Physics of solenoid valves - Two different ways to minimise a magnetic field

The shielding of magnetic fields in applications with solenoid valves is rather rare in practice, but absolutely necessary in some applications. Reasons for this can be that, for example, measuring instruments react sensitively to the magnetic field generated by the solenoid coils and thus lead to incorrect measuring results. There are usually two options for magnetic shielding, whereby the measures for this are almost identical:

1. A magnetic field is to be shielded from sensitive measuring instruments, for example, in order to avoid measurement errors when measuring magnetic fields. Thus, the measuring device assembly is to be protected from external magnetic fields and is packed in a magnetically shielded housing for this purpose.
2. In the second case, the source of the magnetic field should be shielded so that magnetic fields cannot interfere with other devices. This requires shielding of the radiating device to prevent the magnetic fields from escaping to the outside.

Relevant for solenoid valves is the second case considered, for which an alternative approach is briefly presented here. Usually, shielding of magnetic fields is achieved by surrounding the radiating device e.g. with a casing that has very good magnetic conductivity, as exemplified in Fig. 1. The effectiveness of the measure is based on the shunt effect of the magnetically well-conducting enclosure.    
An alternative to this approach is shown in the second figure below. In this case, the effect is not based on shielding or on a targeted steering of the magnetic field by magnetically conductive materials, but on the compensation of one magnetic field by another magnetic field. For this purpose, e.g. a second coil is used to generate an opposing magnetic field which, at a sufficient distance from the coils, almost compensates for the magnetic field of the first coil, provided that both coils generate the same flux (𝛩=𝐼∙𝑁).

In practice, both options are used to minimise the magnetic field, whereby an enclosure with a magnetically conductive material is the best technical solution.