[BLDG-SIM] On-site U-value measurement

[Brandon Nichols]

Hello Gunes,
 
What I was originally thinking was along the lines of tuning the UAΔT equation with a correction factor, based on a wall area with known U-value somewhere in your plant, so that relative U-values could be calculated directly from thermal image ΔT data elsewhere.  
 
Given a known indoor temperature, this correction factor would account for a minor offset in camera calibration, or alternatively could be used to "tune" the camera calibration, with the objective of obtaining U-values accurate out two or three decimal places.
 
The application you describe is a bit different than what I was first envisioning.  If you're scanning residential walls, with less of an emphasis on accuracy and greater emphasis on surveying large areas quickly, then you may consider this approach:
 
Since U is inversely proportional to ΔT, select an area of a given scanned wall with an estimable U-value, e.g. window, door, or framing elements.  Knowing the indoor temperature Ti and neglecting effects of radiation, convection and thermal inertia (apologies in advance to all heat-transfer academics), then U-values of unknown areas are essentially a product of the inverse of the differential temperatures times the known U-value:
 
  U2 = U1 * ((Ti-T1)/(Ti-T2)) 
 
Lets say the indoor temperature is measured at 20°C, a window in the thermal scan is estimated at a U-value of 1.0 W/m2*°C, and the external scan shows the window and adjacent wall temperatures to be 16°C and 10°C respectively:
 
  U2 = 1.0 * ((20-16)/(20-10)) = 0.4 W/m2*°C 
 
...for the U-value of the wall.  Here's where your thermal imaging postprocessing software will come in handy...exclude the window, and calculate the average surface temperature of the remaining wall to use in the equation above.
 
Hope that helps...
 
 
Regards
 
Brandon Nichols, PE
Mechanical
HARGIS ENGINEERS
600 Stewart St
Suite 1000
Seattle, WA 98101
d | 206.436.0400 c | 206.228.8707
o | 206.448.3376 f | 206.448.4450
www.hargis.biz

 
 
 
 


  _____  

From: BLDG-SIM at gard.com [mailto:BLDG-SIM at gard.com] On Behalf Of Gunes Inan
Sent: Wednesday, May 09, 2007 7:00 AM
To: BLDG-SIM at gard.com
Subject: [BLDG-SIM] On-site U-value measurement



Dear All,

 

Thanks for your replies.

 

We currently have a thermal camera but I never used it to record quantitative results. Brandon, you mentioned about “tuning” the heat transfer equation. If we write a steady-state surface heat balance equation for the inner side of the wall:

Q_convection=Q_conduction

h*(T_air – T_wall_in)=k*(T_wall_in - T_wall_out)/x_wall 

where radiation is negligible and h is known, than we should be able to calculate k and u-value, if I am not wrong. Is this what you mean by tuning? 

 

The u-values that I obtain either by calculation or measurement need not to be accurate in the academic level, instead it should be practical so as to analyze more houses and reach more people. We actually intend to improve the energy efficiency of houses for which neither we nor the inhabitant have information about the constituents of the wall.

 

Prof. Sibilio, thanks for referring me the Hukseflux. As I called the company, they told me that each heat flux measurement takes about 24 hours. Is this the way you do it or do you know any tricks to get reasonable results in a shorter time.

 

Regards,

 

Gunes Inan

 

  _____  

Kimden: BLDG-SIM at gard.com [mailto:BLDG-SIM at gard.com] Yerine Brandon Nichols
Tarih: 08 Mayıs 2007 Salı 20:58
Kime: BLDG-SIM at gard.com
Konu: [BLDG-SIM] On-site U-value measurement

 

Hello,

 

You may find an infrared thermal imager to be a powerful tool, useful for the application you have indicated and many others.  Here's a link to Fluke Corp. thermal imaging notes for building systems, there are many other manufacturers...

 

http://us.fluke.com/usen/support/appnotes/default?category=AP_TH_BM(FlukeProducts)&parent=AP_THERMO(FlukeProducts)

 

For building systems, the thermal imager is useful for finding voids in insulation, infiltration losses, calculating the overall heat loss correction due to framing framing elements, etc..

 

Using the thermal imager, you will have to "tune" heat loss equations based on observed temperature differential data to derive U-values. These instruments typically have the ability to scan large areas and quickly calculate a weighted average and other statistical factors for each image.  

 

Unfortunately they are not cheap, and probably overkill for just measuring the U-values of specific materials.  Personally I don't know any way to measure U-values directly with a hand-held instrument, but it seems such a device wouldn't be hard to put together with a data acquisition unit and a couple of thermocouples.

 

 

Regards

 

Brandon Nichols, PE
Mechanical
HARGIS ENGINEERS
600 Stewart St
Suite 1000
Seattle, WA 98101
d | 206.436.0400 c | 206.228.8707
o | 206.448.3376 f | 206.448.4450
www.hargis.biz

 

  _____  

From: BLDG-SIM at gard.com [mailto:BLDG-SIM at gard.com] On Behalf Of Gunes Inan
Sent: Tuesday, May 08, 2007 7:46 AM
To: BLDG-SIM at gard.com
Subject: [BLDG-SIM] On-site U-value measurement

Dear All,

 

We are looking for a hand-held instrument for on-site measurement of the U-value or thermal conductivity of walls. Any experience about the measurement techniques and recommendations about instruments and brands are greatly appreciated. 

 

Thank You,

 

Gunes Inan

 

R&D Engineer 

Betek Paint and Chemical Corp., Inc. 

Kavakpınar Mah., Tandoğan Cad. No: 175, 

Kaynarca, Pendik 

Istanbul, Turkey 

Tel: +90.216.396.2070 / Ext: 177 

Fax: +90.216.396.6989

 

 
==================
You received this e-mail because you are subscribed 
to the BLDG-SIM at GARD.COM mailing list.  To unsubscribe 
from this mailing list send a blank message to 
BLDG-SIM-UNSUBSCRIBE at GARD.COM
 
 
==================
You received this e-mail because you are subscribed 
to the BLDG-SIM at GARD.COM mailing list.  To unsubscribe 
from this mailing list send a blank message to 
BLDG-SIM-UNSUBSCRIBE at GARD.COM

==================
You received this e-mail because you are subscribed 
to the BLDG-SIM at GARD.COM mailing list.  To unsubscribe 
from this mailing list send a blank message to 
BLDG-SIM-UNSUBSCRIBE at GARD.COM
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.onebuilding.org/pipermail/bldg-sim-onebuilding.org/attachments/20070509/66d3911b/attachment-0002.htm>