Hobo Water Level Recorders
Construction, Deployment and Data Management
7-21-95
Contents
Introduction...................................................................................3
Parts List and Ordering Information.........................................4
Fabrication Notes...........................................................................
Deployment Notes .........................................................................8
Calibration Notes..........................................................................1
Data Management.......................................................................12
Introduction
The Hobo Water Level Recorder is based on the Motorola MPX series pressure
transducer coupled with a miniature data logger called the Hobo and manufactured by Onset Computer Corporation. The Hobo logger as ordered from Onset is capable of measuring water depths of up to about 1.5 meters. It is possible to order the Hobo loggers without the stock pressure transducer and subsequently install a transducer with a larger (5 meter) range. However, availability of transducers is intermittent at best.
Once received, the loggers are modified to make them more durable in the salt-air environment in which they are often deployed. Furthermore, a small amount of work is required to fabricate other associated equipment prior to deployment. This manual includes a complete parts list and construction information.
Deployment of the Hobos is straight-forward. A small amount of linear drift is typically seen in the data, this may be quantified and removed with minimal effort. The final sections of the manual describe this work as well as a suggested log-sheet format and in-field calibration checks to perform.
Parts List and Ordering Information
1. Hobo Data Loggers:
Onset Computer Corporation
P.O. Box 3450
Pocasset, MA 02559-3450
(508) 563-9000
Part # HWD-003 is the 1.5 meter logger, order with one RS-232 cable per logger
2. Potting Supplies:
Newark electronics (catalog available)
8002 Discovery Drive
Richmond, VA 23229
(800) 261-3538
item quantity for one Hobo stock number
clear potting epoxy 1.5 tubes 00Z1163
0.9" x 3" x 2" plastic case 1 95F996
3.6 volt Lithium 1/2 AA battery
with axial pins 1 50F1557
applicator for epoxy (one time purchase) 93F6943
nozzle for applicator (disposable) 93F6944
Note: The MPX series pressure transducers are also available through Newark Electronics
3. Sensing Tube:
Thomas Products (catalog available)
(609) 467-2000
item quantity for one Hobo stock number
1/8" Norprene Tubing 5' per logger 9563-B42
McMaster-Carr (catalog available)
P.O. Box 440
New Brunswick, N.J. 08903-0440
(908) 329-3200
item quantity for one Hobo stock number
1/8" i.d. brass hose barb 1 per logger 545K12
0.256" - 0.323" nylon
hose clamp 2 per logger 9579K62
Fabrication Notes
1. Sealing the Hobo Circuit Boards (Potting)
In order to prevent corrosion of circuit board components and wires, it is essential to encase the entire Hobo logger in a clear acrylic epoxy. This is done using the epoxy listed in the previous section. A few important steps must be included.
a.) Remove the factory supplied battery using a flat screwdriver. It will not be used in this modification. Desolder the RS232 jack on the circuit board, it too is not used.
b.) Stand the transducer up vertically on its 4 "legs".
c.) Cut off the jack end of the Onset supplied RS232 interface cable. Strip back the insulation to expose two leads and a bare copper shield. Solder the leads to the circuit board as per the diagram at the bottom of this page.
d.) Solder the new battery in place, attach the negative terminal and solder it first. Be sure that the battery leads are well inserted in the circuit board lugs. Refer tot he drawing below for positioning and location of the positive and negative battery lugs.
e.) Test the Hobo for proper operation prior to further work on it.
f.) Seal the transducer pressure ports with electrical tape to prevent contamination with potting epoxy.
g.) Using double sided tape, position the Hobo inside of a plastic case. Take care to maintain the pressure ports above the rim of the case so that the epoxy cannot flow into them. Position the hose-barbed port so that the Norprene tubing will fit over it easily. Be sure that the first inch or so of the RS232 cable will be buried in the epoxy as a strain relief. Place the case on a level surface and fill with potting epoxy. Allow to cure overnight.
2. Making a Cap for the Sensing Tube
a.) Use 1/2" diameter Schedule 40 PVC pipe and end cap. Drill and tap the end cap for a 10-32 threaded hole.
b.) Coat the thread of a brass 1/8" hose barb with epoxy cement and screw into place, allow to cure overnight.
c.) Hobo may be connected to the sensing tube via Norprene tubing. Use a nylon hose clamp at each end of the tube to insure an airtight fit.
3. Miscellaneous Information
a.) female DB9 RS232 connector pin-out:
b.) Hobo RS232 jack:
Deployment Notes
1. Standpipe and field mounting
Typical standpipes, into which the sensing tubes are inserted, should be of at least 1-1/2" diameter. Schedule 40 PVC pipe is ideal for this application. It is recommended that the sensing tube be of 1/2" PVC pipe. This pipe can be attached to the stand pipe via a pair of hose clamps inserted through 1/2" diameter holes drilled in the upper (dry) portion of the standpipe. See diagram below:
2. Attaching Hobo to the well
Typically, Hobo loggers will be started with a computer prior to travel to the field site. However, it is possible to do this in the field as well. As the pressure transducer senses positive pressure relative to atmospheric pressure, it is important that the sensing tube contain a pressurized column of air above the water in it. To achieve this, one need only attach the Hobo to the sensing tube prior to placing the sensing tube into the well. If the bottom of the tube is above water level, this pressurization is not possible at deployment, but will occur naturally should water levels rise within the standpipe. To insure that the sensing tube has not shifted between site visits, a small amount of spray paint can be applied to the sensing tube - standpipe juncture to mark their relative positions. Hobo loggers should be enclosed in a waterproof container - either several ziplock bags (short-term deployment) or a PVC case (long-term deployment).
3. The following log sheet is recommended for keeping track of deployment, etc.:
Hobo Water Level Recorder Logsheet
7-5-95
Launch Information:
Hobo Serial Number :_____
Date: __/__/__ Time :________(EST)
Duration :___________ Sampling Interval :_________________
Recovery Information:
Date :__/__/__ Time :________(EST)
# Readings :_________
Calibration Information:
Well name/ location :_____________ Closest GPS Monument :______________
Standpipe rim vs. GPS Monument :_____________meters
Calibration Checks:
Date Time Water to Standpipe Rim (meters)
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Calibration
(refer to drawing on next page)
Calibration information is essential to understanding the data logged. Consequently, space for this information is allocated on the logsheet for each well/ data set. Calibration involves equating the output of the Hobo with actual water depths within the well, as well as equating these depths with a know GPS monument to tie the data into the larger body of data archived by the VCR/LTER.
Although there are several versions of Hobos currently in use, calibration is essentially the same for each one. Some loggers will store a "depth" in either inches or centimeters, others will be storing a depth in "bit counts" (the output of an eight bit analog to digital converter internal to the Hobo). There are three steps to "calibrating" a Hobo - and this calibration is specific to the logger-well combination on which it is performed. These steps are:
1. Shoot in the level of the well's standpipe rim relative to the nearest GPS monument and record this information, along with the well ID and the monument ID on the logsheet. Mark the relative position of the sensing tube and standpipe by spray-painting the juncture of the two at the lip of the well.
2. Record (on the logsheet) a time-series of measurements of the distance from the rim of the standpipe to the surface of the water within the standpipe. That is, over a period of several hours, record the distance from the lip of the standpipe to the water inside the well and the time at which it was measured. This table of readings can then be correlated with the Hobo's recorded water depths for the same time period.
3. Finally, one must adjust the recorded Hobo data to reflect actual water depth vs. the GPS monument. This can be done as followed using any spreadsheet-type program such as LOTUS 123:
a.) Use the spreadsheet to examine the Hobo data, make note of the Hobo readings for each manual reading taken in step #2 above. Create a simple table showing manual and Hobo readings side-by-side.
b.) Recall that the manual readings are distances down to the water whereas the Hobo readings are actual depths of water. The two may be equated by considering changes in level over time. That is, by simple subtraction, determine the manually recorded change in water level between two time markers. Then, equate this known difference in depth with the Hobo's recorded difference in depth for the same time period. This will allow you to correct the Hobo's data to reflect actual changes in depth, converting from either bits counts or "depths" recorded by the Hobo.
c.) Finally, this new spreadsheet of depths must be adjusted for the GPS monument - that is, tied into a known elevation. This is straight forward as one simply calculates the height of the top of the standpipe above mean sea level using the GPS monument as a known height above sea level. Then, Hobo data can easily be adjusted to
reflect this known elevation. Of course, several of the above calculations could be combined in one step to save time.
Data Management (under construction)