There have been a number of attempts to quickly determine if the oven is performing to its specified power. However, the simplest and perhaps the most reliable involve heating a known volume of water for a specified time, and noting the temperature change following the heating process.

There two arrangements for this type of test; JIS and IEC705.

The JIS system was originally provided for ovens manufactured up to 1990, while the IEC705 arrangement was established for ovens manufactured after 1990.

The IEC705 arrangement is aimed more at a laboratory environment where mains voltage and ambient conditions are controlled.

JIS

Equipment required:

2 x 500ml plastic beakers
1 x Accurate thermometer
1 x Flat stirrer

1. Fill each beaker with 500ml of water at 20 C° +/- 5 C°
2. Check the precise temperature of each and if there is a difference find the average by adding the two values together and dividing the result by 2.
3. Place the beakers in the centre of the cooking area and set the oven to full power and switch on. Allow the oven to operate for precisely 60 seconds, allowing 2 seconds for the magnetron to reach operating temperature.
4. Remove the beakers from the oven and stir each beaker immediately before taking the temperature of each, repeating step 2.
5. Subtract the average starting temperature from the average final temperature and multiply the result by 70 to give the value representing the oven power in Watts (JIS)

Example:

Starting temperature: (19.3 + 19.9) /2 = 19.6 C°
Final temperature: (27.1 + 30.5) /2 = 28.8 C°
Difference: 28.8 – 19.6 = 9.2 C°
Power output: 9.2 x 70 = 644 Watts JIS

IEC705

1. Proceed as for JIS test, but with water at 10 C° +/- 2 C°
2. On reaching step 5, multiply the difference between the averaged values by 71.15 to give the power output in Watts IEC705

Example:

Starting temperature: (8.8 + 11.0) /2 = 9.9 C°
Final temperature: (19 + 20) /2 = 19.5 C°
Difference: 19.5 – 9.9 = 9.6 C°
Power output: 9.6 x 71.15 = 683.04 Watts IEC

a) Analogue
b) Digital
c) LED

a) This arrangement is usually moving coil meter with a linear or modified response, the quality of which is determined by the price of the instrument. Some technicians prefer this form of display as they feel it is easier to determine the average value of a varying signal. However, as with all displays of this type, they do not respond well to shock and there is always the possibility of parallax error when reading.

b) The display in this case is commonly LCD with the option of a back-light for greater contrast. The importance of achieving the correct signal averaging cannot be overstated. However, when this is accomplished the advantage of a direct unambiguous reading is not to be overlooked. With no moving parts and well positioned they are perhaps more resilient to shock than their analogue counterpart.

They also offer the option of including additional information within the display at little extra cost. e.g. low battery indication, overload condition etc.

c) There are a number of instruments available within the UK in which LEDs form the display arrangement. The LEDs are arranged to illuminate if the instrument detects a signal above a given level. Although simple in concept, difficulty arises when attempts are made to calibrate as unlike options (a) and (b) no truly accurate indication is available on the instrument to show how much above or below the nominal setting the signal is before the LED illuminates.

Options (a) and (b) offer the prospect of resolution at 0.1mW, although in the case of (a) the cost rises whereas (b) provides a cost effect arrangement. The resolution in option (c) is not relatively coarse. Owing to the nature of the display arrangement.

RF diodes are perhaps the most commonly used device in cost effective microwave power flux density instruments used to measure oven leakage

If the response is too slow a low level, varying signal may be missed. Conversely, if the response is too quick a rapidly varying signal may cause difficulty for the user as the display becomes difficult too read. This is particularly true in the case of digital displays and care must be taken in the design of the software that drives the display.