De-coupling.

[beingobserver]

Ok, i will try with seperate power source.

[beingobserver]

I shot a short video with my elementary level english

PH circuit with Arduino.
PH circuit with External Power source.
TMP with external power source.

You can see the results.
http://www.youtube.com/watch?v=_Xx1StvEvBE

[alnath]

OK
I saw you disconnect the GND wire . Why ?
GNDs should be connected together

In your video you didn't try Arduino + TMPsensor with one power supply (say arduino USB powered and TMP sensor powered from arduino 5V pin) ,
and PH-circuit with its own 5V power supply
But : All GNDs connected together
That's what I would try .

BTW, these are not really advice, I'm more on a  "here is what I would try ...... "  mode 

[polymorph]

All those big loops of wire are going to act like antennas. A microcontroller is a noisy thing. And your instability is very small.

Are you running that 5V regulator into the barrel jack on the Arduino? If so, that is meant to have at least 7V into it, as it is connected to a linear 5V regulator on board.

Also, the output from switching power supplies is notoriously noisy. Certainly the manufacturer of a charger isn't worried about making a source of clean 5V.

The sockets on an Arduino are made for those big square pins, and the thin wires meant for a protoboard are likely to not make the best electrical connection.

Ground wires are providing a path for both signal return and power return. This will affect what shows up at an analog pin, relative to ground at the AVR chip itself.

Plugging and unplugging wires from an Arduino without powering it off first is a risky thing to do.

[alnath]

All those big loops of wire are going to act like antennas. A microcontroller is a noisy thing. And your instability is very small.
......
......
Plugging and unplugging wires from an Arduino without powering it off first is a risky thing to do.

at least 2 points I should have pointed out     especially with that ph-circuit wich seems very sensitive to noise
always good to have "new eyes" on this 

[beingobserver]

@alnath Understand the point, there must be common GND between external Power Source, Arduino and the circuit.

@polymorph Thanks for your suggestions. I am now powering off before wiring.

Now. I solved something
I burned that digital temperature sensor which includes an onboard memory.
(This is the second atlas-scientific part i burned)
Fortunately i had another one which is a basic analog model.

I am not using any external power, just using the Arduino for all PH, ORP, DO and TMP sensors.
They worked with the first try!!

Money is not my first concern actually but i am distressed because i don't know how i burned these 2 circuits (EC and Digital TMP).
I used maximum of 9v for the Arduino. And 5v for the external power sources.

Thank you

http://youtu.be/VqUmIBhD2wI

[Grumpy_Mike]

First off those capacitors are not ceramic ones, and even if they were the legs are far too long. The legs should be cut as short as possible and they should be mounted as close to those sensor boards as possible, not just stuck anywhere like you have them.

Next all four sensors are in the same liquid, can you do that? What happens if you connect all the sensors up but only put one at once into the water, does that change things?

You might have burned the sensors by messing about with the wiring when the power is on like you did in the video. Never change the wiring with the circuit when it is powered.

[beingobserver]

First off those capacitors are not ceramic ones

This mistake belongs to the seller not me

Next all four sensors are in the same liquid, can you do that?

Actually there are things i don't understand about the circuits. For example, if i don't connect the BNC connector, i can get the results anyway instead of saying "Check the Probe". So i am not so sure about the values. I will re-calibrate them with their solutions after finishing these things.

You might have burned the sensors by messing about with the wiring when the power is on

Yes, it was an expensive lesson for me.

So, i need to buy some ceramic caps. I have a long long long way to go.

I need to go to bed now because tomorrow is the first school day of my little princess

Thank you so much.

[Grumpy_Mike]

For example, if i don't connect the BNC connector, i can get the results anyway instead of saying "Check the Probe".

Not sure what that means.

Good luck to the little princess. 

[dc42]

Is the temp sensor dipped in the same liquid as the ph sensor? (Please excuse me if that should be obvious from the photos, I'm using a smartphone and it is difficult to view them). The ph sensors I've seen only work when the liquid they are in is electrically isolated from ground, unless you isolate the ph sensor itself. Dipping a metal cased grounded temp sensor into the same liquid as the ph sensor would break the isolation.

[Grumpy_Mike]

Is the temp sensor dipped in the same liquid as the ph sensor

Yes it is.

I have been trying to say that in this and the other thread the OP has.

[beingobserver]

I am really surprised. And i found something confirming this. http://www.eutechinst.com/techtips/tech-tips7.htm

By the way ORP and DO sensors were affected also.

Analog TMP sensor also has a metal case but they are working. But the values might be inaccurate.
Hmm, i think i need to use seperate liquids. This info was critical. Thank you.