Vibration conversion 4

[bohica]

I am in the electronics repair and calibration field and am required to calibrate vibration (a first for me).&nbsp;&nbsp;The calibration is no problem, but I need to convert the units my engineers gave me in vibration (mV/g) in to what my test equipment reads (mm/s).&nbsp;&nbsp;Does anyone know of a good vibration convertion calculator on the web, or know the conversion constant from mm/s to mV/g?<br><br>Thanks,<br><br>Bohica

 

[Hatch]

It is not a straight conversion because frequency is in the equation.&nbsp;&nbsp;The general formulas are:<br><br>Acceleration A = 2 pi f V&nbsp;&nbsp;&nbsp;(pi = 3.14, f = frequency V = velocity<br><br>You have&nbsp;&nbsp;mV/g which is a transducer sensivity and need to convert to mm/s. which is velocity, but your sensivity is in acceleration.<br><br>If your analysis instrument is a voltage measuring device the easiest way to calibrate it is to use a vibration calibrator (Bruel & Kjaer) which will give a direct 1g rms readout at a particular frequency which is easily converted to velocity since you have all the terms for the equation above.<br><br>If you don't have a vibration calibrator, you have to input a known signal such as 1 V rms (sine wave) at say 160 Hertz to your instrument.&nbsp;&nbsp;Assuming that the transducer is accurate in producing say 100 mV/g do the following.<br><br>Now 1 V rms = 1000 mV rms&nbsp;&nbsp;then transducer if it produces the same signal will be equal to 1000/100 = 10 g rms acceleration @ 160 Hertz.<br><br>10 g rms = 10 x 9.81 = 98.1 m/s2 rms&nbsp;&nbsp;(1g= 9.81 metres per second squared)<br><br>Velocity V = A/2*pi*f<br>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;= 98.1/(2*3.14*160) = 0.10 m/s rms<br><br>0.10 m/s rms x 1.414 = 0.14 m/s peak&nbsp;&nbsp;(peak is convention for vibration velocity)<br><br>0.14 m/s peak = 141.42 mm/s peak.<br><br>Now just substitute you numbers to calibrate.

 

[VibrationSpecialist]

Dear bohica,The mV/g is a unit for sensitivity factor relating the voltage output of an accelerometer to the acceleration unit g = 32.3 ft/sec*2 = 386.4 in/sec*2 = 9.81 m/s*2.The mm/s is a unit of velocity (one parameter of vibration parameters, i.e., acceleration, velocity and displacement)Now to convert the acceleration output to velocity reading, you need to know your accelerometer sensitivity output first, a value that most of the time is written on the accelerometer.&nbsp;&nbsp;If not, refer to the accelerometer vendor.&nbsp;&nbsp;I can assist you in this (you can e-mail me the model type and I can find it out for you.&nbsp;&nbsp;Most (not all) accelerometer has a sensitivity of 100 mV/g.&nbsp;&nbsp;i.e. for every 100 mV (mille-Volt=0.01 Volt) output, the vibration measured is 1 g.Once you find out your acceleration amplitude, you need then to convert it to velocity (knowing your frequency component).&nbsp;&nbsp;You cannot convert an Overall acceleration to Overall velocity, but you can convert a frequency component of acceleration to velocity.&nbsp;&nbsp;Use the conversion mentioned in Hatch posting or you could visit wicoxon.com.&nbsp;&nbsp;This web contains a conversion program that you can download free (about 1 MB).<br><br>I hope this is of help to you.<br>Take care.

 

[brin]

Can you give us some details on the test equipment?&nbsp;&nbsp;Are you checking calibration on just the sensor (accelerometer?) or both sensor and instrument?&nbsp;&nbsp;<br><br>Also be aware that if you are providing a calibration service, you should check the output/response at several different frequencies over the (normal) range of measurement.&nbsp;&nbsp;Most manufacturers provide a certificate of calibration from 0-10 KHz, etc for accelerometers as well as comparison to a NIST reference accelerometer.&nbsp;&nbsp;&nbsp;

 

[dgallagher]

Check out this appnote on Comparing Vibration Readings:

http://www.reliabilitydirect.com/appnotes/cvr.html