Wet Bulb Globe Temperature

[AbbyNormal]

WBGT seems to be the definitive value used in heat stress situations. If conditions are dangerous to work in etc.

Various sources give ratings where people could work continuously as long as they are hydrated, and then as activity increases or the WBGT increases, hourly rest periods are prescibed.

Ultimately very minimal work is allowed per hour, and finally lethal levels are defined.

Anyways for indoor applications, with no sun beating down on you, my research finds a WBGT defined as:

WBGT = 0.7 x WB + 0.2 x GT +0.1 x DB

where WB is wet bulb, GT is globe temperature and DB is the dry bulb, all in degrees F.

My problem, I do not have a globe thermometer, I would have to fly one in and to use it in a continual data logging situation, it is going to be an expensive instrument, compared to something I could take spot readings with.

The globe thermometer is basically a 5 inch diameter hollow copper sphere, painted black, with a thermometer in the centre of the sphere. I guess the balck absorbs radiant heat, the sphere gives any air currrents a chance to cool it a bit. It would be nice to have a Globe Thermometer if you are truly into ASHRAE 55 studies etc, as those comfort zone charts never use the dry bulb temperature.

Anyways, I have a large concrete structure, people are working inside of it, there is not a lot of light, they do not have the sun beating on them.

I can easily come up with the same Heat Index the American weather channel or NOAA predicts, or the Humidex they use in Canada.

I have a lot of data logged 24/7 every minute, temperatures, dew points etc do not fluctuate that much. Not a lot of difference between the concrete temperature and the air temperature, very few spot cooling fans used. Its brutal in there, some floors averaged 90F with an 83 dew point.

I was wondering, how far off would I be, if I just used the dry bulb value in there as the globe temperature?

I think if anything, it would be making an error on the low side, I would get a low WBGT value?





The way we build has a far greater impact on our comfort, energy consumption and IAQ, than any HVAC system we install

 

[AbbyNormal]

make that WBGT = 0.7 x WB +.3 x GT for indoors

The way we build has a far greater impact on our comfort, energy consumption and IAQ, than any HVAC system we install

 

[MintJulep]

The globe temperature is a measure of radiant heat exchange with the surrounding surfaces. Dry bulb temperature doesn't give you that.

In a pinch, a trip to the local cookware store and a can of flat black spray paint....

 

[AbbyNormal]

Well mint, would you agree that if I used the dry bulb that if anything I would tend to be on the low side.

Basically surrounded by concrete and air about 8 degrees cooler than a body's core temperature?





The way we build has a far greater impact on our comfort, energy consumption and IAQ, than any HVAC system we install

 

[DRWeig]

AbbyN,

Yes, you'll err on the low side -- as long as the stuff in the field of view is warmer than the surface temp of your instrument.

If your building walls got cold, you might err on the high side, but that doesn't sound like your case.

I agree with MintJulep, you can approximate a globe with a flat-black painted copper or aluminum cup. It'll certainly reduce the error lots. I'd be hard-pressed to calculate how much the error is, though... Depends mostly in your case on the temperature of the surrounding walls and any artificial lighting that might radiate heat to your device.

Good on ya,

Goober Dave

 

[DRWeig]

One more thought --

Our business includes outdoor air temperature sensing devices, and testing in the sun can yield beyond 20°F error for an RTD inside a shiny stainless steel probe. Sticking the same probe in a black plastic (unventilated) box will raise that error well beyond 40°F. A black metal box does better, but not by much. Shiny metal box is better still, but again, not by much.

Conditions for those tests were high noon in Memphis (35°N Latitude), clear skies, 95°F dry bulb, still air.

(It's been tough to get installers to find a shady spot for those things, and even the diffuse reflection of solar radiation from green grass can be significant).

Good on ya,

Goober Dave

 

[MintJulep]

If the concrete temp is lower than body temp then yes, I think using simple DB temperature would be conservative.

If you know the surface temps, emissivity, etc, I suppose that you could calculate what the globe temperature would be.