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How hot is it?

By Chuck Newcombe

July 2012

I got a call the other day from a friend who was having difficulty with an air conditioning system in a facility where occupancy varied from 20 or 30 people to crowds of hundreds at various times of the day. It seems the a/c was having difficulty dealing with the wide range of heat load. He wanted to record room temperatures at various points over several days and compare it to occupancy numbers.

I recalled a rule of thumb from my past, working in a small laboratory where we had a bank of 100-watt light bulbs near the door as we entered. Using the estimate of 100 watts as the average thermal radiation of an inactive adult human body (like a lab technician at a bench), each technician would turn off one of the lights as he entered. Then, he would turn it back on as he left. The idea was to keep the overall heat load the same, thus keeping the lab temperature stable.

Back to my friend's problem

Using the radiation estimate mentioned above, I quickly came to the conclusion that my friend was dealing with variations in heat load from zero, when the facility was closed, to 37.5 kW when 375 people might be present in a one-hour period. The problem is complicated by the fact that this load was not spread evenly over the facility floor, but might be clustered in a few popular locations in the facility.

I could easily see the need for zone control using thermostats with anticipator capability.

How to make the measurements

I rummaged through my collection of Fluke test tools and came up with the perfect answer to his problem - the Fluke 289 True RMS Logging Multimeter, coupled with a K-type thermocouple temperature probe such as the 80BK-A, which has a shrouded dual banana connector compatible with the digital multimeter's (DMM's) input.

The plan is to place the meter and probe in an area of interest to record the temperature every 10 minutes or so, for several days - a task easily within the capabilities of the 289. Then, after saving the results in DMM memory, he could move the meter to other areas of interest and do the same.

When done, the results can be correlated to facility attendance figures and rough estimates of the heat load requirements to be dealt with, using FlukeView® Forms Documenting Software to make reports with comparative graphs.

Dr. Science

Now I'll put on my Dr. Science hat and tell you about a possible exercise I'm thinking about at the children's museum where I volunteer.

This temperature recording exercise got me to thinking about making a calorimeter to measure the heat radiated from a human body. I soon envisioned a project design to make a small enclosure out of rigid foam insulation panels that could house an exercise bicycle and its rider. Such a facility would have to be large enough that the oxygen in the trapped air would not be depleted during an exercise period. (Wouldn't want our test subject to pass out.)

Then, using the 289 with its temperature probe placed in such a chamber, we could record the heat rise of a resting body. Then, the subject could exercise on the bicycle while we continued to record the temperature. Finally, we could do the necessary math to estimate the thermal radiation under the two scenarios.

Maybe something smaller at first…

Initial Recording Setup

Initial Recording Setup

First Trend Graph

First Trend Graph

Second Trend Graph in REL Mode

Second Trend Graph in REL Mode

As an alternative to this rather ambitious experiment, I then considered the construction of what is known as a coffee cup calorimeter to evaluate the energy consumed or released by various chemical reactions, again using the 289 as a recorder to collect the test results.

Using a Styrofoam cup as a test vessel takes advantage of the insulating qualities of the cup walls to make what is known as an adiabatic calorimeter - one in which the temperature change is contained within the container at a constant pressure.

Remembering that plaster of paris gets warm when you mix water with it to harden the mixture, I decided to measure the heat rise for a small sample contained in a Styrofoam cup.

I put equal weight samples of the water and plaster, each at an ambient temperature of 74.8°F, into the cup and mixed them together quickly. The temperature rose 0.7 degrees to 75.5°F before beginning to fall slowly due to losses through the cup. The graph wasn't too impressive, so I decided to use the REL mode of the 289 and try again.

As you can see in the second graph, the change in temperature was about the same, 0.7 degrees, but we've zoomed in on the area of change - from a plateau at 0.3 degrees at first mix, to a peak of 1.0 degrees as the reaction is completed.

I'll have to refine my experiment at bit, and I still have to perform the calculations, but this first attempt to test the feasibility of my idea only took an hour or so because the 289 is so easy to use.

Find out more about the Fluke 289 True RMS Industrial Logging Multimeter with TrendCapture »