Hi, I’m Dr. Colin Campbell. I’m a senior research scientist here at METER Group, and in this course, we’re going to be talking about water potential. So let’s spend just a little bit of time going through the differences between a tensiometer, the TEROS 32 and a solid matrix equilibration sensor, the TEROS 22.

So which one is the most accurate? The TEROS 32 is the most accurate because it’s a first principles method.

Which is the least expensive? Well, the TEROS 22 is going to be your less expensive sensor. So that’s those are a couple easy comparisons.

Now let’s talk about some of the other things we’re interested in. The TEROS 32 is also the first principles method, but it comes with an inherent challenge.

In an earlier lesson, I mentioned the WP4C vapor equilibration device. It can’t measure in the wet range, the optimal range. Well, the TEROS 32 the tensiometer can’t measure in the dry range. In fact, it doesn’t even come close.

It cavitates, meaning it stops working around negative –100 kPa. That’s a challenge. The tensiometer gives you the most accurate reading, but there’s a lot of maintenance to do. If we want the widest measurement range, the TEROS 22 can read all the way to dry and all the way almost to saturation, about –0.5 kPa.

Which has the least maintenance? The TEROS 22 requires no maintainance at all. The TEROS 32 will take some time if your soil has dried out.

Which output water potential directly? Because it’s a secondary measurement, people ask me sometimes if the TEROS 22 outputs water potential.

Well, every sensor here at METER Group goes into an individual calibration process where we store the coefficients to that relationship directly on each sensor and they measure in water potential. So if you use a ZSC, one of our readout devices, you get water potential, not some water content value that you have to convert.

Now I want to talk specifically about the TEROS 21 and the TEROS 22 water potential sensors.

What’s the difference between the two? Well, the TEROS 21 just has a different form factor. They both have the same accuracy. They’re both basically designed in the same way.

But what sets these two sensors apart from other sensors on the market? We can quickly go through some of these things: their affordability, the fact that we publish accuracy specs online—plus or minus 10% of water potential—the fact that these sensors don’t have any maintenance, and their factory calibration.

METER has a special calibration process where we are able to calibrate each of these sensors, as I mentioned before, using our proprietary method that allows them to be directly connected to the output of a tensiometer, so we get as close as we possibly can to that first principles method. Another thing to talk about is the easy installation, the TEROS 21 we made that was a little bit difficult to install the TEROS 22 we actually designed with a masonry drill bit in mind, one of those long drills that you might install electrical lines in a house. We can use that into the soil. We can very easily push our sensor in the soil and get great soil to sensor contact.

The longevity of these sensors is also important to just mention, from our work and our testing in the lab and in the field, these sensors can last past 10 years. That was something we could only dream about several years ago. And finally, both of them are compatible with the SDI 12 communication protocol. So I want to cover just a few more things before we end.

First of all, what sensors should I use in what situation? Now, if you want to get the most accurate water potential measurement possible, you’re going to want to use a tensiometer, something like the TEROS 32. Why is that? Well, it’s a first principles method, and it’s certainly our most accurate way of doing this.

Now you might be worried, well, what if I have to do a lot of maintenance on it? The answer is, you are going to have to do that. The ideal situation is if you’re in a fairly wet environment perpetually, and that tensiometer just is cycling up towards zero. It can even go above zero water potential, kind of under a pressure head and down to –100 kPa. If you go lower than that, you’re going to have to refill your tensiometer, and the tensiometer just may not be a great choice for you if you have to do this over and over again.