I'm Bruce Bugbee, president of Apogee Instruments,
and today I'd like to give you a brief overview of how we measure surface temperature.
Now this is non-contact surface temperature, so it's done with Apogee infrared radiometers.
I'm joined in this effort by Mark Blonquist, he and I have worked together on this for many, many years to refine this measurement.
It's a powerful measurement, but yet quite challenging.
And what I especially want to do in this talk is described what the Apogee Instruments' sensor does.
It looks like this—here it is with the sensor—
compared to much lower-cost sensors. Here's a little one, you can buy these for under $100.
What's the difference in research-grade and hobby-grade?
And this is a lot like taking a picture with your cell phone camera versus a professional-grade camera.
When you need accurate measurements, you need research-grade sensors.
So there are seven things in here that I'm going to quickly go through that establish a research-grade sensor.
The first one, obvious, is accuracy.
And accuracy isn't just a matter of simple accuracy, because if this is the difference between the sensor and the target,
any sensor can typically get that right if the sensor is 77°F or 25°C and the target is also 77°F or 25°C.
It's not that hard to measure it.
Now try to measure a hot surface or a very cold surface with the cheaper sensor and they have significant errors.
So this is a temperature difference down here—temp diff—
and we're gonna put plus 30°C, and over here we're gonna put minus 30°C, and this is the error.
So here's a zero error.
This is a low-cost, or a hobby-grade sensor. The error looks like that.
And these errors up here can be 5°C as these temperatures reach the extremes.
So how good is an Apogee sensor? The Apogee sensor comes in right along here.
The accuracy specification for Apogee is ± 0.2°C and that's over the entire temperature range.
That's an enormous difference. People just do this and they say "this is almost as good as the Apogee."
Yes, but you haven't tested it in extreme conditions. So accuracy is a big deal.
#2: Field of view
All of these sensors—if we put a sensor right here—have a cone-shaped field of view.
Draw it like this.
A low-cost sensor has an enormous field of view. It sees the edges, it sees the sky, it sees the ground.
It averages everything.
If you really wanted to average everything, that be fine, but typically what you really want to see is a small area in there.
So the Apogee sensors have variable fields of view, but they're typically much tinier, depending on the type of sensor.
So you can pinpoint the temperature of a spot, rather than average a big area.
This causes a big change in the signal and it's one of the reasons that Apogee has work hard to get this.
Some of the Apogee sensors have a rectangular field of view.
It depends on what you're looking at, so there's multiple options for field of view.
But that's a big difference between low-cost and research-grade sensors.
#3: Spectral sensitivity
Here's a graph of long wave radiation.
Long wave radiation.
And that typically goes from about 4,000 nanometers—there's a broken axis here—all the way out to 100,000 nanometers.
Radiation from a typical blackbody looks something like that.
A low-cost sensor integrates all of this, which would be OK,
except there are huge windows in here where water vapor in the air absorbs radiation.
So a low-cost sensor partly measures the air temperature and partly measures the target temperature,
particularly as you get farther away from the target.
For this reason, Apogee sensors—and I'm gonna show this in green—measure only what's called the atmospheric window.
A-T-M-O-S window.
In this range right here, water vapor does not interfere with this.
The air is transparent when you use a research-grade Apogee sensor, because they're limited to this window right here.
Again, it's a smaller signal, so it's harder to measure, but it's important not to measure the air.
#4: Response time
If this is time here—let me get that there—signals can look like this.
A cheaper signal, and that's a lot of noise.
This might be plus one degree and minus one degree and zero is right here in this.
So if this signal is bouncing around, what do you do?
You do a running average, and I'll show that right here in green.
You might have a running average that looks like that that's much more accurate, but you have to wait to get your data.
You have to leave it on there for several seconds to average this.
Apogee sensors have much smaller noise over time, so they don't need to do a running average.
It can make measurements over short time intervals. Much faster response time. For many applications that's critical.
#5: Emissivity
This is a complex topic, but if this is a surface—say the ground—not all of the radiation coming off the ground is emitted.
Typically, for many surfaces, we'll say 90% is emitted—this is a parameter usually represented with a lowercase 'e'.
For a surface like this, what happens to the other 10%?
That's reflected.
So it's averaging—in this room it'd be averaging the ceiling temperature with the floor temperature—
and it's weighted to the floor temperature.
Cheaper sensors don't account for this. Apogee mathematically can account for this and correct for problems with emissivity.
We'll talk more about this in a more detailed seminar,
but this is can lead to significant errors if it's not properly corrected for in the sensor and Apogee sensors allow for that.
#6: Durability
A sensor like this is not designed for continuous outdoor field use.
The Apogee sensors are designed for year-after-year measurements in the field.
So if you make the investment for good sensors, you sure want it to work and you want it to work reliably through harsh weather conditions.
And the seventh, and final one, is the output of the sensor.
You have to get the signal accurately to a datalogger, so this is shielded, twisted-pair cable with multiple output options.
This comes with SDI-12 options, it's soon to come with Modbus options, Bluetooth options.
There's many ways to get the signal from this to a datalogger.
Cheaper sensors read it on the sensor, but that's it.
So those are seven reasons that research-grade sensors give you better accuracy and better long-term stability than the much lower cost sensors.
Thanks for listening.