Automatic Systems Laboratories

Tel: +44 (0) 1737 649300
Fax: +44 (0) 1737 649301
info@aslltd.co.uk

page location: home > multi-channel systems

Multi-Channel Systems

Switchboxes

ASL's thermometry bridges can be used with up to six 10 channel switchboxes. The SB148 switchboxes, available as stand-alone units or as part of a fully integrated system as shown, can be operated manually or under remote control via the SB158 driver. RS232C or IEEE interfaces are optional. For harsh environments, these complete systems can be provided with filtered ventilation and/or built-in cooling.

SB148 Switchbox

The SB148 10 channel switchbox provides full 4 wire plus ground switching and has two unique features:

Keep warm current

When in use the temperature of a PRT is increased slightly by the "self heating effect" of the constant current. This effect, which can be many mK, is permanent, and therefore in use is accounted for by calibration. The problem arises if you wish to take a measurement a soon as you switch to a PRT. Probes can take a minute, sometimes more to stabilise once turned on. The solution is not to turn them off in the first place, so as the SB148 disconnects a PRT from the bridge it reconnects it to an identical current from its own power source. Next time that PRT is re-connected to the bridge it is already at "working" temperature and you can make an accurate measurement immediately! Any value up to 10mA may be factory set, individually for each channel.

Optimise bridge performance

To optimise bridge performance when using PRT's of different Ro values, for example 25 and 100 Ohms, measurements are made against reference resistors of matching values. Up to 4 channels of the 1st 148 scanner can be configured to switch reference resistors (Rs) rather than PRT's so that as thermometers are selected, the correct value of Rs can also be automatically selected. Usual configurations (Rt:Rs) are 10:0 (10 PRT's, 0 reference resistors), 8:2, 7:3 and 6:4.