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Addendum: improving
the pressure sensor
As detailed in the last two articles, the Water
Tank Level Meter uses a pressure sensor to
measure the water level. Here’s a few tips on
improving the set-up plus an improved method
for mounting the pressure sensor externally.
A
S ORIGINALLY DESCRIBED in
November 2007, the Water Tank
Level Meter used a pressure sensor
that was mounted inside the case (ie,
on the PC board). This sensor was connected to a tube that was then inserted
into the tank, with one end close to
the bottom.
The resultant air pressure within the
tube thus provided a measurement of
water level.
An alternative method was subsequently described in the December
issue and this involved mounting the
pressure sensor in a sealed box at the
bottom of the water tank. The electrical
output from the sensor was then fed
back via a cable to the Level Meter.
Since publication of these articles
it has come to our attention that the
“tube in tank” method is only valid for
short-term water level measurements.
Unfortunately, the measurements will
become inaccurate after an extended
period of time. This is due to some
diffusion of the air into the water,
resulting in loss of pressure.
As a result, our first measurement
technique (ie, where the sensor is
mounted on the PC board) is no longer
recommended for long term monitoring. By contrast, the in-tank sensor
measurement technique described
in the December article is suitable,
because this is not affected by pressure loss due to the diffusion of air
into the water.
Making it better
Assuming that you do mount the
pressure sensor inside the tank (see
pages 86-87, Dec. 2007), there are a
few things you should do to improve
reliability.
First, a short squirt of silicone water
repellent (eg, Selleys Water Shield)
should be directed into both sensor
ports, to improve water protection at
the sensor’s gauge. In addition, a few
drops of mineral oil should be placed
in the tube, so that an air pocket and
oil trap is formed just above where the
tubing is clamped to the box. This is
to prevent direct contact between the
sensor and the water at port 1.
Note that there should be a small
amount of air left between port 1 and
the oil. The oil repellent action of the
silicone spray is also helpful here.
Note that mineral oil is available from
pharmacies as baby oil.
Water in the vent tube
One problem is that the vent tube for
port 2 may ultimately contain water in
the lower portion of the “U”-shaped
section in the sealed box at the bottom of the water tank. This is due to
water condensation from the air and
if enough water condenses to close off
the tube, this will lead to inaccuracies
in the pressure reading due to incorrect pressure at port 2 with barometric
changes.
As a result, if the water level readings appear to be inaccurate, the con-
This external sensor assembly
is designed to connect directly
to the outlet at the base of the
water tank via a T-piece. Refer
to Fig.1 for the assembly details.
siliconchip.com.au
January 2008 89
Fig.1: follow these diagrams to build and install
the external pressure sensor. As shown, the sensor
is mounted in a waterproof box and connects to
the tank’s outlet via a T-piece made up using a
metal tube and a cable gland. Don’t forget to drill
a small hole in the underside of the box, so that
atmospheric pressure is applied to port 2 (P2) of
the sensor (see text).
densation will have to be drained from
the tube. To do this, it’s simply a matter
of removing the sensor assembly from
the tank and tipping the water out.
Having said that, this degree of condensation is unlikely to occur except
in very humid climates. If necessary,
the effect can be minimised by placing
90 Silicon Chip
a water-absorbing desiccant (eg, silica
gel) within the tube.
Alternative sensor placement
Another recommended technique
for water level measurement has the
sensor located outside the water tank.
This arrangement is shown in Fig.1
and involves connecting the pressure
sensor input directly to the outlet connection at the base of the water tank.
The main advantage of this scheme
is that because the sensor and its wiring are now located outside the tank,
there is no need to fully seal the inside
of the box.
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Parts List
1 IP65 sealed enclosure 64 x 58
x 35mmmm (Jaycar HB-6120
or equivalent)
1 31 x 26mm sheet of 18g
Aluminium
2 3-6mm waterproof cable
glands
3 M3 x 6mm screws
2 M3 x 20mm screws
4 M3 nuts
1 4-way pin header
1 50mm length of 4mm PVC
tubing
1 4-way sheathed cable (length
to suit application)
1 set of fittings suitable for water
tank 4mm tubing connection
The connection to the tank’s outlet
can be made using a “Tee connector”.
This can be obtained from an irrigation
supplier (eg, a T-piece as used for drip
irrigation) or you can fashion your
own fitting. An alternative is to use a
small metal tube inserted into a metal
tap fitting, which would then accept
the PVC tubing from the sensor. This
metal tube can be brazed, soldered or
glued in place.
If you are using 25mm or larger poly
pipe at the tank outlet, then a T-piece
can be made by first drilling a hole in
the side of the pipe close to its end,
to accept a 3-6mm cable gland. This
hole needs to be positioned close to
the end so that access is available to
flatten down the tube at the mounting
area and to provide access to the gland
nut inside the tube.
The seal between the gland and poly
pipe can be improved by using an ‘O’
ring (as shown in Fig.1) or by using a
silicone sealant that’s suited for wet
areas (or you can use both). A 3mm
OD metal (or hard plastic) tube then
needs to be placed inside the PVC tube
so that the gland will not close off the
PVC tube when it is tightened down.
Metal tubing this size can be salvaged
from a telescopic antenna.
As shown, the sensor is mounted
on a small aluminium plate within
a sealed enclosure. This baseplate is
made up using sheet aluminium measuring 31 x 26mm and is attached to the
two central internal mountings posts
using M3 x 6mm screws. The sensor
itself is attached to the baseplate using
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two M3 x 20mm screws and M3 nuts.
As shown in Fig.1, the port 1 connection comprises a 3mm PVC tube
that connects to the T-piece in the
water tank’s outlet. Port 2 vents to the
atmosphere.
A 4-way cable (eg, telephone cable)
is connected to the four sensor pins
and exits from the top or side of the
box through a cable gland. Note that
you must orient the sensor so that port
1 is connected to the tubing. As shown,
the sensor is mounted with pin 1 (the
notched pin) to the left. Make a note
of the wire colour used for each connection to the sensor.
At the other end, this wiring connects to the socket in the Water Tank
Level Meter via a 4-way pin header
connection. Make sure that this connection is made with the correct
orientation and don’t get the wiring
mixed up.
Making it water-tight
Note that if you are using a flat 4-way
cable, it will not form a watertight seal
within the gland. This can be fixed by
applying a small amount of silicone
sealant around the wire at the entry
and exit points of the gland, so that
the box is waterproof.
In addition, a small hole must be
drilled in the box to allow the air
pressure to vary inside the box for the
sensor’s P2 port. This hole should be
drilled so that it is in a bottom panel
when the box is mounted in position,
to keep water out.
A hole size of just 1.5mm is all that’s
required.
The same recommendations we
made above for the in-tank sensor
installation also apply here. These
are to improve reliability from the
sensor – ie, use a squirt of silicone
water repellent into both sensor ports
and place a few drops of mineral oil
in the tube so that air and oil can then
be trapped just above where the tubing
is clamped in the gland.
As before, the oil is to prevent direct
contact between the port 1 sensor and
the tank water. Don’t overtighten the
cable gland for the port 1 tube – you
don’t want to close off the tube completely.
Mounting it in place
The box can be mounted on the side
of the tank, so that the port 1 tube sits
vertically. This keeps the mineral oil
floating on top of the water.
Note that it’s important to keep the
port 1 sensor input as low as possible,
so that it sits just above the tank’s outlet. That way, the full range of the tank
can be measured. Also, try to prime
the tubing with water up to the gland
before attaching to the tank T-piece,
so that the initial calibration will be
correct. If there is too much air, you
may need to recalibrate after the air
has diffused into the water.
Note that the box has two mounting points that are effectively outside
the box’s sealed section but are still
covered by the lid. So to access the
mounting holes, you have to remove
the lid.
The unit can be mounted on brackets or directly onto a wall or the tank.
To mount it to the side of the tank, first
mount two lengths of 19 x 19 x 70mm
hardwood, spaced apart to match the
box’s mounting holes. This timber can
be secured to the tank using builder’s
adhesive or silicone sealant. The box
is then attached using suitable wood
screws into the timber (but not so
long that they can penetrate the wall
of the tank).
As with the in-tank sensor, temperature compensation is not required
and the unit should be left at the zero
compensation setting. As for the Water
Tank Level Meter itself, this can be
mounted in any convenient location,
even if it is exposed to sunlight (since
the temperature sensor no longer has
to correct for the air temperature in
SC
the tube).
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