There are several ways in which a device can be calibrated which can be daunting and hard to understand. Calibration labs use their experience, equipment, the inaccuracy of the device under test (DUT), and the amount of time required to make a determination.

In most cases, they choose one of three options:

• Zero or span adjustment (one point)
• Zero and span adjustment (two points)
• A multipoint adjustment (three to eleven points)

One-Point Calibration

The fastest calibration of the three is using a zero-point adjustment.  They typically do this in the lower 20% of the transducer range.  It uses a single point to calculate the difference between the reference value and the DUT reading to create an offset correction.

For a gauge transducer, this is as simple as venting the device to the atmosphere on both pressure and reference ports.  Zeroing an absolute transducer may require the instrument to be pumped down in the vacuum range.  This depends on the full span pressure.

One-point calibration is ideal for transducers that have a constant offset.  This is because the adjustment applies to all the points across the range.

Two-Point Calibration

Another common procedure is zero and span adjustment.  This two-point calibration uses the same process as the one-point, but it requires pressurizing the instrument to the top 20% of the range in order to get the span or second point reading.  The span adjustment is used to create a multiplier that is factored in at every point within the measured pressure.

They do this type of calibration on transducers that have a zero error and a linear drift throughout the range.

Multipoint Calibration

While it is normal for transducers to have a zero shift or span drift, occasionally it will have inconsistent linearity throughout the range.  There are times that it will have no offset detected at the zero or span point.  However, it will still have errors at various points throughout the range.  For instruments with this type of behavior, it will need a multipoint adjustment.

In order to perform this type of adjustment, the calibrator can use anywhere from three to eleven reference points.  We can think of each set of points as individual 2-point calibrations.  The multi-point calibration always gives the best results.  It requires the most time to perform.

How to Know What Calibration it Needs?

It is not possible to know the exact type of calibration required until an as-found calibration is performed on an instrument.  This often includes a zero-point adjustment as part of the process.  Once this is done, a calibration technician will understand which type of procedure needs to be performed in order to get the instrument back in service.

The importance of understanding the differences between the calibration types and when to use each of them can play an important role in the efficiency of a calibration lab.  For your calibration needs, SRP control systems ltd is your local one-stop lab with over 40 years of experience in the industry.