Help w Water Flow Sensor?
Jack Van Olst
Help w Water Flow Sensor?
I need help with a design problem. I need to get an analog signal
from 200 water pipes that I can use to estimate flow rate. I have a
great analog-digital system from B&B Electronics that will give 12 bit
accuracy from a 0-5 volt signal for only about $6 per channel, but I
cannot decide what type of sensor to use. Forgive the length of this
question, but I am trying to get ideas from anyone that has experience
in this field. My biggest problem is COST - I cannot use traditional
technology, because we donąt have the $$$.
The flows are in 8˛ diam horizontal PVC pipes. The most common way of
doing this is to read water velocity in a full pipe with aSignet style
paddle wheel sensor, and convert the frequency of the pulses from the
prop to water velocity, and convert that to flow. This solution is
too expensive.
All the other methods we have thought of depend on using a rectangular
weir cap on the end of the pipe, and calculating flow from a simple
non-linear formula relating water depth behind the weir to flow rate.
This method works great visually - we can engrave a 0-8 inch depth
scale next to the weir opening, and read depth and GPM within 5%
accuracy - fine for our needs.
But computerizing this depth measurement has proved difficult.
Commercial solutions are usually sonic transducers mounted above the
water surface - too expensive for us to buy 200 of them. One
possibility is to buy consumer style SONIN sonic distance meters for
$36 each and try to gut them to get a useful signal - anyone tried
this?
A second commercial method is to read conductivity, but our water is
too variable to allow this.
Another commercial solution is a łbubbler˛ style water pressure gauge,
in which air is bubbled through a small vertical tube and the back
pressure that develops is converted to depth. The commercial versions
are too expensive. I built a system with a $60 Data Instruments 0-1
psi pressure transducer. It works great, except it is hard to set the
bubble rate so that it wonąt be too rapid for shallow depths, or too
low for deeper water. Perhaps there is a cheap constant-flow valve
for air systems? Like a pinhole?
All other ideas involve some method of directly measuring the 0-8 inch
depth by using a float on the water surface. Here are some ideas:
1) Linear position potentiometer (8˛) - $200
2) VDO Marine fuel tank sender - $18
3) VDO tube type fuel sender - $46, but the łpot˛ is just a thin
strand of wire - very delicate
4) VDO Boat trim plate sensor - $60, but the pot is too stiff
5) Wirewound adjustable power resistor with a sliding tap - $18, but
the linkage to get a tap to slide along the 8˛ coil is awkward
All these methods would give a variable resistance via a pot linked to
the float - the electronics would be simple. But they all mean I
would have to build a lot of mechanical linkage gear to link the float
to the pot with shafts, etc. I donąt have a machine shop, and it would
be expensive to have the linkage fabricated commercially. Also,
each apparatus would need a large dustproof cover box to protect the
sensitive pot and linkage from the environment.
6) Muti-turn pot & retracting cable. UniMeasure in Oregon makes a
cute $88 5-turn pot with a retracting cable I could hook to the float.
This is a tidy solution, but more money than we would like to spend.
An alternative is to try to gut a Starrett consumer electronic tape
measure ($30) to get a usable signal - anyone tried this?
7) Buy high-resistance wire and build a linear pot from scratch.
Anyone have a source for high resistance wire? Comments?
Then come the łhigh-tech˛ concepts:
8) Build a metal detector circuit, put some metal on the float, and
try to get a proportional signal with distance.
9) Use a Hall Effect transducer , put a magnet on the float - but
will Hall Sensors work 8-10˛ away?
10) Move a magnet in an 8˛ fixed coil, and measure the change in
voltage due to disturbing the coil field - possible?
11) Strain gauge - One last idea is to have an external chamber near
the pipe and let water fill it through a flexible tube. Then weigh
the chamber with a strain gauge. Comments?
Again, sorry for the length of this letter. Any comments would be
appreciated.
Jack Van Olst
ia...@gabhsann.demon.co.uk
In article: <31A6DF...@west.net> Jack Van Olst <jvan...@west.net>
writes:
>
> Help w Water Flow Sensor?
>
> I need help with a design problem. I need to get an analog signal
> from 200 water pipes that I can use to estimate flow rate. I have a
I've seen a neat flow switch that relied on heat being carried away by
the fluid. A resistor and two thermistors was all that was required for
the sensor, The resistor was fed with a constant current to heat the
fluid, one of the thrmistors is in close proximity to the resistor, the
other is to allow for temperature changes in the fluid. The resistance
of the thermistor close to the resistor should be proportional to flow,
after canceling out the common resistance due to fluid temperature.
This was then used to activate a relay at a variable set-point, but
theres no reason why the same principle shouldnt be used to give a, say,
0-5V output dependant on flow. Just need some simple op-amp circuits.
This should work well for low flow rates, but it would probably be
impractical at higher rates due to the power the heater would need...
> Again, sorry for the length of this letter. Any comments would be
> appreciated.
>
> Jack Van Olst
>
>
--
Cheers,
Iain.
Henry Spencer
In article <31A6DF...@west.net> jvan...@west.net writes:
>...The most common way of
>All the other methods we have thought of depend on using a rectangular
>weir cap on the end of the pipe, and calculating flow from a simple
A possible alternative is to apply the "hot-wire anemometer" technique.
Each pipe gets fitted with two analog temperature sensors. The upstream
one just measures water temperature, to provide a base value for the
reading from the other. The downstream one is run at a current (or with
an attached resistor) that produces significant internal heating, and its
temperature thus measures the cooling effectiveness of the water flow,
which is related to flow rate. No moving parts, and temperature sensors
are quite cheap nowadays.
This method is used quite a bit for air; it ought to work for water.
--
Unix was a breakthrough. | Henry Spencer
Windows 95 is more like a smash-and-grab. | he...@zoo.toronto.edu
Henry Spencer
>scale next to the weir opening, and read depth and GPM within 5%
>But computerizing this depth measurement has proved difficult.
Outfits like Sensym sell solid-state pressure sensors that might be able
to give you a depth reading at reasonable cost. For example, one of the
Sensym sensors has a full-scale range of 0-4in of water. Don't know what
they cost, offhand.
Howard R. Hansen
>11) Strain gauge - One last idea is to have an external chamber near
>the pipe and let water fill it through a flexible tube. Then weigh
>the chamber with a strain gauge. Comments?
>
>Jack Van Olst
My first suggestion is a semiconductor differential pressure transducer.
You can use a differential pressure transducer to either measure the
pressure drop across an orifice on to measure the water level in your
weir. Using a pressure transducer to measure the water level is very
similar to your suggestion of using a strain gage to weigh the amount of
water in a tube. Twelve inches of water is equal to 0.43 psi. Hence your
full scale output is 2/3 times 0.43 or 0.29 psi. I made a quick search of
the world wide for a suitable semiconductor pressure transducer but so
far I haven't found one.
My other suggestion is post this question to the sci.eng.control newsgroup.
After monitoring the sci.eng.control newsgroup for the last 6 months I
conclude many of the sci.eng.control participants have considerable
experience with process control instrumentation.
H.R. Hnansen
Mayes uk
> Help w Water Flow Sensor?
>
> I need help with a design problem. I need to get an analog signal
> from 200 water pipes that I can use to estimate flow rate. I have a
> great analog-digital system from B&B Electronics that will give 12 bit
> accuracy from a 0-5 volt signal for only about $6 per channel, but I
> cannot decide what type of sensor to use.
[snipped]
> All the other methods we have thought of depend on using a rectangular
> weir cap on the end of the pipe, and calculating flow from a simple
> non-linear formula relating water depth behind the weir to flow rate.
> This method works great visually - we can engrave a 0-8 inch depth
> scale next to the weir opening, and read depth and GPM within 5%
> accuracy - fine for our needs.
[snipped]
> But computerizing this depth measurement has proved difficult.
[snipped]
Presumably the weir is a V-notch type - these are commonly used for flow
measurement and there will be references in many geophysical texts.
I did this using an LVDT connected to a float inside a four inch pipe
for protection, simply because the parts were available, but I guess
this would be too expensive in quantity. A less expensive way is to use
a toilet type ball valve assembly, and mount a shorter stroke pot
near the pivot. RS components in the UK (Not Radio Shack) sell a
suitable 10mm stroke spring loaded sensor for 12 pounds - their part no
is 317-780, but I have seen cheaper (Japanese) devices.
As Leonardo would have drawn it ...
Ball Pot
__ | Pivot
(__)----------+--X
|~~~~~~~~~~~~~~~~~~~~~~~~~~
| <-- Water
Depending on your required sample rate and the amount of turbulence
experienced, a capacitor across the pot output is often useful.
Sloppiness in the pivot arrangement is the biggest enemy here, and it
may pay to throw out the supplied arrangement and use a small brass
hinge instead.
Robert
--
Mayes uk
Adrian Jansen
At least you have done most of the hard work, working out all the
possible techniques !
Sounds to me that the cheapest is probably to measure the pressure,
either at the wier, or across a flow restrictor, depending on which
is easier mechanically.
Motorola pressure transducers can be had for under $10, plus 1
precision opamp, and a couple of 'ordinary' opamps for temperature
compensation, should give you a DC output proportional to pressure
for less than $30 per unit, I would think.
Good luck.
--
Regards,
Adrian Jansen aja...@adelaide.DIALix.oz.au
Design Engineer J & K Micro Systems
PH (08) 271 4126
Microcomputer solutions for industrial control
Spehro Pefhany
Howard R. Hansen (h...@srv.net) wrote:
[..]
: weir. Using a pressure transducer to measure the water level is very
: similar to your suggestion of using a strain gage to weigh the amount of
: water in a tube. Twelve inches of water is equal to 0.43 psi. Hence your
: full scale output is 2/3 times 0.43 or 0.29 psi. I made a quick search of
: the world wide for a suitable semiconductor pressure transducer but so
: far I haven't found one.
Motorola makes some that may be suitable, as does Sensym (a spin off
from NS some years back). You probably don't want one designed for
industrial use (4-20mA output and $200-$400 each), but there are plenty
designed for HVAC and automotive applications that are cheap enough. In
100 quantity, $15-$25 or so is possible. These are silicon diaphram
(rather than stainless steel as in the industrial units)
and require reasonably controlled fluids to be in contact with them .. no
hot sulphuric acid.. and have wimpy connections generally that require
some beefing up for field use.
Good luck!
BTW, response of a pressure transducer using drop across an orifice to
flow is square root, as I recall, so keep that in mind.
--
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
Spehro Pefhany "The Journey is the reward"
sp...@io.org -------------------------------------
Fax:(905) 332-4270 (small micro system devt hw/sw + mfg)
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
Robert Macy
JV>From: Jack Van Olst <jvan...@west.net>
JV>Help w Water Flow Sensor?
JV>I need help with a design problem. I need to get an analog signal
JV>from 200 water pipes that I can use to estimate flow rate. I have a
JV>great analog-digital system from B&B Electronics that will give 12 bit
JV>accuracy from a 0-5 volt signal for only about $6 per channel, but I
JV>cannot decide what type of sensor to use. Forgive the length of this
JV>question, but I am trying to get ideas from anyone that has experience
JV>in this field. My biggest problem is COST - I cannot use traditional
JV>technology, because we don+t have the $$$.
...clip....
JV>All other ideas involve some method of directly measuring the 0-8 inch
JV>depth by using a float on the water surface. Here are some ideas:
If you make the float opaque, you might be able to take advantage of
this inexpensive technique....
Use two parallel lucite rods with an opaque float between them. Mount
an LED on one end of one of the rods to light it up and on the other end
of the other rod mount a receiving diode.
Opaque item (larger than the whole gap) moving up and down between the
two rods will linearly modulate the light transfer (more or less).
The reason for the light source at one end and the light receiver at the
other is to "linearize" the transfer function as the opaque object moves
along between the rods and blocking the light between the two.
If the transfer function of the system is not too nonlinear, you can
always make decent corrections in software. Five bits will give about
5% accuracy. And you said you have 12 bits to start with.
- Robert -
* OLX 2.1 TD * The only way to reform some people is to chloroform them.
ktone
In article <31A6DF...@west.net> Jack Van Olst, jvan...@west.net
writes:
>All other ideas involve some method of directly measuring the 0-8 inch
>depth by using a float on the water surface. Here are some ideas:
How about this.
Measure the volume of the cavity in (0-8 inch) tube by sweeping an audio
frequency from a tranducer and measuring the output signal level with a
mic. It will peak at resonance. One sweep generator could be used to
sweep all sensors and one simple linear AC/DC converter (for your B&B
Electronics analog-digital card) with a multiplex system to scan all mics.
________________________________________________________
"Risk your life on every note" C Hayden
........................................................
David L. King |kt...@chitown.com
H-Tech Enterprise Presents |Chicago Psyberview
Chicago Music and Arts WWW |http://www.chitown.com
________________________________________________________
Bruce Bostwick
ktone <kt...@chitown.com> wrote:
>Measure the volume of the cavity in (0-8 inch) tube by sweeping an audio
>frequency from a tranducer and measuring the output signal level with a
>mic. It will peak at resonance.
Or use the air column as the resonant element of an oscillator and count
the frequency of the oscillator -- less latency time that way. Either of
these approaches will require an acoustically insulated tube and a baffle
on the tube vent to keep from driving you bats listening to the howl
though .. ;-)
<BGB> http://ccwf.cc.utexas.edu/~lihan/ mailto:li...@ccwf.cc.utexas.edu
Douglas Dwyer
>Help w Water Flow Sensor?
or voltage the other with minimal dissipation, with rapid flow the two
thermistors will have a similar resistance without flow one will be
hotter (lower resistance than the other) select power/ resistance and
software to linearise.
--
Douglas Dwyer
Adrian Jansen
This technique is always suggested for water flow, but when you do
the numbers, you find that it only works for VERY low flow rates,
or at VERY high power levels.
Its a great idea for GAS flows, and can be made very accurate and
sensitive, but NOT for liquids.
In article
<Wtsj9AA0...@ddwyer.demon.co.uk>, Douglas Dwyer
(ddw...@ddwyer.demon.co.uk) writes:
>In article <31A6DF...@west.net>, Jack Van Olst <jvan...@west.net>
>writes
>>Help w Water Flow Sensor?
>> Try a pair of similar NTC thermistors one heated with a fixed current
>or voltage the other with minimal dissipation, with rapid flow the two
>thermistors will have a similar resistance without flow one will be
>hotter (lower resistance than the other) select power/ resistance and
>software to linearise.
>
>--
>Douglas Dwyer
>
--