Why Calibrate?

It is often asked “my tube is metal, sturdy and has no moving parts. What can go wrong?”

True, the orifice probably hasn’t changed enough to make a difference, but…

  • Over time, corrosion and dust buildup can make the orifice ID smaller, making a duct
    system appear to be leaking more than it actually is [that’s the last thing you want].
  • Dents and/or bent pressure taps can lead to erroneous readings.
  • Calibration gives piece of mind. The validity of your reading is no longer in question.
  • The orifice plate may have been damaged without your knowledge (see picture below).

Damaged Orifice

An orifice plate that was severely damaged. This orifice tube should be replaced, but if it was just used for leak
testing a system, it should be calibrated since the original calibration data would be meaningless.

It is common to require laboratory instruments to be calibrated annually. The required calibration
interval is actually a function of usage. Instruments that are used excessively should be calibrated
more often than an instrument used infrequently. The best way to determine an appropriate
calibration interval is to obtain three annual calibrations and note the variations between them. If
the calibration data is fluctuating between the accuracy statement limits (say 2%), then the
calibration interval can be extended.

We recommend either one of the following calibration schedules for aluminum orifice tubes
manufactured or constructed from someone other than ORIFLOW:

BEST

Obtain three annual calibrations and note the variations between them. If the calibration data is
fluctuating between the accuracy statement limits, then the calibration interval can be extended to
two years.

GOOD

Have orifice plate recalibrated every two years. Tubes also may be recalibrated at that time, but
isn’t necessary unless there has been damage to the tube such as a bent pressure tap or
significant dents that might change the flow near the orifice.