In a world of endless consumer choice, it becomes increasingly more difficult to find exactly the right tool for the job. Even measuring the pH of water brings up a head scratching question of “where do I start?” A quick Google search returns many solutions from pH test strips to advanced sensor systems, maybe erring more on the light-hearted consumer side. Thankfully, in this article, we will break down the types of pH sensors you need to know, and how the different variations will satisfy any pH application you seek.
What are the different types of pH Sensors?
The combination sensor is the stone wheel to cars today. The materials may become more robust as the years pass but the foundational science remains the same. By utilizing a reference electrode and a measurement electrode, the combination sensor produces an electric signal from the difference between the two electrodes, which can then be converted into a pH value.
The differential sensor jumps one leg up by utilizing not two, but three electrodes in the measurement system. With the addition of a third metal grounding electrode, this differential sensor reduces background noise caused by contamination or other sources of electronic potential to be used in more intense, heavy-duty environments.
Consumer-grade pH sensor is the baseline combination sensor with a pencil-like structure to detect more neutral and mild temperature substances. Perfect for everyday use in pool monitoring, household water, and low-impact environmental sampling.
Lab-grade pH sensor is a step up from the consumer grade, using higher end materials in the combination electrode setup to measure more intense pH levels and temperatures that can arise in research and wastewater environments.
Micro pH sensor isfor small volume samples, like studying microfluidics, with the ability to screw into small closed circuits for continuous monitoring. This sensor also comes in consumer and lab grade variations to fit a specific use.
Spear tip pH sensor utilizes a sharp glass tip to penetrate substances like food and soil to achieve more accurate pH readings in cases that dull combination sensors cannot reach.
Industrial pH sensor again uses combination sensor setup but with more robust materials and a stronger external housing. This sensor can go through the ringer and still hold up for continuous monitoring, especially useful for screwing directly into piping.
Now, hold your pH sensors. Many variations of pH sensors exist, utilizing the same technology to satisfy a specific application or environment. The point is that the fundamental sensor technology remains consistent between them but the different sensors will better apply to the specific applications you seek.
Combination pH Sensors
As stated above, the combination sensor uses two electrodes, a reference and a sensing, to measure the relative difference in signal between the two. Since pH is a measure of the ions in water, these sensors measure a small electrical difference between the two electrodes. The positive or negative strength of this signal is interpreted by a circuit which then spits out a pH reading on some sort of digital screen. If you’re thinking about an electric outlet, the two electrodes would be the two plug prongs and the water would be the source of electricity, albeit a small source. Then the strength and sign of this electricity could be translated into a pH value. A fairly simple concept but a strong application to the physical world is most commonly seen in pencil-like, regular pH sensors.
Differential pH Sensors
Have you ever heard of the three legged race? That’s the differential pH sensor. This type of sensor uses a similar reference and sensing electrode in the combination sensor, but pairs it with a third metal grounding electrode to maintain accurate readings should any contamination or quick shifting pH throw off the combination sensor system. Almost like a safety net electrode to reduce background noise on pH readings. Overall, if maintained properly, the industrial pH combination sensors will have no problem in similar harsh environments, but it is worth noting the differential 3-electrode system since most pH sensors are combination sensors as discussed above.
Different pH Sensors for Various Applications
Everyday consumer applications from testing your tap water to seeing how the local lake is holding up would be satisfied by a consumer-grade sensor. Funny how names can actually be useful sometimes. Simple combination, consumer-grade sensors can be fairly inexpensive and last for a handful of years if they are kept hydrated and cleaned of contamination—which is not a large undertaking for those everyday test scenarios in relatively clean water.
However, for applications that become more heavily acidic or basic, or highly polluted samples, you will want to upgrade to a lab-grade sensor to adequately measure reactions or a resulting chemical solution. Be sure to check the pH range specification of the allotted sensor to ensure it will hold up in the field, which is the largest difference between the consumer and the lab grade. Another difference lies in the temperature range and stems from the external housing material to be discussed later. This lab-grade sensor also comes in a mini sensor version!
Two other sensors in this combination sensor family appear in action for specific and smaller applications. A micro pH sensor has a separate cord for each measurement and sensing electrode so that the sensor tips can achieve signal readings in very small and shallow amounts of liquid. Most commonly used in microfluidic studies, these micro pH are well equipped for handling different ranges of pH and liquid environments. In other words, who doesn’t want a travel size pH sensor to pass through TSA checkpoints.
Additionally, the pH spear sensor is a fantastic tool and measurement device to test the pH of soil, food, and other puncture-worthy applications. Equipped with a pin-point glass tip, this sensor allows for easy insertion and pH measurement no matter the sample (besides rock and other hard materials of course). As expected, you would need to rinse the sensor tip off after each use to prevent cross contamination or build up on the sensor tip. Even though “spear” is in the name, no puncturing is required to achieve a pH measurement; additionally, this spear sensor from Atlas Scientific can measure wide pH ranges just like the lab-grade sensor.
Last but not least, the industrial pH sensor belongs in a wheelhouse of its own. Let’s not forget the same combination sensor technology applies here. However, the EXR advanced sensing glass and bulkier sensor casing allows this device to perform in virtually any environment with these advanced materials—and the sensor can be submerged indefinitely for continuous pH monitoring. A true one stop shop. Also, the continuous monitoring goes one step further because this industrial sensor can screw directly into a piping system (no need to hold the sensor and count down the minutes any longer). Pretty much any scenario that arises, the industrial pH sensor can handle it.
What material is the pH Sensor made of?
Immediately, looking between the consumer grade and lab grade and industrial sensors it’s easy to tell that the external casing is different. The consumer grade and lab grade have polycarbonate and Ultem exteriors, respectively, and the industrial grade sensor uses cpvc for an external casing. The difference lies in the strength, temperature resistance, and chemical resistance. As the price and grade of the sensor go up, so do the materials and their ability to endure more harsh environments. For example, the chlorinated pvc (cpvc) highly increases the temperature durability of the pH sensor.
Next up, the glass sensor tip, or sensing material, comes in different grades and coatings to allow variation in temperature and application. Higher grade pH sensor materials allow for wider ranges in pH and different sensitivities to common substances in water that could affect pH reading like sodium or chlorine. Additionally, some sensors have ISFET (ion-specific field effect transistor) electrodes which provide an alternative to glass and do not need continued moisture for storage.
Lastly, the internal chemicals in the sensors and the junctions that separate them from the external world come in a few different material variations; but for the sake of application most sensors utilize silver chloride or potassium chloride and some sort of ceramic junction. Increasing the robustness of these materials, or adding more junctions, would again be for more harsh applications and contaminated environments.
Why are different types of pH Sensors important?
Water pH probably receives the most press of any other measurement in the water world and for good reason. Thinking briefly about common electrolyte drinks flaunted by athletes, water has the same principle of ions bouncing around on the molecular level. The concentration and overall activity of these ions are exactly what is measured by a pH sensor, to then produce a pH value with the correct setup. This pH scale ranges from 0 to 14, packed with insight about the general quality of the water being tested. Standard “quality” water will remain smack dab in the middle of the scale at a neutral pH value of 7, and healthy values of pH only range from 6.5 to 8.5 according to the Environmental Protection Agency on municipal drinking water.
What about the rest of the numbers on the scale? Well, a lower value corresponds to a more acidic substance and a higher value to a more basic substance—increasing in intensity the farther away the number drifts from neutral 7. This most often one-digit number can provide key indicators about water quality and whether or not corrective action needs to take place (i.e. reducing pollution or counteracting hard/soft water).
Water also has a characteristic called alkalinity, or the ability to resist acidification. This parameter is commonly confused with describing a basic liquid (pH value greater than 7). Since it would be bad if the body’s neutral pH drifted toward a more acidic level (causing numerous health complications), key in the rise of the health fad: alkaline water. The thought is by introducing extra minerals and alkaline substances that your body will more effectively resist changes to its overall pH (anti-aging, weight loss, etc); however, no scientific evidence has supported these claims. Although, introducing alkaline compounds is exactly what is done at wastewater plants, and what the ocean does naturally to maintain its pH. That said, a pH sensor cannot directly measure alkalinity but it can provide insight into water having a higher concentration of alkaline substances and/or pollution. However, measuring the oxidation-reduction potential (ORP) will provide more details into the alkaline content of water if the ORP is more negative. The Atlas Scientific industrial pH/ORP/Temp sensor will allow a user to obtain this information in a convenient 3-in-1 application.
If you have a very specific application for a pH sensor it would be worth reading the specification sheets to ensure the sensor materials will not interfere with your pH readings. Although, in general, you should not have to worry down to that level of detail. Simply decide the pH range and temperature range of your application to choose the correct grade and type of pH sensor.
In summary, the two main types of pH sensors come down to the combination sensor and the differential sensor. When searching for the sensor that will meet your needs, that’s when the list expands to consumer and lab grades, spear and micro sensors, industrial sensors, and other pH sensors which all mostly utilize the combination type of pH sensor. That said, be sure to understand your application and pH measuring environment to purchase the correct pH sensor.
If you are unsure exactly which type of pH sensor will best suit your needs, do not hesitate to reach out to the world-class team at Atlas Scientific.
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