Decagon Devices VP-3 User manual

VP-3
Vapor Pressure, Temperature &
Relative Humidity Sensor
Operator’s Manual
Version 0

Decagon Devices, Inc.
2365 NE Hopkins Court
Pullman WA 99163 USA
[e-mail]
tel: [phone]
fax:[phone]
www.decagon.com

VP-3 User’s Manual
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . 1
Customer Support . . . . . . . . . . . . . . . . . . . . . . .2
Specifications . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
Warranty. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Seller’s Liability . . . . . . . . . . . . . . . . . . . . . . . . . 6
2. Measurements . . . . . . . . . . . . 8
3. RH Sensor. . . . . . . . . . . . . . . . 11
RH Sensor Stability . . . . . . . . . . . . . . . . . . . . . 11
RH Sensor Recalibration. . . . . . . . . . . . . . . . . 13
4. Connecting Sensors . . . . . . . . 14
Connecting to an Em50 Series logger . . . . . . .14
Connecting to a Non-Decagon Logger . . . . . 16
5. Communication . . . . . . . . . . 18
Decagon Serial String . . . . . . . . . . . . . . . . . . . 18
SDI-12 Communication . . . . . . . . . . . . . . . . . 19
SDI-12 Commands . . . . . . . . . . . . . . . . . . 19
SDI-12 Sensor Bus . . . . . . . . . . . . . . . . . . 19
SDI-12 Address. . . . . . . . . . . . . . . . . . . . . 20
Power . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
Reading. . . . . . . . . . . . . . . . . . . . . . . . . . . 21

VP-3 User’s Manual
6. Installation . . . . . . . . . . . . . 22
Installation in a Radiation Shield. . . . . . . . . . . 22
7. Troubleshooting . . . . . . . . . .24
8. References . . . . . . . . . . . . . . . 25
Declaration of Conformity. . . . . 26
Index . . . . . . . . . . . . . . . . . . . . . . 28

VP-3 User’s Manual
1. Introduction
1
1. Introduction
About the VP-3
Thank you for choosing Decagon’s VP-3 for measuring vapor
pressure, temperature and relative humidity (RH). This manual is
designed to help you understand the sensor’s features and how to
use this device successfully. The Decagon VP-3 sensor is an
accurate tool for monitoring vapor pressure, temperature and
relative humidity. The VP-3 utilizes a single sensor chip that
measures both temperature and RH. Each RH/T sensor chip is
individually calibrated with temperature and RH calibrations
stored on board and applied before data are output, ensuring best
possible accuracy. A micro-processor within the VP-3 calculates
vapor pressure from the RH and temperature measurements and
outputs all three parameters using an RS232 (TTL) string and the
common SDI-12 communication protocol.
The VP-3 sensor is packaged in a rugged Delrin housing with the
sensor electronics potted in marine grade polyurethane encapsulant.
The RH/T sensor chip is protected by a hydrophobic porous Teflon
filter that is water and dust proof, but has an extremely high vapor
conductance allowing fast sensor equilibration with the surrounding
atmosphere. An additional stainless steel screen protects the Teflon
filter and RH/T sensor from impact and abrasion. The resulting
ultra-rugged design allows the VP-3 to withstand permanent
exposure to hostile conditions, making it ideal for a wide range of

VP-3 User’s Manual
1. Introduction
2
applications including standard meteorological/weather monitoring,
evapotranspiration measurement, micro-environment monitoring,
greenhouse monitoring and control, concrete moisture monitoring,
and building humidity monitoring for mold prevention/
remediation.
Customer Support
If you ever need assistance with your VP-3, or if you just have
questions or feedback, there are several ways to contact us.
Customer service representatives are available to speak with you
Monday through Friday, between 7am and 5pm Pacific time.
NOTE: If you purchased your VP-3 through a distributor,
please contact them for assistance.
E-mail:
[e-mail] or [e-mail]
Phone:
1-509-332-5600
Fax:
1-509-332-5158
If contacting us by email or fax, please include as part of your
message the sensor’s name, your name, address, phone, and fax
number.

VP-3 User’s Manual
1. Introduction
3
Specifications
Relative Humidity
Range: 0 to 100 % RH
Resolution: 0.1% RH
Accuracy: Sensor measurement accuracy is variable across a
range of RH. Refer to the chart below to determine the accuracy
specification for the VP-3 sensor:
Response time (τ, 63%): <40 s (response time in 1 m/s air
stream)

VP-3 User’s Manual
1. Introduction
4
Hysteresis: < 1% RH typical
Long term Drift: < 0.5% RH/year typical
Temperature
Range: -40 oC to 80 oC
Resolution: 0.1 oC
Accuracy: Sensor measurement accuracy is variable across a
range of temperatures. Refer to the chart below to determine the
accuracy specification for the VP-3 sensor:
Response time (τ, 63%): <400 s (response time in 1 m/s air
stream)
Long term drift: < 0.04 °C/year typical

VP-3 User’s Manual
1. Introduction
5
Vapor Pressure
Range: 0 to 47 kPa
Resolution: 0.01 kPa
Accuracy: Sensor measurement accuracy is variable across a
range of temperatures and RH. Refer to the chart below to
determine the accuracy specification for the VP-3 sensor:
General
Dimensions: 1.96 cm (dia) x 5.4 cm (h)
Power requirements: 3.6 - 15 VDC, 0.03mA quiescent, 4mA
during 300ms measurement
Output: RS232 (TTL) or SDI-12

VP-3 User’s Manual
1. Introduction
6
Operating Temperature: -40 to 80 °C
Connector types: 3.5 mm (stereo) plug or stripped & tinned lead
wires (Pigtail)
Cable Length: 5m standard; custom cable length available upon
request
Datalogger Compatibility (not exclusive):
Decagon: Em50, Em50R, Em50G (Firmware 2.11+)
Campbell Scientific: Any logger with serial I/O (CR10X, CR850,
1000, 3000, etc.)
Handheld Reader Compatibility
ProCheck (rev 1.5C+)
Software Compatibility
ECH2O Utility (rev 1.65+), DataTrac (rev 3.6+)
Warranty
All Decagon products have a 30-day satisfaction guarantee and a
one-year warranty on parts.
Seller’s Liability
Seller warrants new equipment of its own manufacture against
defective workmanship and materials for a period of one year
from date of receipt of equipment (the results of ordinary wear
and tear, neglect, misuse, accident and excessive deterioration due
to corrosion from any cause are not to be considered a defect);

VP-3 User’s Manual
1. Introduction
7
but Seller’s liability for defective parts shall in no event exceed the
furnishing of replacement parts F.O.B. the factory where
originally manufactured. Material and equipment covered hereby
which is not manufactured by Seller shall be covered only by the
warranty of its manufacturer. Seller shall not be liable to Buyer
for loss, damage or injuries to persons (including death), or to
property or things of whatsoever kind (including, but not without
limitation, loss of anticipated profits), occasioned by or arising
out of the installation, operation, use, misuse, nonuse, repair, or
replacement of said material and equipment, or out of the use of
any method or process for which the same may be employed.
The use of this equipment constitutes Buyer’s acceptance of the
terms set forth in this warranty. There are no understandings,
representations, or warranties of any kind, express, implied,
statutory or otherwise (including, but without limitation, the
implied warranties of merchantability and fitness for a particular
purpose), not expressly set forth herein.

VP-3 User’s Manual
2. Measurements
8
2. Measurements
Relative Humidity
The VP-3 utilizes a capacitance type RH sensor to measure the
relative humidity of the surrounding air. Relative humidity is
measured at the same location as the temperature sensor. For
this to be an accurate representation of the atmospheric humidity,
it is critical that the humidity sensor be at air temperature. For
most measurement scenarios, the VP-3 should be housed in a
radiation shield with adequate air flow to allow the sensor to
come into equilibrium with air temperature. This is not as critical
for non-greenhouse, indoor monitoring applications where
radiation loading is small.
The VP-3 sensor provides a RH measurement that is referenced
to saturation vapor pressure over liquid water, even at
temperatures below freezing where ice is likely to be present
instead of super-cooled water. Although this is the standard way
to define RH (WMO, 2008), it has the disadvantage of providing
incorrect RH values below freezing when referenced to ice. The
figure below shows the maximum RH that will be measured by
the VP-3 sensor (at saturation, 100% RH) at temperatures below
zero. RH values below saturation can be corrected using the
correction shown in the figure for a given temperature.

VP-3 User’s Manual
2. Measurements
9
Temperature
The VP-3 has a band gap temperature sensor integrated into the
sensor electronics. The temperature sensor is co-located with the
RH sensor and will accurately measure the sensor temperature.
If the VP-3 is adequately radiation shielded and aspirated, the
sensor temperature will be very close to air temperature.
Vapor Pressure
Vapor pressure is calculated from the primary measurements of
sensor RH and sensor temperature. The saturation vapor
pressure (es) is calculated from the sensor temperature using the
Magnus-Tetens equation for calculating saturation vapor pressure
over liquid water formulated by Murray (1967)
esab
1T1
Tc
a
+
---------------



exp=

VP-3 User’s Manual
2. Measurements
10
with coefficients described by Buck (1981): a = 0.611 kPa, b =
17.502, c = 240.97 °C, and T is temperature in °C. Vapor
pressure is simply the product of saturation vapor pressure and
RH, with RH expressed as a unitless ratio ranging from 0 to 1.
Vapor pressure = es* RH
Unlike relative humidity, vapor pressure does not depend on
temperature, and is relatively conservative over small spatial
scales. This means that the vapor pressure of the atmosphere
near the VP-3 is the same as the vapor pressure at the VP-3
sensor, even if the VP-3 isn’t at the same temperature as the
atmosphere. Additionally, it is the vapor pressure of the
atmosphere (not RH) that controls the rate of vapor phase water
transport (e.g. evaporation, transpiration, and re-distribution of
water vapor).
As discussed above, RH measurements below a temperature of
0°C will have error introduced due to the use of liquid water as
the reference. However, because the Buck (1981) formulation
for liquid water is used to calculate vapor pressure over the full
temperature range, the vapor pressure values output by the VP-3
will be correct over the full temperature range.

VP-3 User’s Manual
3. RH Sensor
11
3. RH Sensor
RH Sensor Stability
Each VP-3 sensor is verified as accurate before leaving our
facility. However, all capacitance RH sensors will drift over long
periods of exposure to environmental conditions. The VP-3 RH
sensor typically drifts less than 0.5% RH per year. We
recommend that VP-3 sensors be re-calibrated every 1 to 2 years
under normal use conditions to ensure best possible accuracy. To
have your VP-3 fully calibrated or re-calibrate contact
[e-mail] and ask about our calibration service.
The polymer RH sensing element in the VP-3 can also be
“poisoned” by exposure to volatile organic compounds, solvents,
and other chemicals. The effects of exposure to these chemicals
can range from subtle loss of accuracy to catastrophic failure. If
you suspect that your VP-3 has suffered chemical exposure or
notice questionable RH measurements, you can check the sensor
accuracy using known RH conditions.
A convenient method for generating known RH conditions is
through the use of salt solutions. For an initial check, we
recommend preparing a saturated NaCl solution, which has an
equilibrium RH of 0.75 (75%). To prepare the salt solution, start
with lab grade NaCl and mix in enough water that there is a thin

VP-3 User’s Manual
3. RH Sensor
12
layer of liquid water present over a thick slurry of NaCl crystals.
The VP-3 sensor can either be sealed into a chamber or bell jar
with the salt solution or the opening of the VP-3 can be sealed
into a small chamber that contains the salt solution. Whatever
method is used, it is critical that the VP-3 sensor be at the same
temperature as the salt solution or large errors in the measured
RH will occur.
Salt solutions prepared at a wide range of RH are available from
Decagon (see chart below). It is possible to prepare your own
solutions using the mixing ratios shown below, but great care and
precision are required to obtain accurate results. It is especially
important that the salt used be pure and dry. Decagon’s salt
solutions are specified accurate to within ±0.3% RH. As
mentioned above, it is critical that the VP-3 sensor be at the same
temperature as the salt solution or large errors in the measured
RH will occur.
RH Sensor Reconditioning
If a VP-3 sensor has been exposed to solvents or other chemicals,
the following reconditioning procedure may bring the sensor
back to the original calibration state. First, bake the sensor in dry
EquilibriumRH
(%saturation)
Salt Molality(m)
(molsalt/kgwater)
25% LiCl 13.41m
50% LiCl 8.57m
76% NaCl 6.00m
92% NaCl 2.33m

VP-3 User’s Manual
3. RH Sensor
13
heat at 100 – 105 °C for 10 hours. Then re-hydrate the sensors at
20 – 30°C under ~75% RH for 12 hours. A 75% RH
environment can be conveniently established by sealing the
sensor in a headspace over saturated NaCl prepared as described
above.
RH Sensor Recalibration
Decagon offers a service to recalibrate VP-3 sensors (contact
Decagon Support at [e-mail] for more
information). Sensors are calibrated at 25%, 50%, and 76% RH.
The calibration values are stored onboard the VP-3 sensor,
allowing the data to be corrected before being output, thus
eliminating the need for calibration during post-processing.
We recommend that VP-3 sensors be re-calibrated every 1 to 2
years under normal use conditions to ensure best possible
accuracy. For safety-critical or especially high accuracy
applications, more frequent recalibration is recommended.
Additionally, if sensors have been poisoned by chemical exposure
and reconditioning fails to restore accurate measurements, the
sensors should be sent back to Decagon for evaluation and
possible recalibration.

VP-3 User’s Manual
4. Connecting Sensors
14
4. Connecting Sensors
The VP-3 sensor was designed to work most efficiently with
Decagon’s Em50, Em50R, Em50G dataloggers, or the ProCheck
handheld reader. The standard sensor (with 3.5 mm stereo
connector) quickly connects to and is easily configured within a
Decagon logger or ProCheck.
The VP-3 sensor incorporates several features that also make it
an excellent sensor for use with third party loggers. The sensor
may be purchased with stripped and tinned wires (pigtail) for
terminal connections. Visit our website at www.decagon.com/
support/literature to get extensive directions on how to integrate
the VP-3 sensor into third party loggers.
The VP-3 sensor comes standard with a 5 meter cable. Sensors
may be purchased with custom cable lengths for an additional fee
(on a per-foot fee basis). Decagon has tested its digital sensor
successfully up to 1000 meters (3200 ft). This option eliminates
the need for splicing the cable (a possible failure point).
Connecting to an Em50 Series Logger
The VP-3 has been designed to work specifically with the Em50
data logger. Simply plug the 3.5mm “stereo plug” connector
directly into one of the five sensor ports.

VP-3 User’s Manual
4. Connecting Sensors
15
Figure 1: 3.5mm Stereo Plug Wiring
The next step is to configure your logger port for the VP-3 and
set the measurement interval, this may be done using either
ECH2O Utility or DataTrac 3 (see respective manuals). Please
check your software version to ensure it will support the VP-3.
To update your software to the latest version, please visit
Decagon’s free software download site: www.decagon.com/
support/downloads.
The following firmware and software support the VP-3 sensor:
Em50, Em50R, Em50G Firmware version 2.11 or greater
ProCheck Firmware version 1.5C or greater
ECH2O Utility 1.65 or greater
ECH2O DataTrac 3.6 or greater
To download data from the logger to your computer, you will
need to use the ECH2O Utility, DataTrac 3 or a terminal
program on your computer.
Ground
Power
Data

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4. Connecting Sensors
16
Connecting to a Non-Decagon Logger
The VP-3 sensor may be purchased for use with non-Decagon
data loggers. These sensors typically come pre-configured with
stripped and tinned (pigtail) lead wires for use with screw
terminals. Refer to your distinct logger manual for details on
wiring. Our integrator’s guide gives detailed instructions on
connecting the VP-3 sensor to non-Decagon loggers. Please visit
www.decagon.com/support/literature to reference the complete
integrator’s guide.
Pigtail End Wiring
Connect the wires to the data logger as shown, with the supply
wire (white) connected to the excitation, the digital out wire (red)
to a digital input, the bare ground wire to ground as illustrated
below.
Ground(Bare)
Data(Red)
Power(White)
3.6-15V DC
G
Supply Digital
out Ground
Datalogger
Digital
In