Super ted
It really depends on the type of instrument and the method it employs to achieve the desired result.
The altimeter uses pure reaction of a sealed capsule to changes in static pressure surrounding it, this is coupled via sector gear / cog / lever arrangments and the lengths of the levers are altered for calibration purposes. Also coupled to the system is a datum shift to account for baro correction. As you can understand all this wizardry attached to a capsule will cause a degree of load which will be smoothed by the use of a vibrator.
Other instruments use a pedulous weight to assist the capsules reaction ie. IVSI., or they may employ more dynamic means as in an eddy current drag cup which is used in tachometer type instruments.
The variety is endless but the requirements will generally be dictated by the type of sensing element.
Regards Conehead
[ 13 January 2002: Message edited by: IMA Conehead ]</p>