<P> External concrete vibrators attach, via a bracket or clamp system, to the concrete forms . There are a wide variety of external concrete vibrators available and some vibrator manufacturers have bracket or clamp systems designed to fit the major brands of concrete forms . External concrete vibrators are available in hydraulic, pneumatic or electric power . </P> <P> Vibrating tables or shake tables are sometimes used to test products to determine or demonstrate their ability to withstand vibration . Testing of this type is commonly done in the automotive, aerospace, and defense industries . These machines are capable of producing three different types of vibration profile sine sweep, random vibration, and synthesized shock . In all three of these applications, the part under test will typically be instrumented with one or more accelerometers to measure component response to the vibration input . A sine sweep vibration profile typically starts vibrating at low frequency and increases in frequency at a set rate (measured in hertz per second or hertz per minute). The vibratory amplitude as measured in gs may increase or decrease as well . A sine sweep will find resonant frequencies in the part . A random vibration profile will excite different frequencies along a spectrum at different times . Significant calculation goes into making sure that all frequencies get excited to within an acceptable tolerance band . A random vibration test suite may range anywhere from 30 seconds up to several hours . It is intended to synthesize the effect of, for example, a car driving over rough terrain or a rocket taking off . A synthesized shock pulse is a short duration high level vibration calculated as a sum of many half - sine waves covering a range of frequencies . It is intended to simulate the effects of an impact or explosion . A shock pulse test typically lasts less than a second . Vibrating tables can also be used in the packaging process in material handling industries to shake or settle a container so it can hold more product . </P>

State the disadvantages of keeping vibrator of mobile unit on for a long time