TURBIDITY METER PROGRESS REPORT II

fiercebunElectronics - Devices

Nov 2, 2013 (3 years and 11 months ago)

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James Berg

10 April 2005

TURBIDITY METER

PROGRESS REPORT II


The last month has been spent on two major developments: finding an appropriate
micro controller and other associated circuitry and determining if a smaller sample
container can be used.


My pa
rtner Jesse Adland, who is an electrical engineer, has decided that, at least
the for the purpose of development, the Atmel AVR MEGA32 micro controller will work
best. This controller will do the conversion from analogue to digital and will talk directly
with the LCD screen. The MEGA32 has far more features then is needed for the
turbidity meter but because it is what Jesse is most familiar with it will be easier to
develop the turbidity meter using the MEGA32. We would switch to a cheaper micro
controll
er for a production model. The basic design of the electronics will be two
photodiodes feeding into the micro controller, the micro controller will then feed directly
into a small LCD screen that will output the turbidity and instructions during calibrati
on.
A 9V battery will provide power for the micro controller and the laser via a voltage
controller.


Tests to determine if the 1” cuvettes from the Hach 2100 could be used in this
turbidity meter have been quite successful. Though there are several p
roblems with the
graph below it shows clear distinctions can be seen at low turbidities. Since the 1”
cuvette is a preferable to 2L bottle the turbidity meter was modified. These modifications
came in the form of a square box in the middle of the turbid
ity meter that was just big
enough for the cuvette to slide down into. Three holes were drilled in the sides of the
box, one for the laser, one for the 180° sensor and one for the 90° sensor. A similar
design would be applicable for the final design. Po
ssibly reducing the whole unite to a
3x3x4 inch box.


As I have said there are several problems with the above graph but despite these it
does show that the photodiodes are capable of being very sensitive at low turbidities. At
higher turbidities the re
adouts from both sensors become rather horizontal. This is a bit
worry some but I am not satisfied with the validity of the numbers after 435 NTU. As
you can see between 435 and 495 there is a dramatic change in the sensor readings.
There was a problem
with the Hach at this point, the sample after 435 should have been
around 500 NTU but the Hach read the sample as around 225 NTU. Several attempts to
correct this problem were unsuccessful. I mixed continually dirtier samples until finally
the Hach gave
a value above 250 NTU, the Hach read this sample as 495 NTU, but I
would estimate, based on the NTU values the Hach gave before the 435 NTU sample,
that the 495 NTU sample should have been somewhere around 650 NTU. I am still
unsure what caused this probl
em. Another problem with this data is that up to 74.9 NTU
the light intensity for the 180° sensor was to much and the sensor produced a voltage
above what the data acquisition system was set to read. This caused the computer to read
the voltage from the
180° sensor as 0.4998 V, when in fact it was higher. This is an
easily fixable problem; just looking at the data I would guess that the voltage input on the
data acquisition system should be set at
±

0.6 V.


I have also looked into getting a different la
ser, one that is not a disassembled pen
laser. Newark Electronics has several diode lasers, they present a minor cost savings but
they would be much smaller. The down side is that they all run on 5V as opposed to the
3V of the pen laser, and would likely

produce a more
intense beam of light. This is not so much a problem it
would just mean that we would have to gather more
data. This has just been a cursory search, there could
be many more complications with using a diode laser,
but it is certainly wort
h looking into.


Right now the final system is looking very
promising and very cheep. As you can see in the table
to the right the electronics should cost between $29 and
$39. This does not include the cost of case or the cost of the sample containers.
The cost
of the case is very hard to estimate, for me it would probably be in the range of $5 for the
black nylon that I would then machine as needed. In a production design the case would
be a few pieces of black injection molded plastic and I have no wa
y of figuring out how
much that would cost, if I had to guess I would say $2
-
3.


While I am now using the cuvettes provided by Hach for use with their turbidity

Part

Price ($)

Micro Controller





MEGA32

8



MEGA8

3

Voltage regulator

5

LED Screen

8

Buttons (2)

1

Diode Laser

7
-
10

Pen Laser

12

9V Battery

5

Sum

29
-
39


meter, it would be undesirable to use them in a production design as they cost $20 for a
pack

of six. There is also no need to store samples so at most two sample cells would be
needed (one spare in case the first one breaks). Cuvettes are available at a much lower
price but two decisions must be made. Cuvettes come square or round (mostly it s
eems
square) and in glass or plastic. Glass has the advantage of not being as soft as plastic and
thus being less likely to scratch and distort the light but is also more fragile. Another
concern is whether or not the turbidity meter is water tight at th
e very least from
splashing. It would be nice to put all the electronic parts in a sealed portion of the
container but the laser, photodiodes and LED screen all need to be visible to some
unsealed portion of the container. This makes it hard but certainl
y not impossible to
make the turbidity meter water tight.