EMC Technologies was approached by a Public School in NSW to perform SAR testing on suspected “leaking” Microwave Ovens. The Test samples were two well-known brands that had been diagnosed with “leaky” door seals by EMR meter measurements. It was decided that the only way to determine if over-exposures had occurred was to measure SAR levels directly because the Reference Levels are a more a conservative estimate of the Basic Restrictions set by the ARPANSA RPS3 standard.

SAR Measurement Method

The ovens were set to 100% Power and tested with the load consisting of a specially designed container connected to an external water pump for cooling. This was necessary because SAR test takes around 30 minutes by which time the water would be rapidly boiling. Before the SAR testing commenced the amount of leakage of Microwave Oven #1 was measured with a conventional field strength meter and confirmed to be right on the Reference Level limit at 50W/m^2.

SAR Results

Surprisingly, for Microwave Oven #1 even at 0mm spacing, the measured SAR was found to be below the ARPANSA occupational limit of 10mW/g. With the spacing of 10mm, the SAR complied with general public ARPANSA exposure limit of 2 mW/g.  It has to be stressed however that RF power levels inside the microwave ovens are very high and that a microwave oven should not be used the same way as in this test.  (Don’t try this at home!!).

This example demonstrates that SAR is the most accurate measurement of RF exposure to an RF source and that it is possible to ensure the safety of the user even when a fault develops, from normal wear and tear.  

Summary of Microwave Oven #1 2450 MHz at 100% Power with 50 W/m^2 Leakage

Test Number Spacer Height (mm) *1 SAR 1g (mW/g) *2 SAR 10g (mW/g)
1 Horizontal 50 0.388 0.256
2 Horizontal 0 9.72 5.71
3 Horizontal 0 14.5 7.95
4 Horizontal 10 2.74 1.46
5 Horizontal 26 0.847 0.502

*1.  In the USA, SAR is measured in a 1 g cube of tissue and the safety limit is 1.6 mW/g measured in a 1 g cube of tissue.

*2. In Australia and Europe, SAR is measured in a 10 g cube of tissue and the safety  limit is 2.0 mW  measured in a 10 g cube of tissue.

Summary of Microwave Oven  #2 2450 MHz at 100% Power

Test Number Spacer Height (mm) *1  SAR 1g (mW/g) *2 SAR 10g (mW/g)
1 Horizontal 0 5.01 2.62
2 Vertical 0 31.2 15.9
3 45 Degree 0 7.4 3.55

1. In the USA, SAR is measured in a 1 g cube of tissue and the safety limit is 1.6 mW/g measured in a 1 g cube of tissue.

*2. In Australia and Europe, SAR is measured in a 10 g cube of tissue and the safety limit is 2.0 mW measured in a 10 g cube of tissue.

The SAR plots below represent Radio Frequency field distribution in selected places around the microwave oven. Firstly a 2-dimensional measurement is performed and after finding the spot of highest microwave energy leakage, 3-dimensional measurements are done. The measured SAR is the amount of energy a person would receive at that distance. The plot is superimposed on the 1:1 scale 3D model of the microwave oven tested.

Conclusions

  1. It is evident that the Reference Levels limits in the ARPANSA are conservative. The (simple) Field meter measurement method is very useful to detect problems with microwave ovens.
  2. The USA SAR limit is more conservative than the AU/EU limit, but both limits are met at a 50 mm separation.
Top of oven above door seal 0mm Spacing
Top of the oven above door seal 0mm Spacing
Corner of the oven above door seal 0mm Spacing
Side of the oven above door seal 0mm Spacing

We are experts in SAR testing! With dedicated facilities ready to be utilised today! SAR testing is a product compliance requirement for many transmitters used in close proximity to the head and body (torso). Contact our friendly team to find out more about mandated human exposure to RF and transmitter product testing and compliance. We can cover Maximum Permissible Exposure (MPE) testing (North America, FCC certification), Human exposure to electromagnetic fields testing (Europe, EMF, RED, LVD compliance) and ARPANSA SAR testing (Australia and New Zealand, Electromagnetic Radiation compliance) and ISED Canada SAR Procedures (Canadian certification).

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