A Hydrobaric Technique for Depth Measuring
Underground water level monitoring using a hydrobaric sensor
Original article by Dr Dave Merchant, Updated by EPS Ltd
Recording the level of water in a cave or other underground passage over time is
of great use for cavers and hydrologists. You can learn an enormous amount about
a cave's feeders and possible undiscovered passages from watching how a cave reacts
to rainfall. The problem has always been how to measure this reliably, cheaply and
for long periods. I've been using hydrobaric (pressure) sensors for this for a few
years off and on, and these pages show you how to do it. Similar work was done by
Dr. Gareth Jones at Dan yr Ogof. Their system was nice, but over-
Important note to the financially strapped:
Building a pressure monitor and buying a datalogger unit is NOT cheap. Expect to fork out about 150 pounds for components and the logger itself. This is however within the realms of a caving club budget, as the unit, once built, will last for years. There is inevitably the issue of it being pinched, but that's an issue of security not technology. To quote a rich American computer company CEO, "Powerful software is often expensive. However, being powerful does not allow it to lock the office door for you. That task is strictly a hardware problem."
Why measure pressure?
Measuring the depth of a body of water can be done in many other ways:
Optical surface reflection : a light beam is reflected from the surface of the water and the height measured. Works only in water with a flat surface and in controlled light conditions. Equipment is expensive, and can be confused by stray light, dust or ripples.
Optical fibre refractometers : A treated optical fibre is vertically fixed in the water. As the depth changes, the transmission of light through the fibre changes. Works on fast flowing water but can be affected by sediment and is sensitive to light. Complicated to build as the signal is not linear with water depth.
Conductivity probes : Two wires are fixed vertically in the water and the electrical
conductivity between them can be used to infer water depth. Works very poorly on
the pure low-
Mechanical sensors : floats are used to move variable resistors or transformers, usually using a vertical pipe containing the float, fixed by string to the mechanical sensor. Relatively simple to construct, they are immune to water quality, light or temperature but have moving parts so are prone to failure over time. Often the sensor part (the variable resistor or transformer) can be very expensive as it must be very low torque and sealed against moisture.
Pressure sensors work on the priciple that the hydrostatic pressure in a body of water is proportional to depth. Mathematically, (pressure) = (depth) X (density) X 9.81 which for freshwater gives a pressure of 0.43 psi per foot of depth, or 9.14 psi per metre. So, if you measure the pressure at the bottom of the water, you can easily calculate the depth. Since the relationship is linear, it's simple to build a device that reports directly (i.e. one volt per foot, or similar).
The environment in a cave is unique in the problems it presents. 99% of the time
it's dark, but every so often someone will pass by and poke about with a torch. The
water flows can vary from zero to tens of feet, often the flow is very rapid and
turbulent. The water can be clear, or filled with sediments, even the odd tree. A
pressure sensor is not affected (to noticeable amounts) by changes in sediment, light
or the speed of the water flow. The pressure measurement is best done using a bladder
-
The main issue is that the pressures you wil be measuring are small, on the scale
of 'normal' industrial measurements. As the weather changes, the air pressure changes
and the depth-
The datalogger
A datalogger is an electronic device that stores a set of measurements for later reading.
It is used to read a value from the pressure sensor every few minutes/hours/days, then when the equipment is collected, all the values and the times they were taken can be transferred to a computer. It's not worth the effort to try and build your own datalogger, as the cost of the components will be more than the cost of buying one. For my systems, I use the 'Tinytalk' dataloggers from Gemini.
They can store 1800 readings, and are the best value loggers available in the UK
in terms of size, performance and cost. You can buy them direct from Gemini (click
the link above) or from distributors such as RS components. Gemini do a whole range
of loggers with inbuilt sensors (for pressure, temperature, etc), but they also do
a device which just records input voltage -
Input range 0 -
Accuracy 10mV + 0.5%
Input current 0.4mA
Temp range -
Logging interval 1 second to 4.5 hours (1800 readings therefore last from 30 mins to 337 days)
Battery life 2 years, irrespective of if it's used or not.
Delay start 0 -
The datalogger is packed in a 35mm film cannister and the voltage to be measured
is connected via a miniature phono plug. The data can be read using a software package
and a connection cable for a PC serial port. The logger should cost about £70, the
software and cable another £50. This may sound a lot, but it's cheap for a datalogger!
When the 1800 readings are taken, the unit will either stop and wait, or loop round
and start overwriting from the start like an aircraft black box -
Tinytalk units are not waterproof, so they must be protected in an enclosure. Gemini
do make waterproof versions, even explosion-
The amplifier circuit
Measuring the air pressure in the tubing is done by a piezoelectric barometer. These
devices contain a resistive bridge that creates a small voltage as pressure is applied
to the sensor. For reliable measurements, a differential sensor should be used, with
the second pressure being that of the air, as described above. The sensor I use (from
RS again) measures 0-
The output of the sensor must be amplified from 0-
Contact us for PCB prices .
The pressure bladder
Making a pressure bladder is easy, but there are rules to follow! The bladder must
be neutrally flexible -
Also, you'll need a bladder that's got an air volume inside it, as you'll be compressing
it. A rugged, cheap and waterproof bladder is handy too! Solution? Simple. A 5-
The other port of the differential sensor we use must be vented to atmosphere. It's
this differential measurement that makes sure your sensor isn't just recording changes
in weather! All you need is a few more feet of the 5mm tubing, and we put a T-
The bladder should go as deep as possible in the water, and if the flow is likely
to be strong, it may need to be fixed down. In really rapid flows, I fix the bladder
to a steel plate, cover the edges with stones and tape the tubing to a nylon line
to stop it being snapped by debris. In a river-
How our systems perform
The graph below shows a typical trace from one of our tests. The entire system (circuit,
sensor, datalogger and case, battery, etc) costs about £150, which is not at all
bad for what it does! The datalogger is costly, but it can be used for many other
caving-
The Grosvenor CC Milwr Tunnel Project uses these sensors and their website shows
real data collected during the infamous floods of November 2000. At the time the
sensor was fitted with a 0-
Using the 0-
Sensitivity (at input to datalogger) 48 mV/cm depth
One datalogger step (10mV) equals
(which gives the resolution of the system) 2 mm
Max depth (when logger input = 2.5V) 47 cm
Minimum depth 1 cm
Baseline level (at input to datalogger)* 250 mV
* The circuit has a small offset even when the bladder isn't under pressure. This is deliberate to ensure the input to the logger is never zero. If the logger ever records a zero then it tells you the amplifier circuit is dead or the battery's been stolen!
