Our instrument: hot wire thermal conductivity meter

Related services Thermal conductivity

We are a Singapore-based third-party test laboratory, providing lab test services of material optical & thermal properties. We use Xiatech TC3000E transient hotwire thermal conductivity meter, with the following key information.

Transient hotwire thermal conductivity meter
  • Measurement principle: transient hot wire
  • Measurement range: 0.001 – 10 W/(m K)
  • Temperature range: room temperature only

The instrument is equipped with two probes, a thin film probe, and a needle probe.

Thin film probe

The thin film probe is mainly for flat plate solid samples:

  • 2 samples in a pair are needed, with the thin film probe sandwiched between the 2 samples
  • Minimum sample size: larger than 25 mm × 25 mm in length and width, and thicker than 0.3 mm in thickness
Needle probe

The thin film probe is mainly for bulk gel or soft soil samples:

  • Needle size: 1.6 mm in diameter and 12 cm in length
  • The needle is to be inserted in the sample for measurement

Related lab services

Handheld data logger for UVC irradiance measurement

Related instruments Light & radiation

UVC disinfection is used in HVAC systems to eliminate germs and viruses, particularly in the current COVID-19 situation. UVC irradiance measurement is required for UVC radiation level control.

Shown below is a handheld data logger for UVC irradiance measurement, with the following components:

  • HD2102.2: handheld data logger for light and radiation measurement
  • LP471UVC: probe for UVC irradiance measurement

At the customer’s request, an ISO 17025 calibration certificate can be provided at an additional cost.

Thermal conductivity (K-value), thermal resistance (R-value), and thermal transmittance (U-value)

Related services Thermal conductivity

Thermal conductivity, thermal resistance, and thermal transmittance are the 3 commonly used properties related to material or system thermal insulation performance. The 3 properties and their differences are explained below.

Thermal conductivity (K-value)

Thermal conductivity represents the ability of a material to conduct heat.

Thermal conductivity is also called K-value and its unit is W/(m⋅K). The smaller the thermal conductivity, the better the thermal insulation performance.

Listed in the table below are the typical thermal conductivity ranges of selected building material types.

Building material typeTypical thermal conductivity range
Insulation material (e.g. polyurethane/polystyrene foam, mineral wool)0.02 – 0.04 W/(m⋅K)
Wood, plywood, and gypsum board0.1 – 0.5 W/(m⋅K)
Coating (e.g. paint), plastics, and rubber0.1 – 0.5 W/(m⋅K)
Glass1 W/(m⋅K)
Concrete (light weight or heavy weight), brick, and tile0.5 – 2.5 W/(m⋅K)
Metal (e.g. stainless steel or aluminum)15 – 200 W/(m⋅K)

Thermal conductivity is typically for homogeneous materials. For inhomogeneous materials (e.g. concretes or composite panels), their average thermal conductivity is referred to as the apparent thermal conductivity.

Thermal resistance (R-value)

Thermal resistance represents the ability of a material layer to resist heat transmission. Thermal resistance is calculated as:

Thermal resistance is also called R-value and its unit is (m2K)/W. The greater the thermal resistance, the better the thermal insulation performance.

Thermal resistance is always in terms of one or multiple material layers with fixed thicknesses:

  • Material layer: thermal resistance is applicable to layer-by-layer structures only.
    • Example 1: for a wall system with 3 layers: concrete + rock wool insulation + plaster, there is a thermal resistance for each material layer (or multiple layers combined together).
    • Example 2: for studs, frames and fasteners, and other materials without layered structure, there is no thermal resistance for such materials
  • Fixed thickness: thermal resistance is dependent on thickness.

Thermal transmittance (U-value)

Thermal transmittance represents the ability of a wall, roof or fenestration system to transmit heat. Thermal transmittance is calculated as:

Thermal transmittance is also called U-value and its unit is W/(m2K). The smaller the thermal transmittance, the better the thermal insulation performance.

Thermal transmittance is always in terms of a complete wall/roof/fenestration system and air-to-air thermal transmission.

  • Complete wall/roof/fenestration system: thermal transmittance is applicable to a complete wall/roof/fenestration system only.
    • There is no thermal transmittance of an individual material layer unless the wall/roof/fenestration system is formed by 1 material layer only
    • Example 1: for a wall system with 3 layers: concrete + rock wool insulation + plaster, there is a thermal transmittance of the complete system only, but no thermal transmittance of each material layer.
    • Example 2: for a double glazing unit (DGU) system with 3 layers: glass + air gap + glass, there is a thermal transmittance of the complete DGU system only, but no thermal transmittance of each layer.
  • Air-to-air thermal transmission: thermal transmittance includes thermal transmission through both indoor and outdoor air layers.
    • The indoor and outdoor air layers adjacent to a wall/roof/fenestration system introduce some additional thermal resistance too (called surface film resistance). The additional thermal resistances by air layers are always included in thermal transmittance calculations.
    • Thermal transmittance is therefore always dependent on the environmental conditions (such as wall/roof/fenestration orientation, indoor/outdoor airflow speed, and indoor/outdoor surface emissivity).
    • Example: a wall system and a roof system made of the same materials may be with different thermal transmittances, due to the different airflow speeds along a vertical surface (for a wall) and a horizontal surface (for a roof).

OTM provides two online thermal transmittance (U-value) calculators:

Thermal conductivity (K-value) vs. thermal resistance (R-value)

  • Thermal conductivity is independent of thickness; thermal resistance is dependent on thickness.
  • Thermal conductivity is for homogeneous materials (or averaged for inhomogeneous materials); thermal resistance is for material layers.

Thermal resistance (R-value) vs. thermal transmittance (U-value)

  • Thermal resistance can be in terms of an individual material layer in a wall/roof system; thermal transmittance is always in terms of a complete wall/roof/fenestration system and there is no thermal transmittance of an individual material layer (unless the wall/roof system is made of a single material layer only).
  • Thermal resistance excludes the effect of indoor and outdoor air layers; thermal transmittance includes the effects of indoor and outdoor air layers (air-to-air thermal transmission).
  • Thermal resistance is independent of environmental conditions; thermal transmittance is dependent on environmental conditions.


The discussions above follow typical engineering practices. In the academic context, the practices could be different.

If we use double glazing unit (DGU) glasses as an example, the engineering practice is to use its thermal transmittance (U-value), instead of the other two, for performance evaluations, though in the academic context it is still correct to calculate the apparent thermal conductivity and thermal resistance of a DGU glass.

Sample retention period at OTM

After testing, a test sample is retained by the lab for a certain period. The following sample retention periods are implemented at OTM:

Sample typeSize rangeRetention period
Small samplesSmaller than 10 cm × 10 cm × 3 cm3 years
Large samplesLarger than 10 cm × 10 cm × 3 cm
Smaller than 30 cm × 30 cm × 5 cm
1 year
Oversize samplesLarger than 30 cm × 30 cm × 5 cm3 months

At the end of the retention period, the sample will be disposed of by the lab. The lab may further retain some samples for internal quality control purposes.

Upon the customer’s request (indicated in the test request form or in writing), a test sample can be returned to the customer.

Wind speed & direction measurement with Delta Ohm HD53LS ultrasonic anemometer

Related instruments Weather stations, Wireless & web data loggers, Air speed & wind

Shown below is a wind speed and wind direction measurement system, with the following components:

  • HD53LS.S 2-axis ultrasonic anemometer with MODBUS-RTU output
  • HD35EDW-MB wireless data logger with MODBUS-RTU input

For this system, OTM also supplied a 2-meter-long stainless steel pole for mounting and wired/configured all instruments before delivery. The last step mechanical mounting was done by the end user. A USB dongle type base unit (HD35APD) was also included for manual downloading of monitoring data in a scheduled interval.

EN 12898:2019 glass emissivity measurement

Related services Emissivity / emittance, Glass optical & thermal properties

We are able to measure glass emissivity according to EN 12898:2019, with the PerkinElmer Spectrum Two FTIR spectrometer and PIKE 10Spec 10-degree specular reflection accessory.

The total normal emissivity (εn) result is reported. The corrected emissivity (ε) is calculated and presented in the appendix. For glass U-value calculation, the corrected emissivity should be used.

Glass UV transmittance calculation

Related services Glass optical & thermal properties

As defined in ISO 9050 or EN 410, the UV transmittance of glass is calculated with the equation below:

In the equation above:

  • τUV: UV transmittance
  • λ: wavelength
  • τλ: spectral transmittance
  • SλΔλ: normalized relative spectral distribution of the UV radiation (part of the standard global solar radiation)

The wavelength range of interest is 300 nm – 380 nm. The term SλΔλ is the weights used in the weighted average of the spectral transmittance. The standard values of SλΔλ are plotted in the figure below.

The peak of the UV radiation distribution is at 375 nm. Glasses with high spectral transmittance near 375 nm are with high UV transmittance.

There are some small differences between the ISO 9050 and EN 410 UV distributions. The UV transmittances calculated according to the two standards could be slightly different.