TIM Tester - Frequently Asked Questions
Question: What is the range of k (thermal conductivity) that can be measured?
Answer: The range of k for the TIM Tester is not specified directly. The associated specification is on the thermal resistance (standard heat transfer definition, K/Watt ) of the samples tested. Thermal resistance is computed as:
R = X / ( k * A) (where X=thickness, k=conductivity, A=sample area)
Samples should always be tested at the specified maximum diameter so the sample area, A, is fixed. The range of thicknesses that can be tested is 0 -1 inch (0-2.5 cm). The specified range of R is 0.01 to 10 K/W. So the upper and lower bounds of k measurements can be computed:
khi= Xhi / (Rlow * A) (upper limit of K)
klow= Xlow / (Rhi * A) (lower limit of K)
In summary, the range of k is limited by the range of available thicknesses of the material samples as well as the specified range of R for the tester.
Question: How can I measure the thermal conductivity a material?
Answer: The method recommended by ASTM D5470 for measuring thermal conductivity of the material while excluding the effects of surface contact resistance is to test a range of material thicknesses, plot the R*A (Y-axis) versus sample thickness (X-axis), and determine the best fit straight line for this data. The measured value of thermal conductivity, k, is the reciprocal of the slope of this line; the combined contact resistance (RA) for the upper and lower test surfaces will be the Y-intercept. A minimum of 3 different sample thicknesses is recommended with as large a span of thickness as is practical. Generally stacked materials cannot be used to achieve this variety of sample thickness since each additional layer in the stack adds additional contact resistance which will become included with the measurement of k resulting a poor accuracy result. The figure contained in Contract Material Test Services illustrates the recommended technique that can easily be performed with WinTIM software.
Question: How can I measure the thermal conductivity of very thin grease samples?
Answer: When testing very thin samples, the influence of the bulk material conductivity is minimized whereas the influence of the surface contact resistance and specifics of the contact surfaces is maximized. In the extreme case where the sample thickness approaches zero, the contact resistance totally dominates the resulting test data. As such, microscopic surface characteristics can control the end result without being explicitly known thus giving the data an unpredictable scatter. An application note for bond-line testing is available upon request.
Question: How long do tests take?
Answer: Test durations range from 10 minutes to 15 minutes under Standard Accuracy option selection.. This duration does include waiting for the coolant reservoir temperature to change when chiller setpoint adjustment is needed. Phase Change Material tests (PCM) also require an additional time for pre-test melting of the sample.
Question: Can I use a chiller that I already in my lab?
Answer: The chiller requirements detailed on the Chiller Requirements page.
Question: What pressure should I use for testing my material sample?
Answer: The answer to this question depends the type of material to be tested. Generally, higher contact pressures yield lower surface contact resistances. In the latest version of ASTM D5470, there are 3 categories of materials:
Type I: Liquid materials that exhibit unlimited deformation when a stress is applied. These include greases, pastes, and phase change materials. These materials exhibit no evidence of elastic behavior or the tendency to return to initial shape after deflection stresses are removed. [Materials of this type are tested at specified thickness without regard to the pressure applied. Here the sample thickness is fixed either by using the TIM Tester in the Specified Thickness Mode or by using various type of spacers or shims to regulate the thickness of the sample. Surface contact resistance for Type I materials are close to zero.]
Type II: Elastic solids where stresses of deformation are ultimately balanced by internal material stresses thus limiting further deformation. Examples include gels, soft and hard rubbers. These materials exhibit linear elastic properties with significant compression deflection relative to material thickness. [Materials of this type can be tested on the TIM Tester in either Controlled Pressure or Controlled Thickness modes depending on the elasticity of the material. Generally, Controlled Pressure is the method of choice although some types soft material exhibit progressively decreasing surface contact resistance and/or thickness over a long period of time. This makes repeatable test results difficult to achieve in a short period of time since the test result depends on the duration of pressure application. For these types of materials, Controlled Thickness Mode testing should be performed. ASTM D5470 recommends the use of 5% deflection from the uncompressed starting thickness NOTE: the 5% deflection method can require higher pressures for thinner samples which causes systemic data errors due the pressures / contact resistance dependency.]
Type III: Inelastic solids which exhibit negligible deflection; examples include ceramics, metals, and some types of plastics. Although these materials are elastic in nature, deflection is insignificant relative to the material thickness.
[Materials of this type require accurate sample preparation to ensure that the hot and cold test surfaces are flat and parallel to within 10 microns (0.0004") which will minimize the surface contact resistance. In addition, the test surfaces are treated with oil or light thermal grease to exclude air from the contact surfaces and further reduce the contact resistance. Controlled Pressure Mode is used with a minimum of contact pressure of 700 kPa (100 psi).]
Question: Can the TIM Tester 1300 test phase change materials (PCM's)?
Answer: WinTIM offers the capabilities to test PCMs at pre-test bake temperatures of up to 80°C.
Question: What is the range of sample area-sizes that can be tested?
Answer: Samples should always be provided in 33 mm (1.3") diameter size for the best accuracy. Samples cannot be larger than 33 mm although samples down to 25 mm diameter can be tested with some loss of accuracy. A sample die-cutter is provided with the TIM Tester 1300 to for cutting 33 mm diameter samples in type II materials. If extensive testing at smaller diameters is absolutely required, area adapters are available for this purpose although some loss of data accuracy is incurred.