For most technicians, calibrating switches becomes somewhat of an afterthought. This might be due to the fact that switches are commonly viewed as an over-simplistic device. A technician may see a switch and think, “What is there to calibrate?” The fact of the matter is there are significant reasons to calibrate switches and multiple ways of doing so.
The three main reasons to calibrate switches:
- Due to regulations from the FDA, EPA or EMEA all regulated instruments, including safety instrumented systems (SIS), require calibration and documentation.
- A faulty switch can be a safety hazard for both employees and customers. If too much pressure builds up in a vessel and the switch is not releasing properly it could explode and seriously injure nearby employees and cause damage to the surrounding factory. For industries producing consumable items, a faulty switch leads to product quality issues. This type of inconsistency in ingredients can prove fatal when dealing with medication or food products.
- A switch that is not functioning properly and is opening and emptying out at the incorrect time wastes material. Ultimately this causes a factory to lose time and money, whether it is due to the cost of raw materials, failing an audit or recalling a product.
With all of these reasons to calibrate, why is it still an afterthought? Maybe it is not an issue of why one calibrates a switch, but how and what is involved?
There is what we like to call the shade tree mechanic way. Here a technician applies pressure and hooks up a meter to the switch and waits until the meter changes. At this point the technician sees the meter change and maybe hears it click and looks at the dial and tries to guess at what pressure the switch tripped. (Not so scientific).
A more accurate way allows one to automatically capture the pressure the switch actuated with precise resolution instead of guessing when the analog meter changed and what value was indicated on the pressure gauge. Before you calibrate you need to know your switches defined set and reset values have a processes tolerance defined. For example, a normally open (NO) rising switch may have a set point of 10 psi and a reset of 8 psi. If our tolerance is ±1 psi, then our set point can be anywhere from 9 to 11 psi and our reset from 7 to 9 psi. Using those two aspects and functions one can then perform a test and determine whether the switch passes or fails.