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Here are the sensors I am trying to measure and then convert into a 0-5V analog output:

pH (acid or base): my understanding is you can use DC voltage to measure this sensor, just not sure what kind it is... impedance / resistive / inductive?

EC (electrical conductivity): Resistive I believe, with an EC of zero giving a 1/1megaohm measurement? has to be AC or you risk the probe wearing out real quick.

10k thermistor (temp): same as EC i believe, a simple resistive sensor

DO (dissolved oxygen): no idea

ion selective probes(nitrates, ammonium, etc): no idea either, possibly the same as EC?

Once I get a grasp of how each type of probe is measured, what resources should I be looking at to tie it all together? How do I reduce noise on the sensors if they are in the same container? What are some of the better methods to test these out if I already have the sensors? breadboarding?

A good example of sensors: http://www.vernier.com

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  • \$\begingroup\$ I updated the post with definitions and a link to one of my sensor resources. there are commercial sensors as well, but they try and sell you their own version of a "transmitter box" that will convert the reading into a 4-20mA Analog output. Frankly id rather develop my own for a lot cheaper \$\endgroup\$ – Zero0ne Nov 22 '10 at 4:28
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    \$\begingroup\$ Let me guess, trying to sense the condition of hydroponic growing solutions? Some of the things you listed are rather complicated to measure. Dissolved Oxygen sensors for instance use some rather exotic materials and trying to measure an absolute value requires constant calibration (same with PH). I'd consider looking at the project goal and seeing which of these you really need. The nitrates,ammonium,etc for instance could be replaced with total dissolved solids (TDS) in many cases. \$\endgroup\$ – Mark Nov 22 '10 at 7:33
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    \$\begingroup\$ In general you need to buy most of the sensors you mentioned. If the ones your looking at are similar to the ones you linked then the signal that comes out of the sensor body has already been pre-processed in some form. That is your not measuring the resistance of those wires (maybe on the temperature) but rather it will be presented in some other form. Voltage, frequency, digital data, there are several ways it could come out, all would be specific to the sensor vendor. \$\endgroup\$ – Mark Nov 22 '10 at 7:35
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    \$\begingroup\$ here is a better company: jencostore.com As they sell just the probes. If EC / pH can be done by a $30 meter I would hope i can get the same accuracy of readings on the PC. A routine that walks through the calibration process would be easy to develop. \$\endgroup\$ – Zero0ne Nov 22 '10 at 8:18
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    \$\begingroup\$ @Zero0ne, you would probably have to have hardware to do the calibration process. \$\endgroup\$ – Kortuk Nov 22 '10 at 16:38
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Thermistor: To measure the temperature of a thermistor, first determine your temperature range that you are interested in, and convert that to delta R of your thermistor.

With this information it will be possible to determine the optimum circuit for detecting that range of temperatures. You may find that a thermocouple may be better suited to your application for larger temperature ranges, at the expense of the circuit and software complexity, involved with instrumentation amplifiers and software to perform cold junction compensation.

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When I use a thermistor it is normally part of a voltage divider. You chose a resistance to place in series with the thermistor that will allow you diverse readings over its operating range. In an Ideal world a thermistor would go from an open(infinite resistance) to a short(no resistance) over the exact operating range you need.

In reality you want to choose a resistance to go with the 10k thermistor to give you readable change. Tie one side to a microcontroller and only turn it on to read it, then turn it off, otherwise the current can cause a significant heating effect. I have a simple schematic, control the 0V or 5V with your micro to stop constant dissipation:

Schematic with resistor and thermistor

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