How to get salinity using the recorded resistance and temperature?

Hi guys! My name is Iris, a Junior High School Student from the Philippines. It's been 3 months already since I began working on my Research Project: Measuring Salinity and Dissolved Oxygen through a low cost Wireless Sensor Network. As stated above, the main idea of our study is to create an inexpensive WSN.

To do this, we have studied several studies relevant to our research. And as my knowledge about the subject expands , I formulated an idea to make my study feasible. First, since buying Salinity And Dissolved Oxygen Sensor cost an abundant amount of money, I suggested to make our own sensor. But before that, I first searched the factors of the two variables. And later on I found out that they had a relationship with pressure and temperature. Knowing this, it lessens the burden of making sensors, since we can now use their relationship to find the Dissolved Oxygen. And to make that possible, we are going to use the DOTABLES SOFTWARE. ( A SOFTWARE THAT CALCULATES THE SOLUBILITY OF OXYGEN IN WATER IF TEMPERATURE, PRESSURE AND SALINITY IS GIVEN). With this available software online, I no longer need to create a Dissolved Oxygen Sensor. And it's a good thing, and a bad thing at the same time, since I'm now going to use temperature and barometric sensor in replacement. But compared to Dissolved Oxygen and Salinity Sensor, Temperature and Barometric Sensor are less expensive and are much more affordable.

I already have the codes for the Temperature and Pressure Sensor. But the problem arises as it comes to Salinity Sensor. As I have said earlier, to make a low cost WSN, I will need to make them. (EXCEPT FOR THE TEMPERATURE AND PRESSURE SENSOR SINCE THEY ARE AFORDABLE TO BUY) The salinity sensor that I'm going to make is based on the study of Samuel H. Russ and Bret Webb to use resistance and temperature to obtain Salinity. But the codes they used are not included in their study. Is there anyone out there who can tell me the what to do?

A DO sensor is expensive, get over it. You're not going to get that cheap.

A salinity sensor is a different story - see GitHub - wvmarle/Arduino_ECSensor: Library to operate EC sensor for Arduino. for an example of how it can be done inexpensively. I'm afraid I forgot to upload the proper schematics of it to the Github page. Here you go:

I'm not formally selling them yet but if you're interested drop me a PM as I have a few fully functional prototype boards on hand that I can sell you, with probe and all.

If you want to build your own, a few important notes on parts selection.
C1 must be a film capacitor, for best stability use a PP one, but a cheaper PET one will do as well. A ceramic cap is a disaster there. The difference between the three types is clearly measurable! Tolerance of the caps I use is 10%, that doesn't matter as you have to calibrate anyway, the most important here is that the value is stable.
C2 and C3 are for galvanic isolation, must be non-polarised, therefore ceramic. Value is not critical as long as they are both much larger than C1. R1 and R2 are for current limiting; use resistors with low temperature coefficient for best stability, I normally use 25 or 50 ppm/°C.
This one works for EC up to about 10 mS/cm, it's designed for use with hydroponics. If you need a higher EC you need a rather different circuit. I have built an EC sensor that's doing the whole range of tap water until sea water (~60 mS/cm - that circuit should be able to handle up to about 100 mS/cm).

You forgot to add a link to the study you refer to...is it this one?
Interesting way to measure salinity. I'm not familiar enough with digipots so can't predict how stable this will be.

wvmarle:
You forgot to add a link to the study you refer to...is it this one?
Interesting way to measure salinity. I'm not familiar enough with digipots so can't predict how stable this will be.

Yes

wvmarle:
A DO sensor is expensive, get over it. You're not going to get that cheap.

A salinity sensor is a different story - see GitHub - wvmarle/Arduino_ECSensor: Library to operate EC sensor for Arduino. for an example of how it can be done inexpensively. I'm afraid I forgot to upload the proper schematics of it to the Github page. Here you go:

I'm not formally selling them yet but if you're interested drop me a PM as I have a few fully functional prototype boards on hand that I can sell you, with probe and all.

If you want to build your own, a few important notes on parts selection.
C1 must be a film capacitor, for best stability use a PP one, but a cheaper PET one will do as well. A ceramic cap is a disaster there. The difference between the three types is clearly measurable! Tolerance of the caps I use is 10%, that doesn't matter as you have to calibrate anyway, the most important here is that the value is stable.
C2 and C3 are for galvanic isolation, must be non-polarised, therefore ceramic. Value is not critical as long as they are both much larger than C1. R1 and R2 are for current limiting; use resistors with low temperature coefficient for best stability, I normally use 25 or 50 ppm/°C.
This one works for EC up to about 10 mS/cm, it's designed for use with hydroponics. If you need a higher EC you need a rather different circuit. I have built an EC sensor that's doing the whole range of tap water until sea water (~60 mS/cm - that circuit should be able to handle up to about 100 mS/cm).

Have you tried the Dotables Software of USGS?

Never heard of it; again please remember to add links for such things.

A quick Google turned up this page, a calculator for the solubility of oxygen in water. I don't know what that really has to do with your project, as the DO value of your water will normally be anywhere between zero and the number this calculator will give.

Also I've never worked on DO myself. Just developed this EC sensor, and more recently designed a galvanically isolated EC/pH/temp water quality sensor board. I don't even know how DO probes actually work.

wvmarle:
Never heard of it; again please remember to add links for such things.

A quick Google turned up this page, a calculator for the solubility of oxygen in water. I don't know what that really has to do with your project, as the DO value of your water will normally be anywhere between zero and the number this calculator will give.

Also I've never worked on DO myself. Just developed this EC sensor, and more recently designed a galvanically isolated EC/pH/temp water quality sensor board. I don't even know how DO probes actually work.

I'm thankful for your help, sir. But sorry if I can't buy your product at this point. We have been focusing our research on an inexpensive way of collecting data. And one of the researches that I found best is the (PDF) Novel low-cost salinity sensor for embedded environmental monitoring. How unfortunate Am I that they did not include their codes.

Well, you have my code & circuit, feel free to build it yourself.