Understanding and Measuring Mean Radiant Temperature

Slideshow (click to launch)

All the way back in 1993, one of my first research projects at the NAHB Research Center—now Home Innovation Research Labs—was assessing the performance of radiant ceiling panels for the Department of Energy’s Advanced Housing Technology Program. (The final report was titled “An Evaluation of Thermal Comfort and Energy Consumption for the Enerjoy Radiant Panel Heating System.”)

I really knew next to nothing about the science of thermal comfort. I distinctly remember the first time I heard the name Ole Fanger. Dan Cautley, a mechanical engineer who was my NAHB Research Center mentor (and subsequently a lifelong friend), handed me ASHRAE Standard 55 – Thermal Environmental Conditions for Human Occupancy and said, “Study this and come to love Ole, the father of measuring thermal comfort.” (The photo at right of Ole Fanger is from a tribute to Fanger and his work.)

Ole Fanger’s work and ASHRAE Standard 55 both focus on the six factors determining thermal comfort:

• air temperature (AT)
• mean radiant temperature (MRT)
• air speed (AS)
• relative humidity (RH)
• metabolic rate (unit of measure: met)
• clothing insulation (unit of measure: clo)

The first four factors are environmental conditions, and the last two are personal factors. And if the last four are in a “typical” range for a commercial office setting—roughly, little to no air speed, RH between 25% and 55%, people seated at their desks (so having a low metabolic rate and dressed for work)—we can characterize thermal comfort just by measuring air and mean radiant temperature, adding them together and dividing by two. This is called the "operative temperature."

An MRT globe measures mean radiant temperature

A thermal comfort measuring station is pictured in Image #2 in the slideshow. It consists of three air temperature sensors (located at different heights) and a mean radiant temperature (MRT) globe positioned at about the height of a seated person. The MRT globe is a 6" hollow copper sphere with a temperature sensor located in the center of the sphere.

You can purchase devices that measure MRT along with air temperature and relative humidity—for example, the Extech HT200 pictured in Image #3 in the slideshow. But the HT200 costs about $250 (retail price). Is there an easier—and more affordable—way to measure MRT, which is so critical to assessing thermal comfort?

My colleague and friend Brian Just from Efficiency Vermont let me borrow his Extech HT200 so I could compare a couple of “homemade” globes and see how they did.

Making your own MRT globe

Image #4  in the slideshow shows an MRT globe built by my good friend Howdy Goudey from the Lawrence Berkeley National Laboratory. Howdy has a reputation as an engineer who can build just about anything; Howdy let me borrow his do-it-yourself MRT globe to test it against the Extech HT200.

But I wanted to go even lower tech, so Howdy referred me to a 1977 research paper by M.A. Humphreys: “The Optimum Diameter for a Globe Thermometer for Use Indoors.” This paper includes an illustration of the simplest, cheapest MRT globe ever (see Image #5).

The required parts include a ping-pong ball and a mercury thermometer. It’s hard these days to come by mercury thermometers, so I thought I would try a “spirit-filled” thermometer, which cost about $7 at a local hardware store (see Image #6).

Testing a homemade MRT globe

Image #7 is a photo of a test I performed. I lined up the three MRT globe devices and then placed the fully energized radiant panel (Image #8) just behind the devices and recorded how well they tracked each other and whether they responded within the same time frame. I figured that if either or both of the do-it-yourself MRT globes were even close to the Extech HT200, that would be terrific.

Image #9 shows how I tracked the background air temperature as I tracked the devices being closely exposed to the heating panel.

Before being exposed to the radiant heating panel, the Extech HT200 and Goudey’s MRT globe were reading within about a degree and a half of each other (see Images #10 and #11). My do-it-yourself MRT was reading just about 60°F, lower than either of the other two devices (see Image #12).

Images #13, #14, and #15 show the temperature readings for each of the devices as they reached peak radiant temperatures with the 141°F radiant heating panel just 2 inches behind all three devices. Good news! All three devices are reading within about 5°F of each other. And in terms of speed of response, the Extech HT200 and my do-it-yourself device responded pretty much equally fast to the radiant heating panel, with Goudey’s MRT globe taking up to two minutes longer to “catch up” with the other two.

The Goudey MRT is the largest of the three devices, and that may explain its slower response time. Don’t be fooled by the matte white color of the Goudey globe compared to the matte black of the other two. Color only matters with shortwave IR, which only happens with radiant exposure to something like the sun; Goudey only uses his MRT globe for indoor settings. And as a heat stress device, the Extech HT200 gets used outdoors quite a bit. Inside buildings, only lower energy, long-wave IR is generated, and color does not matter.

Conclusions

My little test suggests that a do-it-yourself MRT globe built out of a spirit-filled thermometer, a ping-pong ball, and some matte or flat spray paint will do just fine.

I bet Fanger would be proud. You can build your own MRT globe and use it to work with clients to demonstrate how air temperature and MRT affect thermal comfort.