Hall Effect Theory
In an ideal world, a Hall element consists of a small slab of semi-conductor material. Current passes from one end of the slab to the other and the voltage on each edge of the slab is the same when no magnetic field is present. If a magnetic field is now applied through the top to bottom surfaces of the slab, a voltage appears across the sides of the slab which is directly proportional to the magnetic flux density or magnetic field strength. In reality, all practical Hall probe elements are only linear within certain limits, normally 1%-2%. The more accurate and thinner the probes, the greater the expense. Most Gaussmeter manufacturers approach this problem by selecting current and Hall probe load resistance to minimise these non-linearity errors.
The Hall probes are connected to Gaussmeters. The Gaussmeters are designed to be linear to make the best use of the non-linear Hall probes.
The philosophy behind the design of the Hirst Gaussmeters is that both the probe and the Gaussmeter will contain non-linearities and errors. The difference between a theoretical, perfect Hall probe and that of an individual Hall probe is measured and the difference recorded in an E2prom located in the Hall probe socket (this memory device also contains other information such as serial number and calibration date).
The perfect Hall probe is in fact a calibration process using a technique known as Nuclear Magnetic Resonance (NMR) which gives very high accuracy.
The errors between the GM08/7 and an ideal electronic circuit are measured and recorded in its internal E2prom.
When a measurement is taken the voltage generated by the Hall probe element is amplified by the GM08/7 electronic circuit and digitised in an analogue to digital converter. The software of the GM08/7 then automatically corrects this data mathematically, firstly with the Hall probe calibration information and then with the GM08/7 calibration information. This technique gives superb results and enables calibration of the Gaussmeter to be carried out in software rather than "select on test" resistor values. The GM08/7 includes other innovative techniques which further improve and give additional long term stability.
The GM08/7 also includes a linear analogue circuit for peak detection. This dedicated circuit enables the accurate capture of transient events without the inherent delays of analogue to digital converter sampling times.
Although this captured voltage level will decay, In the analogue circuit, it is converted and displayed from a digital storage giving zero droop digital storage.