Valves and the electronic circuits that they are controlled with as well as the actuators that they are moved with are all vulnerable to the effects of aging after they are installed. Often times this is actually sooner that one would expect without knowing anything about the process. The valve seat doesn't just wear from the repeated seating of the valve by also from the liquid or gas that passes through it on a regular basis.
As the valve can be stroked anywhere from hundreds to tens of thousands of times over a single year depending on the application, the screws will loosen and reposition and the springs will weaken while the mechanical linkages loosen. This causes the results to no longer fully open or close, close early, or operate erratically which can cause other problems within the application. It is commonly referred to as a 'calibration drift'.
In order to prevent prolonged downtime, there are routine checks performed by a preventative maintenance program that mandates the periodic checks of the valve postioners. The process calibrators that are used must be portable so that they can test the valves in the field without shutting down everything and prolonging the downtime that is taken.
Checking the valve's response time is done by plugging in a controller into the valve positioner's input and setting the positioner's input current to a specific level. This allows the visual inspection of the reaction of the valve's position. It does this by using the mechanical position indicator, the flow indicators, or the valve stem position indicators.
Remember that working with process calibrators is not actually the safest job to have. There are several very serious problems the can arise on the job at any given time and even if something is marked 'intricately safe' it only means that it is the safest that the designers and distributors can possibly make it, it does not control natural events or reactions and so it is better to always be on guard when faced with potentially dangerous situations.
It is particularly important for those technicians that work within industries like the petro-chemical and pharmaceutical industries. It is also important for those who work around bulk materials like grain, mining, and any area where explosive gasses have been reported that they are present. It takes a very small amount of energy to cause an explosion. An example of this would be the mixture of hydrogen in the air and this only takes 20 uJ of energy. So always bear this in mind and take the necessary precautions and actions.