The TriField Meter 100XE
Manufactured in the USA by AlphaLab, Inc.
The original TriField ® Meter combines all the features needed for fast, accurate measurements of electromagnetic fields. It independently measures electric field and magnetic field, and is properly scaled to indicate the full magnitude of currents produced by each type of field inside a conductive body. As a result, it "sees" much more than any other electromagnetic pollution meter. [en français]
Depending on where the knob is set, the meter detects either frequency- weighted magnetic fields (two separate scales), or frequency-weighted electric fields in the ELF and VLF range. It has significant sensitivity at 100,000 Hz, well past the 17,000 Hz horizontal scan of video displays. The radio/microwave setting can detect up to three billion Hz (3 GHz), which lets you gauge radio- wave power, CB and cellular phone equipment, and many types of radars.
This meter is the only one which combines magnetic, electric, and radio/microwave detectors in one package, so that the entire nonionizing electromagnetic spectrum is covered. In addition, the magnetic setting and the electric setting measure true magnitude, a feature found elsewhere only in more expensive meters. If you hold the meter in the center of a room and tip it to various angles, the magnetic reading will stay approximately the same regardless of which way you tip or rotate it. The electric reading is similar, although the presence of your body alters the actual electric field, so readings will vary more. The radio/microwave setting reads the full power of radiowaves when the meter is pointed toward the source.
Surprisingly, the standard TriField Meter is one of the least expensive meters available. The few other meters below $200 read only low-frequency magnetic fields and only in one direction (not true magnitude). The magnetic section of the TriField meter has three field-detecting coils pointing in the X, Y, and Z directions. A circuit amplifies these signals and gives them the proper frequency-weighting (sensitivity increases linearly from 30 Hz to 500 Hz, but with some residual sensitivity up to 100 kHz). A unique network combines the three coil outputs nonlinearly to approximate a true magnitude. The meter is sensitive from 0.2 to 100 milligauss full scale at 60 Hz (or 0.1 to 50 milligauss full scale at 120 Hz, etc.) with a resolution of 0.2 milligauss in the sensitive range. Accuracy is +/-20% at mid-range. A version with flat frequency response (instead of linear sensitivity increase) is also available for the same price.
The TriField Meter comes with a 9-volt transistor battery included. This type of battery lasts about 10 hours (total measurement time). When the BATTERY TEST reads low, the battery can be replaced with any rectangular 9 volt transistor or alkaline (which lasts about 50 hours) type. This meter is manufactured in the USA. TriField is a registered trademark of W.B. Lee.
Using The Trifield Meter
A knob on the front has six positions: OFF, BATTERY TEST, two MAGNETIC field sensitivities (0.5-100 milligauss at 60 Hz, and 0.2-3 milligauss at 60 Hz, the second sensitivity to measure weak fields more accurately), ELECTRIC field and RADIO/MICROWAVE power density. The meter face is analog (needle type). A needle reading of one-third of full scale corresponds to either 3 milligauss (or 0.6 milligauss) @ 60 Hz, 3 kilovolts/meter @ 60 Hz, or 0.04 mW/cm2 respectively in the magnetic, electric, and radio/microwave field settings. Average magnetic field strength in North American homes and offices is 1-3 milligauss, so the meter is labeled "HIGH" above these levels.
Magnetic Field Detection
The magnetic section consists of three ferrite-core coils pointing in the X, Y, and Z directions and located in the geometric center of the meter. Non-linear circuitry combines the signals of these three into a true magnitude of the field strength, independent of which direction the meter is pointed.
In most homes and offices, a large fraction of the total magnetic field is at frequencies above 60 Hz. A TriField meter, when exposed to a 3 milligauss field, will read "3" if the frequency of the field is 60 Hz, but it will read "6" if the 3 milligauss field is at 120 Hz. In contrast, a non-frequency-weighted (the flat response version of the TriField) meter will read "3" in both cases, and a 60 Hz-only meter will read "3" and "0" respectively (even though in the 120 Hz case, the current induced in a conductive body is twice as much.)
In most homes or offices, some areas are "hot" spots with readings in the HIGH range. Most often, this is caused by magnetic fields, which come largely from unpaired internal wiring. (Contrary to popular belief, power transmission lines and transformers do not generally contribute as much magnetic field as does internal wiring.) Other magnetic sources include video displays, motorized clocks and other equipment, electric blankets and heaters, fluorescent lights and light dimmers, and the transformers that are inside consumer devices. Much of the total field strength is from frequencies that are harmonics or multiples of 60 Hz (120 Hz, 180 Hz, etc.) and 17,000 Hz of video displays. Cars (especially near the front floorboard of electronic ignition cars) and motorcycles have fairly strong fields that are at frequencies higher than 60 Hz. Magnetic field is difficult to shield, but sheet steel is somewhat effective.
Electric Field Detection
The electric section consists of three metal plates under the meter face. Because the meter housing is plastic, the electric fields can penetrate through to the plates, which are also arranged to detect AC electric fields in the X, Y, and Z directions. Circuitry similar to the magnetic section converts the signals into an electric field signal which is frequency-weighted. Sensitivity is 0.5-100 kilovolts per meter (KV/m) at 60 Hz, with resolution of 0.5 KV/m (1 KV/m =1000 V/m). Accuracy at mid-range is +/-30%.
A few areas in most homes read HIGH in the electric field setting. These include areas near improperly grounded equipment, the front of video screens, and fluorescent lights. Most of these fields can be easily shielded using a grounded metal screen or foil; VDT screens of this type are readily available. You can greatly reduce the strength of an electric field just by placing your hand in front of the source. This effect can be seen using a TriField meter.
Occasionally, certain areas read HIGH in the radio/microwave setting. These include door seals around microwave ovens, and cellular phones (but not regular radio phones, which are very low-power). Radio/microwaves can be shielded in the same way as electric fields, although the lower frequency radio waves are not shielded by your hand as easily as microwaves are. (Metal screens will shield both.) In the United States, radars and FM transmitters can legally expose residents to moderately high power levels (as high as 10 mW/cm2 briefly or .57 mW/cm2 near cellular towers), but such exposure is not common.
Radio And Microwave Detection
Radio and microwaves are composed of a particular combination of electric fields and magnetic fields that is self-sustaining. For frequencies below about 100 MHz (100 million Hz) the principle effect on a conducting body is from the magnetic field part only. This is because the electric field component of radio waves produces much weaker currents in the body than does the magnetic field unless the wavelength of the waves is smaller than the height of the body. Low-frequency electric fields by themselves can be strong enough to create significant current, but only if they are from sources other than true radio waves.
The radio/microwave section has a small L-shaped antenna in the front. The signal is amplified and converted to a power density magnitude, calibrated at typical home microwave oven frequency (2 GHz). It reads 0 to 1 milliwatt/square centimeter. The resolution at the bottom of the range is 0.01 mW/cm2, which is the Russian standard for maximum exposure, and is the most conservative standard of any country. In contrast the US legal maximum is 1000 times higher, at 10 mW/cm2, but only brief exposure is allowed at this level. As mentioned, a true radio wave is a particular combination of electric and magnetic fields. A radio wave strength of 0.01 mW/cm2 has 0.006 KV/m and 0.2 milligauss, respectively, of electric and magnetic field (RMS averaged), while a strength of 1 mW/cm2 corresponds to 0.06 KV/m and 2 milligauss. Typical accuracy is within a factor of two. Variations are caused by reflections off the user's hand and body.
The TriField BroadBand Meter
This was designed as a more general electromagnetic measurement tool. It detects electric fields from power line frequencies up through 2.5 GHz. This fills in the gap from 100 KHz to 50 MHz, which includes many amateur radio frequencies so that antenna radiation patterns can be measured. The Broadband detects magnetic fields up through 100 KHz, as does the standard TriField meter.
The AC magnetic section is identical to that of the standard TriField Meter and has several uses. It can detect which of several wires or transformers is carrying current, and help determine appropriate placement of magnetically sensitive equipment. It can also help to reduce personal exposure to fields. Most users prefer a magnetic section which is frequency-weighted, and calibrated at 60 Hz. However, a flat frequency response version (50 Hz to 500 Hz) is also available for the AC magnetic section. This is the same price as the regular frequency-weighted version (which has sensitivity proportional to frequency from 50 Hz to 500 Hz.)
The AC electric section is also identical to that of the standard TriField Meter and can be used to determine whether an AC line is hot or neutral, even through a wall. It can easily detect by proximity which side of a wall outlet or which wire of an extension cord is hot. It can also detect improper grounding, as well as the extensive electric fields from video monitors and fluorescent lights.
The radio/microwave section detects vertical electric fields parallel to the long axis of the meter. Range is 10V/m to 1 KV/m (0.026 mW/cm2 to 260 mW/cm2) and is frequency-flat from 100 KHz to 2.5 GHz. Accuracy is +/-30%. It is used to detect ON-AIR status of transmitters (or to detect concealed transmitters), to check microwave ovens or other microwave equipment for leakage, and to look for RFI sources.
For radio waves, magnetic field strength is proportional to electric field strength. Therefore only electric or magnetic need be measured. In a radio wave, one milligauss corresponds to 30 volts per meter, which are the RMS magnetic and electric fields for a power density of 0.24 milliwatt/cm2. The power density increases as the square of the field strength, and the field strength decreases as the inverse distance from the transmitting antenna.