It is important to know the difference between the different types of Electromagnetic Radiation measurements. Every type of measurment will have a different focus, different methods and in some cases you should use different meters. The right meter should be chosen according to its charataristics and not only according to the frequency that it can measure.
Electromagnetic radiation sources can be divided into 2 frequency types, low frequency (power line, power transformers) and high frequency (RF, DECT, Mobile phone, microwave). Some meters can measure both types, some can measure only one.
When the radiation level is changing rapidly (like in some digital radiation sources like 3G and 2G mobile phones, WiFi routers and etc.) it is very important to use electromagnetic radiation meters with a high sampling rate. If the sampling rate is slow, for example once a second like in some simple meters, the measurement outcome of a WiFi router will be very low, when in reality it picks to high levels several times in a second. I usually use electromagnetic radiation meters with the highest possible sampling rate.
In some cases it is the custom to make an average out of the levels of radiation that were measured. It is recommended (but not a must) to make a note of the minimum and maximum values that were measured. When average outcomes are reported it will usually mean that the picture revealed by the report will be very partial and limited since the peaks and high values that happened in some of the time were averaged by low values that were measured most of the time. In most cases I don't average my measurements. I try to get a clear and full picture and try to understand what causes the high peak levels.
When the Radiation levels are changing throughout the day there is a need to measure them during all the hours of the day, sometimes for several days. In most cases when this is the need you have to use electromagnetic radiation meters that has the option to monitor the radiation levels throught the day and night, and to store all the results in an internal memory, which can later be reviewed, examined and processed . The most recommended meters for this operation are meters that can be connected to a PC for data acquisition, extraction, realtime and offline analysis and display of all the information gathered during the measurement.