Ask the Experts
Q: How can I use the model 136 with µV/g transducers, when the resolution of the enterable sensitivity is 1µV/g?
A: Some customers, using the 136 three channel PR Signal Conditioner and low output Piezoresistive devices (100 µV/g down to 1µV/g) have expressed concern that they cannot input the sensitivity of their devices with good resolution. The 136 does not allow numbers less that 0.001 mV/g to be entered.
The gain structure of the 136 allows for a convenient workaround, provided that the result does not violate the internal gain maximum (1000). The 136's front panel shows the transducer's Sensitivity expressed in mV/EU. For the sake of clarity in this article, we'll use g's instead of EU (Engineering Units). PSI can be substituted for EU as well, or any other physical measurand that can be measured using Piezoresistive (or PR-like transducers, such as Microtron®) devices.
In any formula, where the same unit of measure is used in the numerator and the denominator of the gain formula (as in the 136), it can be substituted for any other unit of measure, as long as it is used on both sides of the equation. This allows us to state the Sensitivity as µV/EU, as long as the Output Scaling is also expressed in µV/EU (instead of the marked "mV/EU"). This gives us the required sensitivity resolution.
The side effect of this is restrictions on output level. The Output Scaling will now allow entries up to 1000µV/g, max. The Auto-Zero will still set the output offset level to within 10mVDC, though with the scaling change this will now seem to be a rather high number. For transducers that need this scaling, these tradeoffs are probably acceptable, since their full scale is generally in the high thousands of gs.
As an example, we have an Endevco 71-20K with a sensitivity of 9.565 µV/g. Using what is stated above, we enter it as 9.565 mV/EU. Since we want to see 10,000g as 10V at the output, we will set the Output Scaling to 10,000,000 µV (10V) over 10,000 g or 1000 mV/EU (really 1000 µV/g). Since the calculated gain, 104.55 (see the 136 instruction manual) is less than 1000, were okay to enter these values, and run the test.
Endevco produces high performance equipment for extreme environments as well as normal laboratory environments. We provide the above information to help you accomplish accurate and reliable measurements. We want to be your source for vibration, shock and pressure monitoring needs.
[ Return to front page ]
