Specialized Sensor Series #2 – Water Quality Sensors

At Phidgets, we try to make a wide variety of sensors available to our customers. If we can’t make the sensor ourselves, we try to find a manufacturer so we can sell a third-party sensor to fill the gap. The purpose of this series of blog posts is to highlight a specific class of unique sensors and walk through what options are out there and which ones will work with your Phidgets devices.

Last week, we looked at flow sensors and in this post, we’ll have a look at water quality and water property sensors: a broad category of sensors including pH, conductivity and dissolved oxygen probes. These types of sensors have obvious laboratory applications, but are also used in many hobbyist and DIY applications such as hydroponics, micro-breweries or aquarium control systems.

pH probes are the most popular water quality sensor, measuring the acidity or basicity of the solution it’s immersed in. They measure the pH by creating a small voltaic cell between the test fluid and the internal buffer fluid. The stronger the reaction, the higher the voltage. ORP probes function similarly, but they measure oxidation-reduction potential, a property closely linked to the pH.

Features

• Measure the pH/ORP of a solution
• Must be stored in appropriate buffer solution
• Price range: $10-$200

Compatibility with Phidgets

Given the popularity of pH/ORP measurement, we already sell a pH Phidget. It is compatible with any pH or ORP probes we sell, but it will also work with any third-party pH probe with a voltage output range of ±0.4V DC or an ORP probe with an output of ±2V DC. But be careful- just because a sensor has a BNC connector that fits with the pH/ORP interface, it doesn’t mean it’s automatically compatible. One such example is the conductivity probe.

Conductivity probes measure how easily electricity passes through a solution. The principle of how they work is the same as that of an ohmmeter: A test current is applied in the material tested, and the voltage is measured in order to find the resistance of the material via Ohm’s Law. Instead of being electrically connected to the material, the conductivity probe has two self-contained parallel plates that are driven by an AC power supply. The ions in the solution move back and forth between the plates as they change polarity, and the amount of ions (which is proportional to the conductivity) has a direct effect on the voltage measured across the plates.

Features

• Measure electrical conductivity of a solution
• No need for buffer/electrode solution
• Price range: $40-$200

Compatibility with Phidgets

Even though many conductivity probes have the proper BNC connector on the end, they are not compatibile with the pH/ORP adapter because a conductivity probe requires both a power supply and an ohmmeter in order to be read. There is no Phidget device that fulfills these requirements, so the only way you could interface a conductivity probe with Phidgets is if you had some other device to do the measurement and then connect it to the Phidget with an analog or digital output.

Often times a temperature measurement is needed to calibrate and correct various other properties being measured in a liquid. There are numerous ways to measure the temperature of a liquid, but the easiest and most repsonsive way is using a thermocouple probe. Thermocouples determine temperature by measuring the voltage difference between the two different types of metal in the probe. For more information on using thermocouples to measure the temperature of liquids, take a look at this blog post. Thermocouples generate such a small voltage that a specialized interface is required to accurately record the signal and convert it into a digital value.

Features

• Measure temperature of a liquid
• Requires high resolution ADC or interface
• Price Range: $10-$100

Compatibility with Phidgets

Using a Phidgets thermocouple interface, you can read a J, K, E or T type thermocouple with ease. You can also use other types of thermocouples if you’re willing to figure out the conversion formula. Since thermocouples are relatively simple devices compared to these other probes, you shouldn’t have any compatibility problems here.

Dissolved oxygen probes can be used to measure the oxygen saturation of a fluid. Measuring dissolved oxygen content is important when dealing with aquatic organisms or when managing fluids in pipes or containers that are susceptible to corrosion. The probe works similarly to a pH probe; a chemical cell is set up to complete a reaction that consumes oxygen and produces electricity. Oxygen from the solution is absorbed through the gas permeable membrane on the bottom of the probe. There are two common types of probes available: polarographic and galvanic. Each one uses a different chemical reaction, but the important thing is that galvanic probes do not need an external power source and require no warm-up time as opposed to their polarographic cousins.

From my time spent on searching for dissolved oxygen probes online, it would seem that the vast majority of these sensors are designed and sold for industrial applications. However, more and more customers have been asking about dissolved oxygen sensors for their own projects. The main difficulty in trying to find a sensor that only has an industrial form factor is figuring out how to get data from it. When a hobbyist buys a simple sensor designed for general use, the output is usually clearly defined. In the case of industrial sensors, however, you may find that the manufacturers do not specify exactly what sort of signal is being generated by the sensor. Instead, they’ll sell a meter that is specifically designed to interface with that sensor, usually with its own LCD screen and controls.

Features

• Measures oxygen pressure of a solution, which can be used to calculate dissolved oxygen concentration
• No power supply required for galvanic probes
• Price Range: $75 – $500

Compatibility with Phidgets

Like the conductivity sensor, you probably won’t be able to get one of these working with Phidgets without a fair amount of hacking. There is woefully little information about these probes online, and the high cost discourages users from trying to get them to work in their applications. Most of the projects I’ve seen that use dissolved oxygen probes just buy a meter and find a way to interface that with the rest of their project, instead of attempting to use the probe directly.

Just like temperature, sometimes pressure is required to calculate other properties of the fluid. As usual, there are a wide variety of methods for measuring water pressure. The most common type of water pressure sensor is simply a pressure sensor (either strain gauge or piezoelectric) with a chamber for the water to press in on. Strain gauges are flexible pads whose resistance changes depending on how much they deform from pressure. Piezoelectric materials generate small amounts of electrical charge when compressed.

• Measure the pressure of a fluid
• Must be installed carefully and securely
• Price Range: $25-$200

Compatibility with Phidgets

Like usual, the sensor’s compatibility with Phidgets has to do with the type of output the sensor provides. If you end up with a piezoelectric water pressure sensor, chances are it’ll have 0-5V output or 4-20mA output. In the first case, you can just hook it up to any Phidget with an analog input. In the second case, a 4-20mA Adapter will also be needed to convert the 4-20mA signal to a 0-5V signal. If you have a strain gauge water pressure sensor, then you’ll need the load cell interface to read it, just as you would read an ordinary strain gauge or load cell.

To test water pressure sensors with Phidgets, I purchased a piezoelectric pressure sensor for $25 online. Since it was a 5V sensor, I soldered a Phidgets sensor cable to it and plugged it in to a PhidgetInterfaceKit. Since this was just a quick test, I didn’t bother making the connection leak-proof. I shielded the InterfaceKit when I turned on the tap. Always be careful when using electronic components and water.

To the left is a plot of the SensorValue over time. When I turned on the tap, the value went from 100 to 400. According to this sensor’s data sheet, this corresponds to a rise from 0 to 375 kPa. This makes sense, since most of the ordinary buildings in Canada are rated for 345-500 kPa. The disturbance at the 30 second mark was when the toilet was flushed to test its effect on the faucet’s pressure.

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I hope this post has been an informative look into the wide variety of water quality sensors that are available. If you’ve made a project that uses water sensors, tell us about it in the comments! Check out our third article, Specialized Sensor Series #3 – RFID