We are pleased to announce that digitally signed test reports will be issued by OTM, with the following benefits:
Better image quality
Smaller file size
Our digital signature is auto-trusted in Adobe Reader. The signature certificate can be verified by simply clicking the signature.
During the transition period, we will still provide manually signed hardcopy reports by default. After the transition period, only the digitally signed test reports will be issued, and manually signed hardcopy reports will be provided on request only.
1 x HD35APD USB dongle type base unit: for wireless communication between the computer and the wireless data loggers; 1 base unit can support multiple wireless data loggers
The model HD35EDWPM is an upgraded version of the system shown in this post. The new model integrates the sensing part and the data logging part into a single unit.
Shown below is the software screenshot:
The system can be deployed for construction site dust level monitoring. The data logger is rain-resistant and can be deployed outdoors. A clamp is included for each unit and the mechanical mounting is very easy. If the data logging interval is set to 5 min or 10 min, the internal memory can store years of measurement data.
We were asked why the U-value of a glass is different when the glass is installed horizontally.
There are 3 heat transfer modes: conduction, convection, and radiation. The convection part is dependent on the glass tilt and it affects the glass U-value.
By default, we evaluate the U-value of a glass with the vertical tilt, which is the most common position of glasses. For a horizontally tilted glass, the U-value is significantly greater than the U-value of the same glass with the vertical tilt.
Besides the dependency on tilt, the U-value is also dependent on the glass height. Other thermal properties (e.g. SHGC) are dependent on the tilt too.
However, it does not mean that the glass U-value shall be evaluated with different tilts. There are primarily two applications:
Glass performance rating
Fenestration performance rating
For glass performance rating, it is sufficient to evaluate the glass U-value with the vertical tilt only. With this standardized tilt, fair comparisons can be performed conveniently.
For fenestration performance rating, the glass tilt is considered in the evaluation by default.
Typically, an solar reflectance index (SRI) test is performed on a fresh sample. Such SRI can also be called fresh SRI. It is often required to determine the aged SRI, to understand the decrease of SRI after weathering, due to soiling and material degradation.
We’ve helped a few customers determine the aged SRI of their materials. Below are the typical steps:
Step 1. Fresh SRI measurement: a fresh sample is measured before weathering
Step 2. Weathering: the fresh sample is returned to the customers for weathering
Step 3. Aged SRI measurement: an aged sample is measured after weathering
For the fresh and aged SRI measurement part, our usual SRI measurement practices are followed and there are no differences on the lab side, except that a sample is tested twice.
For the weathering part, there are 3 options:
User weathering: the customers perform the weathering following their in-house methods (which are determined internally or are mutually agreed by all relevant parties). For all aged SRI testing conducted by us so far, this option was employed.
Weathering according to ANSI/CRRC S100: in ANSI/CRRC S100, detailed natural weathering (field exposure) and laboratory soiling and weathering methods are defined. The natural weathering requires a 3-year weathering duration. The laboratory soiling and weathering is based on ASTM D7897.
We typically advise our customers that the maximum sample size is 300 mm × 300 mm. Most of our customers can provide samples smaller than 300 mm x 300 mm and this size is convenient to handle for both us and our customers, but it is not a technical limit.
Technically, we can test samples greater than 300 mm × 300 mm. The maximum size we can test is limited by the weight (due to safety reasons, samples that are too heavy cannot be lifted and aligned properly) and our lab space (due to the space limit, samples that are too large cannot be handled in the lab).
The maximum sample sizes tested by us so far are:
For spectral transmittance testing: a glass of the size 1.0 m × 1.9 m
For spectral reflectance testing: a solar panel of the size 1.1 m × 2.3 m
Due to the extra effort required to handle such large samples, some oversize surcharge is applicable to samples larger than 300 mm × 300 mm.
It is well known that there are three heat transfer modes:
Emissivity is the key material surface property related to radiative heat transfer. In radiative heat transfer, a surface exchanges heat with the surroundings via radiation (electromagnetic wave):
The surface emits radiation to the surroundings (characterized by its emissivity)
The surface absorbs radiation emitted by the surroundings (characterized by its absorptivity)
The surface reflects radiation emitted by the surroundings (characterized by its reflectivity)
There are two relationships:
Emissivity = Absorptivity (Kirchhoff’s law of thermal radiation)
Absorptivity + Reflectivity = 1 (conservation of energy)
It is easy to calculate the absorptivity and reflectivity, when the emissivity is known.
Most natural surfaces are with high emissivity, around 0.9. Reflective metal surfaces are with low emissivity, around 0.05 or lower. Listed in the table below are the performances of high emissivity and low emissivity surfaces:
High emissivity surface
Low emissivity surface
Radiation emission to surroundings
Emits more radiation
Emits less radiation
Absorption/Reflection of radiation from surroundings
Absorbs more radiation Reflects less radiation
Absorbs less radiation Reflects more radiation
Overall radiative heat transfer with surroundings
Stonger radiative heat transfer
Weaker radiative heat transfer
In summary, emissivity is a material surface property characterizing its radiative heat transfer ability. A surface with high emissivity has stronger radiative heat transfer with the surroundings; a surface with low emissivity has weaker radiative heat transfer with the surroundings.
For insulation applications, surfaces with low emissivity are preferred, due to the weaker radiative heat transfer (and therefore better insulation).