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bohica (Military) (OP)
12 May 00 18:17
I am in the electronics repair and calibration field and am required to calibrate vibration (a first for me).  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).  Does anyone know of a good vibration convertion calculator on the web, or know the conversion constant from mm/s to mV/g?

Thanks,

Bohica
Helpful Member!  Hatch (Mechanical)
17 May 00 16:54
It is not a straight conversion because frequency is in the equation.  The general formulas are:

Acceleration A = 2 pi f V   (pi = 3.14, f = frequency V = velocity

You have  mV/g which is a transducer sensivity and need to convert to mm/s. which is velocity, but your sensivity is in acceleration.

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.

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.  Assuming that the transducer is accurate in producing say 100 mV/g do the following.

Now 1 V rms = 1000 mV rms  then transducer if it produces the same signal will be equal to 1000/100 = 10 g rms acceleration @ 160 Hertz.

10 g rms = 10 x 9.81 = 98.1 m/s2 rms  (1g= 9.81 metres per second squared)

Velocity V = A/2*pi*f
            = 98.1/(2*3.14*160) = 0.10 m/s rms

0.10 m/s rms x 1.414 = 0.14 m/s peak  (peak is convention for vibration velocity)

0.14 m/s peak = 141.42 mm/s peak.

Now just substitute you numbers to calibrate.
Helpful Member!  VibrationSpecialist (Mechanical)
17 May 00 23:45
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.  If not, refer to the accelerometer vendor.  I can assist you in this (you can e-mail me the model type and I can find it out for you.  Most (not all) accelerometer has a sensitivity of 100 mV/g.  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).  You cannot convert an Overall acceleration to Overall velocity, but you can convert a frequency component of acceleration to velocity.  Use the conversion mentioned in Hatch posting or you could visit wicoxon.com.  This web contains a conversion program that you can download free (about 1 MB).

I hope this is of help to you.
Take care.
brin (Aerospace)
18 May 00 15:23
Can you give us some details on the test equipment?  Are you checking calibration on just the sensor (accelerometer?) or both sensor and instrument?  

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.  Most manufacturers provide a certificate of calibration from 0-10 KHz, etc for accelerometers as well as comparison to a NIST reference accelerometer.   
Helpful Member!  dgallagher (Mechanical)
1 Mar 01 10:52
Check out this appnote on Comparing Vibration Readings: http://www.reliabilitydirect.com/appnotes/cvr.html

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