REPORT: DOE MICROWAVE OVEN ACTIVE MODE TEST PROCEDURE INVESTIGATIONS September 2010

Summary

During its most recent energy conservation standards rulemaking for microwave ovens, the U.S. Department of Energy (DOE) evaluated test procedure methodology to measure microwave energy use in cooking ( i.e. , active) mode. Specifically, DOE conducted measurements of output power and cooking efficiency according to the International El ectrotechnical Commission (IEC) test procedure incorporated by reference until recently in the DO E microwave oven test procedure, as well as the most recent IEC test procedure addressing microwave oven active mode. Test data were used to investigate the suitability of these test procedures for measuring microwave oven active mode energy use in an accurate, representative, and repeatable manner. DOE identified key test conditions, instrumentation requirements, methods, and calculations impacting the cooking efficiency metric, and outlined additional specificity to certain provisions that improved the accuracy and repeatability of the test

m easurements. Based in part on the information developed during this investigation, DOE

r epealed the active mode provisions in its microwave oven test procedure and is currently considering a rulemaking to address new active mode methodology.

Historical Background: DOE Testing According to IEC Standard 705

Prior to the test procedure final rule published in the Federal Register on July 22, 2010 which repealed the active mode test procedure for microwave ovens (75 FR 42579), DOE ’s

m icrowave oven test procedure incorporated by reference testing provisions from IEC Standard 705-1988 and Amendment 2-1993, “Methods for measuring the performance of microwave

o vens for household and similar purposes ” (hereafter “IEC Standard 705”). As discussed in detail below, the test procedure consists of h eat ing 1 kilogram (kg) of water in a borosilicate container from about 10 degrees Celsius ( C) below room temperature to room temperature, using the maximum power setting on the microwave oven. The input power over the duration of
t he test, and thus energy consumed during the test, are compared to the energy absorbed by the
t est load to obtain the efficiency measurement.

As part of the energy conservation standards for residential dishwashers, dehumidifiers, and cooking products (including microwave ovens), and commercial clothes washers (hereafter “Home Appliance Group 1”), the Association of Home Appliance Manufacturers (AHAM) provided cooking efficiency data on 21 microwave ovens tested according to DOE’s test procedure (using IEC Standard 705). In addition, as part of the Home Appliance Group 1
st andards rulemaking, DOE conducted testing on a sample of 32 microwave ovens according to the DOE test procedure. DOE noted that IEC Standard 705 does not precisely specify the
t emperature measurement device, how the temperature measurement device is to be used, or how
to ensure test- to -test consistency. In order to increase this consistency while remaining within the

c onditions specified in the test procedure, DOE included additional provisions for its testing :- Using a slotted, low-mass plastic cup with four thermocouples attached at even intervals

around the circumference of the cup, approximately 3⁄4 inch from the rim. By inverting the cup in the water and swirling the water with it, a multi-point temperature distribution could be measured at a known and repeatable distance from the bottom of the borosilicate test vessel, avoiding direct contact with the thermocouples. The slots in the cup ensured
t hat the water could be well stirred prior to the temperature measurement in order to minimize temperature gradients that could provide misleading results.

- Insulating the glass test vessel from the test bench surface with a small piece of foam to ensure that the bench would not influence the test vessel temperature when it was outside of the microwave oven.

On December 13, 2007, DOE held a public meeting for the Home Appliance Group 1 energy conservation standards Advanced Noticed of Proposed Rulemaking (ANOPR) during whic h DOE presented the measured range in cooking efficiency for individual models tested for the data submitted by AHAM (which followed IEC Standard 705 with no modifications) as compared to the measured range in efficiency for units tested by DOE following IEC Standard 705 with the additional modifications within the conditions of the test procedure (as presented

a bove). The results, shown below in Figure 1 and Figure 2 , show that the test- to -test repeatability could be improved with additional specifications that are within the conditions specified by the test procedure.

Figure 1 AHAM Cooking Efficiency Test Results Using IEC Standard 705 with No M odificationsFigure 2 DOE Cooking Efficiency Test Results Using IEC Standard 705 with Additional S pecifications

At the December 2007 ANOPR public meeting presentation, DOE also presented modifications to IEC Standard 705 that could be considered for improving repeatability, which included: 1) increasing the water sample weight to 2 kg for better test vessel coating and longe r

run time, and 2) requiring the test vessel to be insulated from surfaces and hands outside of the microwave oven.

DOE notes that as microwave ovens on the market become more powerful, the alreadyshort cooking times associated with the test procedure will shrink even further because the mass of the water sample (1 kg) becomes smaller in relation to the output of the magnetron. Past research has shown a relationship between tested efficiency and the loading factor. 1 The greater the water mass inside the microwave oven, the better the load’s ability to absorb all the microwave radiation put out by the magnetron. DOE notes that doubling the water sample weight to 2 kg could potentially improve load matching at higher magnetron outputs, and that such an increase in water mass would not necessitate a new test vessel and would roughly double cycle times. This could be beneficial for testing certain microwave ovens whose user interfaces only allow 10 second cooking time increments. Such long increments may make the test sample under- or over-shoot the allowable final water test load temperature on units with high-powered magnetrons. For example, a typical 1,200 watt (W)-rated microwave oven needs only about 42

se conds to heat the present 1 kg sample by the requisite 10 oC.

1 Houšová, J. and Hoke, K

. 2002

. “Microwave Heating – the Influence of Oven and Load Parameters on the Power Absorbed in the Heated Load

. Czech Journal of Food Science , Vol. 20, No. 3, pp. 117 – 124, 2002.In addition, DOE notes that the volume of a 1 kg water load is only sufficient to fill the

test vessel about half full, resulting in about 64 percent of the test vessel surface being in contact with the water sample. By doubling the water sample weight, 94 percent of the test vessel surface

a rea is in direct contact with the water sample during the test. This may reduce calculation error

a nd improve test- to -test consistency. DOE understands that the IEC Standard 705 cooking

e fficiency calculation assumes that the entire test vessel changes uniformly in temperature. Ho wever, based on testing, DOE believes that this may not be the case. Because conductive heat transfer through thin cross-sections of borosilicate glass is relatively slow, the non-wetted portion of the test vessel may have a significant temperature gradient across it. Therefore, DOE believes that the IEC assumption of a consistent vessel temperature may not be valid. By doubling the water sample weight, the test vessel is filled to the point where this temperature gradient effect is minimized, since the sections of the test vessel that are not in direct contact with water would then represent only about 0.2 percent of the heat load instead of 2.2 percent at present. DOE believes that this change could improve test accuracy.

DO E also believes that using thermal insulators whenever the test vessel is handled or measured outside of the microwave oven could improve test accuracy and repeatability. Both the

we ighing scale and the test bench could be thermally insulated from the vessel to limit heat gain via conduction through the base of the test vessel

. In addition, the test vessel should be handled with insulated gloves to limit heat transfer via the hands.

DOE does not have data quantifying the cumulative effect of these potential revisions to th e test procedure.

Comparison of IEC Test Procedures for Measuring Efficiency

As part of the Home Appliance Group 1 energy conservation standards rulemaking, DOE conducted an analysis to compare the cooking efficiency testing procedures in IEC Standard 70 5 and IEC Standard 60705-2006 (hereafter “IEC Standard 60705”). The following analysis was
pr esented in chapter 5 of the Home Appliance Group 1 energy conservation standards’ Technical Support Document (TSD) of the Notice of Proposed Rulemaking (NOPR).

As discussed above, prior to the July 2010 final rule which repealed the active mode test provisions (75 FR 42579), the DOE microwave oven test procedure referenced IEC Standard 705 . However, this IEC test procedure has been declared obsolete by the IEC, and the current IE C test procedure is IEC Standard 60705. Because there are key differences between these two IE C procedures, the impact of which on efficiency measurements were not known to DOE, and because manufacturers which do conduct efficiency testing are likely to be using the current version, a test series was conducted on a sub-sample of the microwave ovens to compare efficiency measurements made using both IEC test procedures as part of the Home Appliance G roup 1 energy conservation standards rulemaking.

The general methodology for each test procedure is largely the same, and consists of h eat ing 1 kg of water in a borosilicate container from about 10 C below room temperature to room temperature, using the maximum power setting on the microwave oven. The input powerover the duration of the test, and thus energy consumed during the test, are compared to the

energy absorbed by the test load to obtain the efficiency measurement.

DOE conducted tests per IEC Standard 60705 on 12 selected microwave ovens from its original 32 unit sample with a representative range of output power, efficiency, and test- to -test variation observed in the DOE testing, which utilizes IEC Standard 705. The table below

su mmarizes key differences noted between the test procedures that can potentially impact the final energy efficiency calculation.

Table 1: Key Differences Between IEC Standard 705 and IEC Standard 60705
IEC Standard 705

- 1988 and Amendment

2

- 1993 IEC Standard 60705

- 2006

Ambi e nt Temp.,T 0 = 20 ± 2 C Starting Water Temp., T 1 = T 0 – (10 ± 1 C) Final Water Temp., T 2 = T 0 ± 1 C Ambi e nt Temp. ,T 0 = 20 ± 5 C Starting Water Temp., T 1 = 10 ± 1 C Final Water Temp., T 2 = 20 ± 2 C Electrical Input Energy neglects the magnetron filament heat

- up time, the
measurement start ing when the input current reaches 90 percent of its final value. Measurement of Electrical Input Energy includes the energy consumed during the magnetron filament h eat

- up time.

No mention of rounding off efficiency or output power calculations Efficiency is rounded off to the nearest whole number , while output power is rounded off to the nearest 50 W Temperature measurement accurate within 0.25 C and linearity better than 1 percent

. Time measurement accurate within 0.25 seconds. No specifications for accuracy of temperature and time measurements

.

It can be noted that, for the efficiency measurement, IEC Standard 60705 includes the energy consumed during the magnetron filament heat-up time, while IEC Standard 705 excludes filament heat-up time. Thus, the IEC Standard 60705 efficiency measurement can be more

a ccurately described as an overall microwave oven efficiency measurement, while the IEC Standard 705 efficiency measurement can be considered strictly a cooking efficiency
measurement. Table 2 shows the results for the test procedure comparison. Per the IEC Standard 60705 test procedure, DOE rounded the measured output of each test to the closest 50 W and then averaged the rounded values.Table 2: IEC Standard 705 versus IEC Standard 60705 Test Results

Th ere is a largely consistent decrease in efficiency of about 1 to 2 percentage points from IE C Standard 705 to IEC Standard 60705 for all of the microwave ovens. This decrease for IEC S tandard 60705 was due in significant part to the requirement that the measured input energy include the energy consumed during the magnetron filament heat-up time, whereas IEC Standard 705 specifies that the measurement of energy input begins when the current reaches 90 percent of its maximum value. Therefore, IEC Standard 60705 factors in a higher energy consumption to
t he efficiency calculation than does IEC Standard 705.

Another significant difference between test procedures is the specifications for accuracy of the test instrumentation. IEC Standard 705 specifies that temperature measurements be accurate within 0.25 °C, with linearity better than 1 percent, as well as that time measurements be accurate within 0.25 seconds. In contrast, IEC Standard 60705 does not provide any accuracy

r equirements for these measurements. Using a standard thermocouple, which typically has an

a ccuracy no better than 0.5 °C, or less accurate time measurement equipment could create significant test- to -test variation in efficiency results.

The variation of test- to -test efficiency results per IEC Standard 60705 for an individual microwave oven ranged from 0 to 5 percent of the average value, which was much greater than the comparable variation for IEC Standard 705, whose test- to -test variation in efficiency results ranged from 0 to 1.5 percent for the same sub-sample of microwave ovens. This larger range associated with IEC Standard 60705 is believed to be attributable to the effects of the procedure’s rounding requirement for the power output to the nearest 50 W and the efficiency to
t he nearest whole number after each individual test, prior to averaging.

The prescribed rounding under IEC Standard 60705 would make it more difficult to achieve an overall efficiency increase via improvements in minor components such as fan/turntable motors and oven light bulbs, which each consume less than 20 W. For any component efficiency improvements to have a measurable effect after rounding the power output and efficiency calculations, power consumption would have to be reduced by at least 25 W.To demonstrate the effect of rounding more clearly, the raw data from the IEC Standard

60705 testing were used to calculate efficiency in the same manner as for IEC Standard 705; i.e

., no rounding was performed on output power or efficiency from the individual tests prior to averaging. As shown in F igure: , the test- to

- test variation in efficiency for the rounded data is on average four times higher than that of the non-rounded data. The effect of rounding output power

a nd efficiency also causes a significant change in the calculated efficiency, as seen in Figure

. The rounded efficiencies varied from the non-rounded values by as much as 0.75 percentage points.

Figure: 3 DOE Non-Rounded versus Rounded Test- to -Test Efficiency Variation Measured Using IEC Standard 60705Figure 4: DOE Non-Rounded versus Rounded Efficiency Measured Using IEC Standard 60705

The non-rounded data obtained via IEC Standard 60705 still show a consistent 1 to 2
p ercentage point decrease in efficiency for all of the units tested as compared to the IEC Standard 705 tests, which DOE believes to be attributable to the differences in the energy input measurement. As with the rounded IEC Standard 60705 results, the non-rounded data for IEC S tandard 60705 also show more test- to -test variation for a given unit (0 to 2.1 percentage points) than the variations test- to -test during the IEC Standard 705 testing (0 to 1.5 percentage points). This remaining increment in test- to -test variation was likely due to the more lenient tolerances on the prescribed ambient and final test load temperatures. The IEC Standard 60705 test procedure
sp ecifies ambient temperature as 20 ± 5 °C, as compared to 20 ± 2 °C for the IEC Standard 705
t est procedure. For the final test load temperature, IEC Standard 60705 specifies 20 ± 2 °C, while IE C Standard 705 specifies it as ambient temperature ± 1 °C.Table 3: IEC Standard 705 versus IEC Standard 60705-2006 Test Results, Without Rounding

Based on observations and analysis of test results, DOE stated in the Home Appliance G roup 1 energy conservation standards NOPR TSD that it believes that IEC Standard 705 is

m ore likely to produce consistent test results than IEC Standard 60705 because the measurement requirements in IEC Standard 705 are more stringent.