Conventional EMF meters work by sensing alternating electromagnetic energy in a detection coil, this is energy that is continually changing polarity.
This energy is then rectified and smoothed into an electrical charge which is then displayed on a LED display or needle meter as an average measurement.
Conventional meters are usually designed to detect 50/60 hertz mains electromagnetic fields.

The Sentinel is designed primarily to detect slow reacting/changing energy fields, these can be electrical, magnetic or other energy types.
The LED display on the Sentinel usually centred with one LED illuminated, when adjusted to a useable detection sensitivity setting, energy change can be displayed as a positive (right) or negative (left) reaction

Depending on the sensitivity setting, a slow changing energy field will illuminate the LEDs to the left or right of the centre LED relative to the actual speed of change.

If the Sentinel detects a fast changing energy, or as the Sentinel is brought closer to a source of energy, an increasing number of LEDs will be illuminated both left and right of centre.
Alternatively if the Sentinel is stationary any approaching energy can cause a 'movement' on the LED display.

The 9 LED display is normally adjusted so that only the centre LED is illuminated,
A change to the left indicate a negative electric field or south magnetic field.
A change to the right indicate a positive electric field change or north magnetic field.

It has a high and low sensitivity setting and a variable adjustment for fine tune setting.
to suit the surrounding environment.
A display centre control allows adjustment of the centre LED position.

The Sentinel can give a triggered audio tone burst response if it detects energy change outside a preset setting, on some models this triggered response can activate a digital to take a picture.(via external equipment)

An internal audio amplifier and speaker in some models allow the user to hear (if within our audio range) sound of any reaction from the selected sensor, electrical, magnetic or external sensor.
A sound/tone generator fitted on some models generates a rising and falling tone relative to changes in energy to be detected, the audio level can be adjusted with this control.

Several energy types can be monitored with the Sentinel (but not all at once) -
Electric fields (built-in sensor)
Magnetic fields (built-in sensor)
Light - external sensor
vibration - external piezo disk
Living plants
Quartz crystal
CCTV interface

The sentinel can detect (if conditions allow) a persons electric body field at up to 2 metres and trigger an alarm or auto photograph.

I have used the Sentinel as a monitoring system during Psychokinesis / psychic testing experiments using adapted external sensors.

I have built several versions of the Sentinel, the single channel versions have two sensors built-in, electric field and magnetic field,
Some of the sentinels are equipped with automatic camera trigger capability.
There are 2 and 4 channel versions that allow several various sensors to be used simultaneously.