United States
Environmental Protection
Agency
Atmospheric Research and Exposure
Assessment Laboratory
Research Triangle Park NC 27711
Research and Development
EPA/600/S3-91/005 May 1991
1&EPA Project Summary
Snow/Rain Collector Sampler
Richard G. Purcell and Rick B. Brown
This report describes a new instru-
ment for collecting chemically accurate
precipitation (liquid and frozen) samples
and measuring precipitation rate and
quantity. The design goals include the
ability to operate in a wide range of
weather conditions at remote and un-
attended monitoring sites for at least
seven days, be easily operated by non-
technical personnel, provide for con-
venient sample and data transport, ex-
clude dryfall, and operate from a lim-
ited power source. The design features
and fabrication of each subassambly
are discussed. The results of field tests
are provided, with particular emphasis
on chemical measurements. Finally,
recommendations for future improve-
ments based on these test results are
provided.
This Project Summary was developed
by EPA's Atmospheric Research and
Exposure Assessment Laboratory, Re-
search Triangle Park, NC, to announce
key findings of the research project
that is fully documented In a separate
report of the same title (see Project
Report ordering information at back).
Introduction
The study of precipitation chemistry im-
plies the ability to collect chemically accu-
rate precipitation samples. In general,
precipitation chemistry varies from loca-
tion to location so geographically disbursed
collection is necessary. Geographical dis-
persion implies a collection device that
will operate continuously and unattended.
Such a device must be able to determine
when precipitation is occurring and, during
non-precipitating periods, prevent con-
tamination of collecting surfaces. Addi-
tionally, both liquid and frozen precipita-
tion will be of interest so the instrument
must have the ability to sense, collect and
melt frozen precipitation for convenient
storage.
Instrument Description
Overview
The precipitation gauge and collector is
a computer controlled electro-mechanical
device for collecting chemically accurate
precipitation samples and measuring pre-
cipitation quantity and rate. It is designed
to operate unattended for periods of up to
one week from a 12 volt DC power source.
The device includes a start of event sensor
that detects the onset of precipitation, a
collector funnel that/is automatically un-
covered during precipitation, a liquid mea-
surement apparatus referred to as the
accumulator, removable bottles for storage
and transfer of precipitation samples, a
mechanism for transferring liquid from the
accumulator to the storage bottles, and a
controller to supervise the actions of the
instrument and collect data. The end of a
precipitation event is determined when the
accumulator has not become full within a
predetermined time period. The controller
records the time of start and end of pre-
cipitation events and the time of every
filling of the accumulator, ft also records
status events such as door open, heater
use, sample bottle number, and instrument
exception information such as low power
voltage and failure of the instrument to
behave, as expected. The majority of the
Printed on Recycled Paper
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instrument workings are contained within
an insulated 28 x 13 x 13 enclosure. Pro-
vision has been made to heat the start of
event sensor, collector, and interior of the
enclosure irr-order to collect frozen pre-
cipitation.
Sub Assemblies
Accumulator
The accumulator is a removable subas-
sembly that is designed to detect a .01
inch increment of precipitation and to
transfer that increment into storage bottles.
It consists of a collection tube of a known
volume, a lower normally closed pinch
valve that traps liquid in the collection
tube, an upper normally open pinch valve
that allows precipitation to enter the col-
lection tube and optical sensors that detect _
the presence of liquid. The controller op-
erates the accumulator by monitoring the
liquid detectors during a precipitation event.
When it observes that both detectors see
liquid at the top of the collection tube it
forces a sample needle and vacuum
needle through a storage bottle septum
by means of the injector assembly, opens
the lower pinch valve and turns on the
vacuum pump to suck the increment of
precipitation into the storage bottle. The
controller determines that a precipitation
event has ended when no liquid has col-
lected in the accumulator for a program-
mable interval of time.
Carrousel Assembly
The carrousel is a removable subas-
sembly provided to hold the storage bottles
and to position them under the injector.
The carrousel is designed to allow the
storage bottles to be installed and removed
as a unit. The rack of bottles may be
transferred without need to keep track of
the bottles on an individual basis. The
carrousel assembly consists of a turntable
that accepts a rack of eight 500 ml bottles,
a motor to drive the -turntable, a motor
heater and a connector. Locator pins and
a locator pin detector switch enable the
controller to determine the position of the
turntable.
Collector
The collector consists of an eight inch
diameter aluminum funnel coated with
Halar to prevent sample contamination.
To preserve cleanliness the collector is
covered between precipitation periods. The
bottom of the funnel is connected to the
accumulator. The controller servos the
heaters at 3°C during precipitation events
when ambient temperatures are below
freezing.
Cover Assembly
The cover assembly is provided to pre-
vent contamination from entering the col-
lector during non-precipitating periods. The
start of event sensor is mounted on the
top of the cover. A mercury bead tilt sen-
sor is mounted to a cover mounting arm.
The controller uses that sensor to deter-
mine that the cover has moved when an
open or close command is given and to
detect when the cover is approaching its
destination in order to turn off the cover
motor. The cover is entirely coated with
Halar for cleanliness.
Injector
The injector is a remoyable_subassern-
bly that is designed to transfer liquid from
the accumulator to the storage bottles. It
consists of a solenoid that forces a dual
needle assembly through a septum in the
cap of the storage bottles, a heater to
prevent liquid from freezing in the needles
and switch that enables the controller to
detect when the needles are retracted.
The septa are cut from a special high tear
strength silicons elastomer to withstand
up to 64 penetrations. One of the needles
is plumbed to a vacuum pump and the
other to the bottom of the accumulator.
The controller transfers liquid from the ac-
cumulator to the storage bottles by plung-
ing the needles through the septum and
applying suction to the bottle by turning
on the vacuum pump for a predetermined
time period. As a safety feature the con-
troller will not automatically operate the
needles when the enclosure door is open.
Start-of-Event Sensor
The start of event sensor (SOES) func-
tions by detecting the lowered resistance
of a printed circuit board grid pattern when
the grid becomes wet. AC excitation of
the grid prevents long term degradation
due to electrolysis. A heater and control
electronics within the SOES housing keep
the temperature of the grid above 3°C in
order to detect frozen precipitation. A short
chimney reduces the chance that frozen
precipitation will bounce out of the SOES
before it melts. The capability to force the
SOES heater on is provided so that the
grid may be dried at the end of a precipi-
tation event.
Materials
Chemistry and material compatibility
questions made material selection one of
the most important aspects of the
sampler's design. Fortunately, the choices
were considerably simplified by selecting
from a list of materials already deemed
acceptable for handling NADP samples.
All wetted surfaces were fabricated from
materials included on this NADP list with
one exception. The collector funnel and
cover were coated with Halar ECTFE, a
copolymer of ethylene and chlorotrifluoro-
ethylene. It gives an inert, stable surface
that should avoid the potential problems
of pinholes and porosity sometimes seen
with Teflon coatings. Both silicone and
pvc tubing were tested in the accumula-
tor. The former was selected based on
less variation in its inside diameter and
less tendency to take a set in the pinch
valves.
--•- The materials were assessed ,in two
ways: 1) laboratory testing of samples,
and 2) chemical performance tests of the
completed instrument.
Results and Discussion
The field tests were conducted in two
modes. In the first procedure, 500 ml of a
fairly clean water standard was prepared
and stored in a polyethylene sample bottle.
At the start of the sampling week 200 ml
was poured into the collector and pro-
cessed by the instrument into sample bottle
#8. At the end of the week, the procedure
was repeated and the liquid was passed
into sample bottle #7. The standard and
two blanks, along with whatever precipita-
tion that was collected in the other sample
bottles, were sent to the laboratory for
analysis. We were looking for differences
in species concentrations between the
standard and the blanks. The second
procedure involved using a not so clean
water standard, in this case tap water. We
were looking for the loss of species.
Conclusions
The instrument described herein per-
formed satisfactorily during field testing. It
demonstrated the ability to operate in a
cold winter mountain environment while
powered by four automobile batteries. The
removable carrousel of sample bottles
proved to be easy to replace and transport.
It is doubtful that the instrument can be
reproduced in quantity for $2,500. That
figure will probably cover the cost of in-
strument components and assembly, but
not test, product support, and profit.
U.S. GOVERNMENT PRINTING OFFICE: 1991/548-028/20216
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Richard G. Purcellis with the Atmospheric Sciences Center, Desert Research Institute,
Reno, NV 89506. Rick B. Brown is with the Energy and Environmental Engineering
Center, Desert Research Institute, Reno, NV 89506.
Steven M. Bromberg is the EPA Project Officer (see below).
TTje complete report, entitled "Snow/Rain Collector Sampler," (Order No. PB91 -
167601/AS; Cost: $15.00, subject to change) will be available only from:
National Technical Information Service ,
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Atmospheric Research and Exposure Assessment Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
United States
Environmental Protection
Agency
Center for Environmental
Research Information
Cincinnati, OH 45268
BULK RATE
POSTAGE & FEES PAID
EPA
PERMIT No. G-35
Official Business
Penalty for Private Use $300
EPA/600/S3-91/005
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