Fluke 810 User Manual page 131

FAQs - Measurement
1. What are the effects of machine load and
condition?
The vibration from a drive train may change
depending on the load and the ambient
temperature of the motor. The one exception to
this rule is machines with misaligned driveshafts.
It is recommended to take vibration
measurements when the machine is running in a
steady state and at normal operating
temperature. For instance, taking
measurements from a machine where the load
regularly cycles or surges on or off is not ideal
(i.e. compressors). In these instances,
bypassing the process will be necessary to
ensure accurate and consistent diagnoses. In
the case of many compressors, the best
opportunity to take measurements is when the
tank is low, and the compressor is switched on.
The compressor will run longer in order to fill the
tank. Machines tested while still cold up may
have significantly different vibration signatures
than those at normal operating temperature as
temperature impacts shaft alignment and
operating clearances due to thermal expansion.
Frequently Asked Questions
FAQs - Measurement
In the case of pumps, cavitation, air ingestion or
discharge pressure will influence vibration
readings and the diagnostic quality will be
affected. Pumps should not be tested with the
discharge valves closed; however, if they must
be tested in a recirculating condition, the
recirculating valve may be partially closed in
order to achieve a normal discharge pressure.
For larger machines, the general rule is to warm
up the machine for at least 30 minutes before
taking a measurement.
2. How do I mount the sensor?
When using a magnetically mounted sensor,
take care when attaching it to the test surface.
The magnet could pull the sensor assembly from
your hand and impact the test surface,
introducing excessive shock that could
permanently damage the sensor. Holding the
sensor firmly, carefully roll the sensor onto the
test surface to minimize the potential for impact.
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