show normal
diurnal variations for summertime. During nighttime hours, a temperature
inversion sets up, resulting in a reversal in sign of the daytime
relative humidity profile. Note the lack of dependence of vapor pressure
on temperature and relative humidity.

![](media/image4.png){width="5.9847222222222225in"
height="3.6222222222222222in"}

[]{#__RefHeading___Toc121187765 .anchor}Figure 4. Southeast site
temperature.

![](media/image5.png){width="5.427083333333333in"
height="3.6506944444444445in"}

[]{#__RefHeading___Toc121187766 .anchor}Figure 5. Southeast side
relative humidity.

![](media/image6.png){width="5.913888888888889in"
height="3.592361111111111in"}

[]{#__RefHeading___Toc121187767 .anchor}Figure 6. Southeast side vapor
pressure.

## User Notes and Known Problems

The elevator systems used on the SGP CF 60-meter tower and the NSA
Barrow 40-meter tower are robust. However, the receiver brackets on the
SGP CF tower have at times bent under frequent use, which causes a loss
of ability to make an electronic connection between an elevator carriage
connector and the tower receiver and therefore some data loss.
Maintenance by the installer has been required to fix this problem on
several occasions.

Fortunately, this situation has not happened at the NSA Barrow tower
yet.

There were significant periods of poor relative humidity measurements on
the SGP CF tower before September 1996, when the Qualimetrics probes
were in use on the west elevator system and did not have the correct
conversion to engineering units in the datalogger programming. Much of
this stemmed from incorrect calibrations performed by the manufacturer.
These inconsistencies were discovered normally in the data after
installation of the relative humidity probe. Sometimes this was
recognized during the six-month calibration and the calibration was
adjusted in the datalogger programming. However, at times, these
adjustments have only corrected the data below about 90% relative
humidity, with no sensitivity resulting for relative humidity above 90%.
One reason for this has been determined to be the calibration procedure
used by the manufacturer, who calibrated the sensors at three relative
humidities, but did not check the performance at 0% or 97%. Since the
sensor is less than linear above 90%, a check above 90% must be used to
ensure that the slope has been properly set for the entire operating
range of the sensor. Since September 1996, Vaisala, Inc. HMP35D or
HMP45D T/RH probes have been used on the tower; these sensors are sent
to Vaisala, Inc. for calibration annually. Each relative humidity probe
is now replaced and recalibrated annually. The Vaisala Inc. probes do
not usually exhibit any abnormal characteristics. However, the Vaisala
probes installed on the west side of the tower in August 2003 had an
abnormally high relative humidity bias, which ramped up to as much as
13% excess relative humidity at 60-meter on the west elevator after a
few months. This probe had to be replaced to ensure accurate
measurements.

Temperature comparisons between the southeast and west sensors on the
SGP CF tower can show some differences. Strong solar heating can cause
the west side temperatures to be slightly higher than the southeast side
temperatures and the west side relative humidities to be lower than the
southeast side humidities. This is seen most frequently during the
summer.

The connections between the SGP CF tower west side tower instruments at
25 meters and 60 meters and the dataloggers were accidentally reversed
from May 19, 1999, until September 20, 2000, (see discussion in DQR
D001011.1). This caused measurements from 25 meters to be recorded in
datastreams, files, and data fields labeled as 60 meters, and vice
versa. Because the data values were valid measurements, but were labeled
incorrectly, it was difficult to identify this error during data
analysis. This data has been reprocessed to place it at the correct
levels; the corrected data are what is found in the ARM Archive.

The sensors on the NSA Barrow tower are subject to freeze-up at times
during the winter. Rime ice easily builds up on the instrumentation,
causing blocking of the inlet to the T/RH probes and interference of the
measurements of the chilled mirror hygrometer, present weather sensor,
and sonic anemometer. The wind speed and direction sensors on the tower
are heated and therefore do not ice up.

The SGP Okmulgee tower is situated in a forest and is therefore subject
to vulture roosting. The bird droppings caused a deterioration of a
number of measurements, including radiometry, precipitation, sonic
anemometry, etc. Numerous deterrents were tried without much success
until bird spikes and rotating shafts were installed; these have worked
well since early 2004.

The original booms on the SGP Okmulgee tower were difficult to operate.
Therefore, new booms manufactured by Tower Systems, Inc. were installed
in 2003 and have proved to work much better.

## Frequently Asked Questions

1.  **What does the aspirator value represent?**

The aspirator status value on the west side of the SGP CF tower
indicates whether aspiration of the T/RH and PRTD probes is sufficient
to produce quality measurements of temperature and relative humidity.
The aspiration rate must exceed a certain value to prevent errors from
heating of the sensors by solar radiation impinging on the aspirator
radiation shield. A complication prevents the aspirator value from
accurately indicating aspiration status; long cable lines on the tower
introduce the need to use a marginal limit value for aspiration status
in the datalogger program that serves the tower temperature and relative
humidity system. Therefore, wind speeds in excess of about 10 meters per
second used to produce (before May 24, 1996) a false aspirator status
indication. A change in the aspiration limit value in the CR10X programs
has effectively resulted in aspiration status being continuously good.
See the discussion of this issue in the note at the end of
Section 5.1.3.

2.  **Which measurements are the most accurate, from the southeast side
    or from the west side?**

Data users are advised generally advised to use the southeast side
measurements, unless only the west side measurements are available or
correct. This is because the aspiration and radiation shielding of the
probes on the southeast side is usually better than on the west side.
The aspiration rate of the aspirators on the west side is barely
adequate, resulting in west side temperatures being up to a degree too
high and therefore relative humidities being too low during low wind and
high solar radiation conditions, particularly during the summer.

# Data Quality

## Data Quality Health and Status

The status of the measurements made by the T/RH/VP system on the SGP CF
tower can be found by going to the ARM Data Quality (DQ) HandS website
at dq.arm.gov/cgi-bin/dqmenu.pl.

## Data Reviews by Instrument Mentor

Monthly reviews of the SGP 60-meter tower data are prepared by the
mentor and submitted to the Instrument Mentor Monthly Summary (IMMS)
report database. These reports are available at "Related Documents" on
the Tower Temperature and Humidity Sensors (TWR) web page.

Beginning in FY2006, Data Quality Reports (DQRs) are not written for
missing data or for situations when QC flags clearly show that the data
are incorrect. DQRs are written for periods when the tower carriages are
down; in this case, QC flags often do not appear in the data and it is
not obvious that the data should have been flagged as incorrect.

## Data Assessments by Site Scientist/Data Quality Office

The following guidance has been provided by the SGP 60-meter tower
mentor for use by the DQ Office in preparing their weekly assessment
report for the SGP 60-meter tower temperature and humidity systems.

**60-meter Tower Data Quality Guidance**

**David R. Cook**

**2 November 2005**

**Introduction:** The best way to tell someone what to look for in
assessing the tower data is to describe conditions that reflect correct
and incorrect data. For the most part, the QC checks provide adequate
guidance. However, there are conditions for which legitimate values fall
outside the QC limits. Therefore, please use the following information
as further guidance.

**Primary Measurements:**

West elevator, 25-meter level (sgp1twr25m, sgp30twr25m datastreams):

temp (temperature)

rh (relative humidity)

vap_pres (vapor pressure)

West elevator, 60-meter level (sgp1twr60m, sgp30twr60m datastreams):

temp (temperature)

rh (relative humidity)

vap_pres (vapor pressure)

Southeast elevator, 25-meter and 60-meter levels (sgp1twr10x,
sgp30twr10x datastreams):

temp_25m (25-meter temperature)

temp_60m (60-meter temperature)

rh_25m (25-meter relative humidity)

rh_60m (60-meter relative humidity)

vap_pres_25m (25-meter vapor pressure)

vap_pres_60m (60-meter vapor pressure)

**Nuisance QC Flags:** The aspirator (aspirator status) QC flag may trip
for the west side 25 meters or 60 meters. However, the aspirator status
levels are set in the CR10X datalogger programming so that they will
always indicate that the aspirators are working. A reliable indication
of aspirator status was impossible with the long cable lengths on the
tower. Therefore, the aspirator status diagnostic variable is useless;
it is maintained only to preserve the datastream form used since the
T/RH system was installed in 1993. Therefore, the aspirator QC flag
condition should not be reported in the DQO assessment reports.

**Comparison of Data Between the West and Southeast Sides:** The
measurements can generally be favorably compared between the two sides
of the tower. However, the west side aspirator shields are made of metal
(the southeast side are plastic), absorb more solar radiation than the
southeast side aspirators do, and tend to cause the west side
temperature to be just slightly higher than the southeast side and west
side relative humidity to be slightly lower during high solar and warm
weather conditions (primarily summer). When the temperature difference
between the two sides and at the same height differs by more than 2°C,
this should be reported in the assessment report. When the relative
humidity difference between the two sides and at the same height differs
by more than 10 percent, this should be reported in the assessment
report.

**Comparison of Data With the Surface Meteorological Observing Station
(SMOS) and Energy Balance Bowen Ratio (EBBR):** Caution must be used in
comparing the tower temperature and relative humidity measurements with
the E13 SMOS and EBBR (both in the CF central cluster) measurements.
First, they usually see different vegetation surfaces; this is true for
virtually all wind directions. Secondly, they are at different heights
(SMOS and EBBR temperature and humidity measurements are at \~2 meters).
At times, the 60-meter level of the tower is de-coupled atmospherically
from the surface, resulting in very large differences in measurements
between the SMOS, EBBR, and the two levels of the tower.

**Comparison of Data With the SONDE:** The SONDE is launched some
distance away from the tower and, depending on the wind direction and
speed; the sonde could be located very far away from the tower when it
reaches the 25-meter and 60-meter heights. Therefore, one cannot expect
that the SONDE measurements of temperature and relative humidity will be
exceptionally similar to the tower measurements. Furthermore, the SONDE
sensors do not respond rapidly to strong gradients in temperature and
relative humidity that often occur in the lower 60 meters of the
atmosphere; they tend to lag the actual temperature and relative
humidity. When the vertical gradients of temperature and relative
humidity are strong, large differences will be seen between the SONDE
and tower measurements.

**Common Conditions Reflecting Correct or Incorrect Data:**

a)  Occasionally, the tower elevators experience hardware or electronic
    problems, such as when an elevator carriage connector does not mate
    properly with the receiver on the tower. In this case, some or all
    of the measurements at that level have been incorrect. If this
    condition seems obvious (especially when comparing data between the
    two sides of the tower), please report it in the assessment report.

b)  If datalogger battery voltage (vbat in the one-minute datastream) is
    less than 10.5 volts, all measurements are reliable and can be
    considered incorrect.

c)  Since the aspirator status for the west side 25-meter and 60-meter
    levels is useless, the data should not be considered correct or
    incorrect based on this value.

d)  Occasionally a relative humidity probe will become saturated with
    moisture, resulting in a measurement of 98% or more for many hours.
    When this condition persists for more than several hours, a
    hysteresis can occur in the recovery of the sensor from saturated
    conditions. The relative humidity may read too high for as much as a
    day afterward. This condition can usually be detected from looking
    at the other relative humidity measurements on the tower or from
    comparison with the EBBR and SMOS measurements.

## Value-Added Procedures and Quality Measurement Experiments

> Tower-related value-added procedures (VAPs), or VAPs that use SGP CF
> tower data as input include:
>
> [**TWRMR**](http://www.arm.gov/docs/vaps/static/twrmr.html) (computes
> water vapor mixing ratio at the surface and at the 25-meter and
> 60‑meter tower levels; needed as input to RL PROF MR, and hence RLPROF
> BE)
>
> [**RLPROF
> suite**](http://engineering.arm.gov/process/vap/docs/rlprof/),
> specifically [**RLPROF
> MR**](http://engineering.arm.gov/process/vap/docs/rlprof/webpage.rlprof_mr.html)
> (25-meter- and 60-meter-level tower in situ measurements of moisture
> are used as input)
>
> Tower-related QMEs, or QMEs that use tower data as input, include:
>
> [**QMEAERIPROF**](http://www.arm.gov/docs/vaps/static/qmeapro.html)
> (25-meter and 60-meter tower in-situ measurements of temperature and
> vapor pressure are used as inputs)

# Instrument Details

## Detailed Description

### List of Components

**SGP CF 60-meter tower:**

Vaisala HMP35D or HMP45D T/RH probes (west side at 25 meters and
60 meters, east side at 25 meters and 60 meters), in aspirators.

Minco Products Inc., Model S853PD60X72 100 ohm PRTD temperature probes
at 25 meters and 60 meters on the west side, in aspirators.

Qualimetrics, Inc. Model 8151-B motorized aspirated radiation shield
with Qualimetrics, Inc. Model T450009 flow sensor at 25 meters and 60
meters on the west side.

R.M. Young Model 43408/43482 motor aspirated radiation shield at 25
meters and 60 meters on the southeast side.

Campbell CR10X dataloggers, and communication interface equipment; one
each per 25 meters west level, 60 meters west level, southeast side.

IRT (Infrared Thermometer), upwelling, 25 meters, southeast side.

MFR (Multi-Filter Radiometer), upwelling, 25 meters, southeast side.

GPS (global positioning system) repeater antenna, 60 meters, southeast
side.

PGS (Precision Gas System), attached to tower structure at 2, 4, 25 and
60 meters, north side.

CO~2~FLX (Carbon Dioxide Flux Measurement Systems), 25 meters and 60
meters, west side.

**SGP Okmulgee 21-meter tower (see individual instrument system sites
for details):**

SMOS (Surface Meteorological Observation Station)

ECOR (Eddy Correlation System)

SIRS (Solar and Infrared Station)

**NSA Barrow 40-meter tower** **(see Barrow meteorology station \[BMET\]
website at**
[www.arm.gov/instruments/static/bmet.stm](http://www.arm.gov/instruments/static/bmet.stm)
**for more details):**

Technical Services Laboratory, Inc. Model 1088 chilled mirror hygrometer
at 3 meters, attached to the north leg of the tower. (previously,
Meteolabor AG VTP6 Ventilated Thermohygrometer chilled mirror hygrometer
at 3 meters, attached to the north leg of the tower, removed in
October 2003).

Vaisala, Inc. FD12P Present Weather Sensor measures visibility,
precipitation, and precipitation type at 2 m, located south of the
tower.

Vaisala, Inc. WS425 Sonic anemometer at 2 meters, northeast side.

Vaisala, Inc. HMP45D T/RH probes measure T/RH/VP/Dew Point at 2, 10, 20,
and 40 meters, northeast side.

Vaisala, Inc. WAA251 cup anemometers with heated cups and shafts measure
wind speed at 2, 10, 20, and 40 meters, northeast side.

Vaisala, Inc. WAV251 wind vanes with heated shafts measure wind
direction at 2, 10, 20, and 40 meters, northeast side.

Vaisala, Inc. PTB-201A Digital Barometer measures station atmospheric
pressure at 1.5 meters, located in an enclosure on the northwest side of
tower.

Scientific Technology, Inc. ORG-815-DR optical precipitation sensor
attached to the southeast leg of the tower at 3 meters (removed from
service in October 2003).

Vaisala, Inc. QLI50 Sensor Collectors used at 2, 10, 20, and 40 meters
to convert wind speed, wind direction, temperature, and relative
humidity signals to digital data.

Vaisala, Inc. WHP25 power supplies used at 2, 10, 20, 40 meters to
service the sensor Collectors and instrumentation.

Campbell Scientific, Inc. CR23X datalogger, multiplexer, and
communication equipment, located in an enclosure on the northwest side
of the tower.

Camera that takes photographs of the tundra surface south of the tower,
installed at 40 meters, south side.

### System Configuration and Measurement Methods

Duplicate sets of instrumentation are used at heights of 25 meters and
60 meters on the SGP CF 60-meter tower. The probes and aspirated
radiation shields are mounted on both the west and southeast elevator
systems. A separate Campbell CR10X datalogger (previously, CR21X
datalogger) is used for each level on the west elevator to measure
sensor outputs, perform conversions to engineering units, and provide
serial output for the remote collection of data from the datalogger
memory. The same functions are accomplished for both levels on the
southeast elevator with one CR10X. The meteorological measurements made
with the system are as follows:

Air temperature at two heights (25 meters and 60 meters)

Relative humidity at two heights (25 meters and 60 meters)

Aspiration status (west elevator only)

### Specifications

The accuracies cited below for the meteorological sensors on the SGP CF
60-meter tower are generally those stated by the manufacturer. They are
sensor absolute accuracies and do not include the effects of system
(i.e., datalogger) accuracies. Although it is not known how some of the
manufacturers have determined sensor accuracy, it is properly the root
square sum of any nonlinearity, hysteresis, and non-repeatability,
usually referenced as percentage of full scale.

The detection limit is normally restricted to the range (sometimes
called Calibrated Operating Range) over which the accuracy applies. Some
manufacturers also specify an Operating Temperature Range in which the
sensor will function physically and electronically, even though the
calibration may not be appropriate for use throughout that range. When
no detection limits have been listed by the manufacturer, none is stated
below.

**West Elevator:**

**Air temperature:** Platinum Resistance Temperature Detector (PRTD),
100 ohm, Minco Products Inc., Model S853PD60X72; Detection Limits -30°
to 40°C; Operating Temperature Range -40° to 50°C; Accuracy +/- 0.2°C.

**Relative Humidity:** Before September 1996: Capacitive element,
Vaisala Inc. Humicap; probe, Qualimetrics, Inc. Model 5120-E or 5134-E;
Detection Limits 0% to 100% RH; Accuracy +/- 2% (0‑80% RH) and +/- 3%
(80-100%) over a temperature range of -20° to 50°C; uncertainty of RH
calibration +/- 1%. After September 1996: Vaisala Inc. HMP35D or HMP45D;
Humicap capacitive relative humidity element; Detection Limits 0% to
100% RH; Accuracy +/- 2% (0-90% RH) and +/- 3% (90-100%) over a
temperature range of -20° to 50°C; uncertainty of RH calibration +/- 1%.

**Motor Aspirated Radiation Shield:** Qualimetrics, Inc. Model 8151-B
with flow sensor; nominal ventilation rate 1.83 meter per sec into
inlet; radiation error 0.05°C; operating temperature range -40 to 55°C.
Wind speeds over 10 meter per sec, particularly from the south can
reduce the ventilation rate in the radiation shield; uncertainties are
thought to be negligible due to this effect as radiation errors from
heating of the radiation shield are reduced in high wind speeds.

**Flow Sensor:** Qualimetrics Model T450009; this sensor is a heated
element type sensor that detects changes in air flow via a relay sensing
of a gap that is dependent on the maintenance of a particular
temperature in a coil.

**Southeast Elevator:**

**Temperature and Relative Humidity:** Vaisala Inc. HMP35D or HMP45D;

**Air Temperature**: Platinum Resistance Temperature Detector (PRTD),
100 ohm; Detection Limits -30° to 40°C; Operating Temperature Range -40°
to 50°C; Accuracy +/- 0.2°C.

**Relative Humidity:** Humicap capacitive relative humidity element;
Detection Limits 0% to 100% RH; Accuracy +/- 2% (0-90% RH) and +/- 3%
(90-100%) over a temperature range of -20° to 50°C; uncertainty of RH
calibration +/- 1%.

**Motor Aspirated Radiation Shield:** R.M. Young Model 43408/43482;
nominal ventilation rate 3 meter per sec into inlet; radiation error
0.05°C; operating temperature range -40° to 55°C. Uncertainties are
thought to be negligible.

## Theory of Operation

Temperature and relative humidity are measured at both 25 meters and 60
meters on both elevators (west and southeast sides) of the SGP CF tower.
However, the aspirators used on the two sides are different; the west
side aspirators are made of metal and have a fairly low flow rate of 1.8
L/min, whereas the southeast side aspirators are made of plastic and
have a flow rate closer to 3 L/min. The temperature sensors on the two
sides are also different; the southeast side uses the internal PRTD in
the T/RH sensor, whereas the west side uses a PRTD that is separate from
the T/RH sensor. Because of these differences, some difference in
measurements from the two sides at the same height is expected.

Ambient vapor pressure is calculated for both sides (west and southeast)
and measurement heights (25 meters and 60 meters) from the measurements
of temperature and relative humidity. This is accomplished using an
instruction in the datalogger that calculates saturation vapor pressure
from temperature. Vapor pressure (in kPA) is then calculated from

Vapor Pressure = Relative Humidity/Saturation Vapor Pressure.

## Calibration

### Theory

Standard calibration procedures are followed for all temperature and
relative humidity sensors used on the SGP CF 60-meter tower. See details
below in Section 7.3.2.

### Procedures

Vendor recommended calibration procedures, and calibration checks by the
mentor are used to maintain the accuracy of the temperature and relative
humidity sensors on the SGP CF tower.

Four-point relative humidity calibrations (at 0%, 12%, 75%, and 97%)
above saturated salt solutions (12% and 75%), zero air (0%), and in
almost saturated air (97%) are performed annually on the SGP CF tower
HMP35D and HMP45D T/RH sensors by Vaisala, Inc. (after September 1996).
The PRTD is also checked for accuracy by Vaisala, Inc. and the
electronics adjusted to provide accurate temperature measurements.
Previously, calibrations were performed by NovaLynx, Inc. (west elevator
only) at 12%, 40%, and 75% above saturated salt solutions. This previous
calibration technique often yielded incorrect calibration slopes, zero
offsets, and a lack of sensitivity at relative humidity above 85%.

The Minco Products, Inc. PRTD on the west side are calibrated annually
by the mentor with an ice bath check in the field (the elevator
carriages are lowered to ground level for this procedure). The PRTD data
are presently acquired with Campbell Scientific, Inc. CR10X dataloggers,
using the built-in resistance-to-temperature polynomial function. The
inverse of a normalized resistance ratio at zero degrees Centigrade is
entered into the datalogger system software as a multiplier.

The T and RH probes are also annually checked against each other and
against ambient values using an aspirated psychrometer as the reference
(both carriages from both sides are lowered to ground level for this
procedure).

### History

All calibration information is kept in the 60-Meter Tower Temperature -
Relative Humidity Manual. Copies of the manual are maintained by site
operations personnel at SGP CF and by the mentor. A large number of
calibrations have been performed by the manufacturers and by the mentor
and/or site operations personnel (in field). Calibration of west
elevator relative humidity sensors was performed by NovaLynx before
October 1996 and both east and west elevator sensors have been
calibrated by Vaisala, Inc. since September 1996. The dates of
calibration are listed below for each sensor serial number and/or
location. W designates the west-facing elevator (side B); S designates
the southeast facing elevator (side A). The T/RH sensor serial number is
the one that was left installed after calibration. MINCO PRTDs do not
have serial numbers. Qualimetrics RH sensor numbers are three digits
long. Vaisala T/RH sensor serial numbers are the long numbers. An
asterisk indicates when a PRTD was replaced.

Temperature (PRTD):

Vendor Calibrations:

MINCO: MINCO PRTD: two sensors Mar 1993

MINCO PRTD: two sensors Mar 1996

Vaisala: HMP35D: S1720002 Jun 1997

HMP35D: S1720003 Jun 1997

HMP35D: unknown Oct 1997

HMP35D: S1720004 Jun 1997

HMP35D: S1720005 Jun 1997

HMP45D: S4520022 Jan 1998

HMP45D: S4520023 Jan 1998

HMP45D: S4930004 Jan 1998

HMP45D: S4930005 Jan 1998

Annual calibration performed on four Vaisala, Inc. T/RH sensors by
Vaisala, Inc. since.

In-field Calibrations (for southeast elevators, see T/RH sensor
replacement dates):

W 25 meters 6 Feb 1996

23 May 1996

21 Oct 1996

5 Feb 1997

17 Jul 1998

19 May 1999

14 Aug 2000

8 Aug 2001

2 Aug 2002

15 Aug 2003

27 Sep 2004

S1720002 5 Aug 2005

S 25 meters S2920009 4 Aug 2005

W 60 m eters 11 Mar 1993

6 Oct 1993

11 May 1994

15 Nov 1994

19 May 1995

19 Sep 1995

6 Feb 1996

23 May 1996 \*

21 Oct 1996

5 Feb 1997

17 Jul 1998

19 May 1999

14 Aug 2000

8 Aug 2001

2 Aug 2002

15 Aug 2003

27 Sep 2004

S1720004 5 Aug 2005

S 60 meters S2920011 4 Aug 2005

Relative Humidity:

Vendor Calibrations:

Novalynx S/N 109 29 Jul 1992

S/N 109 7 Jul 1993

S/N 231 ? Nov 1995

S/N 226 5 Apr 1996

S/N 234 5 Apr 1996

Vaisala, Inc. S/N 226 3 Dec 1996

S/N 234 3 Dec 1996

S/N 109 26 Feb 1997

S/N 231 26 Feb 1997

S/N 226 21 Aug 1997

S/N 234 21 Aug 1997

HMP35D: S1720002 Mar 1997

HMP35D: S1720003 Mar 1997

HMP35D: S1720004 Mar 1997

HMP35D: S1720005 Mar 1997

HMP45D: S4520022 Jan 1998

HMP45D: S4520023 Jan 1998

HMP45D: S4930004 Jan 1998

HMP45D: S4930005 Jan 1998

Annual calibration performed on four Vaisala, Inc. T/RH sensors by
Vaisala, Inc. since.

In-field Calibrations:

W 25 meters S/N 234 6 Feb 1996

S/N 226 19 Mar 1996

S/N 231 25 Mar 1996

S/N 231 22 May 1996

S/N 231 21 Oct 1996

S/N 231 4 Feb 1997

S/N 234 5 Feb 1997

S/N 109 13 Aug 1997

S4930005 5 Nov 1997

S4930004 17 Jul 1998

S4930005 19 May 1999

S4520022 14 Aug 2000

S1720002 8 Aug 2001

S4520023 2 Aug 2002

S1720004 15 Aug 2003

S2920008 27 Sep 2004

S 25 meters S1720003 13 Aug 1997

S1720005 26 Sep 1997

S4520022 17 Jul 1998

S1720004 19 May 1999

S1720002 14 Aug 2000

S1720004 8 Aug 2001

S4520022 2 Aug 2002

S4520015 15 Aug 2003 (replaced 10 Feb 2004, S2920011, no cal.)

S4930004 27 Sep 2004

W 60 meters S/N 109 11 Mar 1993

S/N 231 6 Oct 1993

S/N 231 7 Oct 1993

S/N 231 11 May 1994

S/N 231 15 Nov 1994

S/N 109 19 May 1995

S/N 109 19 Sep 1995

S/N 109 6 Feb 1996

S/N 234 22 May 1996

S/N 226 21 Oct 1996

S/N 109 4 Feb 1997

S/N 226 5 Feb 1997

S/N 231 13 Aug 1997

S1720004 5 Nov 1997

S4520023 17 Jul 1998

S1720003 19 May 1999

S4930004 14 Aug 2000

S4930005 8 Aug 2001

SW0120011 2 Aug 2002

S4930005 21 Feb 2003

S1720002 15 Aug 2003

S2920010 27 Sep 2004

S 60 meters S1720002 13 Aug 1997

S1720002 17 Jul 1998

S1720005 19 May 1999

S4520023 14 Aug 2000

S1720003 8 Aug 2001

S4930004 2 Aug 2002

S1720003 25 Oct 2002

S4930004 15 Aug 2003 (replaced 10 Feb 2004, S2920009, no cal.)

S4520015 27 Sep 2004

## Operation and Maintenance

### User Manual

Copies of the SGP CF tower and elevator user manuals are kept at SGP CF
and at the mentor's office. A copy of the SGP Okmulgee tower user manual
is kept at SGP CF and at the mentor's office. Copies of the NSA tower
and elevator user manuals are kept at NSA Barrow and at the mentor's
office.

SGP CF and Okmulgee maintenance and system checks are performed in
accordance with procedures developed by SGP Site Operations and the
mentor; these procedures are maintained in print form at SGP CF and in
digital form on Site Operations laptops taken into the field and involve
maintenance and checks on the various sensors on these towers. A NSA
tower maintenance procedures document is maintained at NSA Barrow and
includes maintenance and checks on the various sensors on the tower.

Software Documentation:

The SGP CF tower tower25, tower60, and twr2560 programs are maintained
on SGP Site Operations computers at the SGP CF, on laptops used by Site
Operations, in print form by SGP Site Operations, and by the mentor
(print and digital copies).

### Routine and Corrective Maintenance Documentation

Print copies of the annual SGP CF tower in-field calibration reports are
maintained at SGP CF and by the mentor.

### Software Documentation

This section is not applicable to this instrument.

### Additional Documentation

This section is not applicable to this instrument.

## Glossary

See the [ARM Glossary](http://www.arm.gov/about/glossary.stm).

## Acronyms

ACRF ARM Climate Research Facility

ARM Atmospheric Radiation Measurement (Program)

BMET Barrow meteorological station

CF Central Facility

CO~2~FLX Carbon Dioxide Flux Measurement System

DQ Data Quality

ECOR Eddy Correlation System

GPS global positioning system

MFR Mult-Filter Radiometer

NSA North Slope of Alaska

PGS precision gas system

PRTD Platinum Resistance Temperature Detector

RMS root mean square

SGP Southern Great Plains

SIRS Solar and Infrared Station

SMOS Surface Meteorological Observation Station

Also, see the [ARM Acronyms and
Abbreviations](http://www.arm.gov/about/acronyms.stm).

## Citable References

Revercomb, HE, DD Turner, DC Tobin, RO Knuteson, WF Feltz, J Barnard, J
Bosenberg, S Clough, D Cook, R Ferrare, J Goldsmith, S Gutman, R
Halthore, B Lesht, J Liljegren, H Linne, J Michalsky, V Morris, W Porch,
S Richardson, B Schmid, M Splitt, T Van Hove, E Westwater, and
D Whiteman. 2003. "The ARM Program's water vapor intensive observation
periods - overview, initial accomplishments, and future challenges."
*Bulletin of the American Meteorological Society* 84, 217-236.