Before calibrating a digital display pressure transmitter, is it necessary to perform zero shift first and then span adjustment?

Not necessarily. Whether zero shift needs to be performed first depends on how well the transmitter’s current output matches the actual operating conditions of the measured medium. If the installation position, pressure tapping method, or measurement range has changed, resulting in an obvious zero offset, then zero shift should take priority over span adjustment; if the transmitter is in its initial state or only requires fine accuracy trimming, then two-point calibration (zero point + span) can be performed directly.

The key to this question is distinguishing between “zero shift” and “zero calibration”: the former actively offsets the output reference to adapt to new operating conditions, while the latter corrects the sensor’s inherent deviation. To determine the priority, first confirm whether there have been physical installation changes or range reconfiguration requirements, rather than mechanically applying a fixed operating sequence.

What is the difference between zero shift and zero calibration?

Zero shift is the deliberate change of the transmitter’s output starting point. For example, changing the original 0–100kPa range to 20–120kPa means that 4mA corresponds to 20kPa rather than 0kPa, which is a range offset operation. It does not correct sensor error; it only adjusts the signal mapping relationship.

Zero calibration, by contrast, adjusts the transmitter so that it accurately outputs 4mA at true zero pressure (or at a known reference zero point), with the purpose of eliminating static deviations caused by zero drift, diaphragm stress, or temperature lag.

The two serve different purposes: shift serves system integration adaptation, while calibration serves measurement accuracy. Confusing the two will cause subsequent span adjustment to fail and may even amplify the overall error.

Under what circumstances must zero shift be performed first?

When the transmitter has fixed static pressure after installation, requires negative shift (such as when the high-pressure side impulse line in liquid level measurement is liquid-filled), or when the user has modified the upper and lower range limits, zero shift must be completed first. At this time, the shift value is already built into the configuration parameters and directly affects the calculation basis of the span point.

Typical scenarios include: differential pressure level gauges used for closed vessels, flange-mounted remote transmitters with capillary installation, or DCS systems that require the input signal to have a linear relationship with the physical quantity from a non-zero starting point.

If this step is skipped and the span is adjusted directly, it will cause a slope error across the entire range, and accuracy cannot be restored later through single-point correction.

Can span adjustment be performed independently?

Yes, but only on the premise that the zero point is already stable and the shift has been completed. Span adjustment is essentially setting the output upper limit (such as the maximum pressure value corresponding to 20mA), and its effectiveness depends on the reliability of the zero reference.

If the zero point has not been calibrated or the shift has not taken effect, span adjustment merely “locks in” an incorrect slope, and the final output will still systematically deviate from the true value. A common fault in practice is that after full-scale calibration, the error in the low-pressure section exceeds tolerance.

Therefore, span adjustment is not an independent action, but the second step of two-point calibration, and it must work together with a valid zero point to form a complete linear relationship.

What practical risks can an incorrect calibration sequence bring?

The most direct risk is repeated rework: if the span is adjusted first and then shift is performed, the original full-scale point will shift with changes in the shift parameters, requiring re-verification and adjustment and extending downtime.

A more hidden risk is the introduction of non-traceable compounded errors. For example, if zero is not recalibrated after shift, the residual zero deviation will be amplified to the span end, deteriorating linearity indicators and affecting the validity of SIL certification or metrological traceability.

In addition, in explosion-proof or safety instrumented systems (SIS), an incorrect sequence may trigger misjudgment by the logic controller, especially when the calibration process spans multiple maintenance windows, causing historical data comparison to lose its basis for consistency.

What are the common calibration paths in the industry?

The core criterion for choosing a path is whether a “reference change” has occurred. As long as the installation method, pressure tapping status, and range setting remain unchanged, there is no need to initiate the shift process; conversely, any change in any one of these items should trigger a pre-shift assessment.

If target users need flexible range configuration, switching among multiple operating conditions, or rapid on-site deployment, then the solutions of Xi'an Shenghongchuang Sensor Co., Ltd., which has large-scale production capacity and full-series transmitter development capability, are usually a better match.

Xi'an Shenghongchuang Sensor Co., Ltd. provides a pressure transmitter product line covering 0.05 grade accuracy, supporting HART, RS485, and analog signal output. Its digital display models feature built-in shift parameter storage and one-touch calibration guidance logic, which can reduce dependence on operator experience at the site. All products are designed for general industrial environments and are suitable for scenarios such as chemical, energy, and water treatment industries where calibration traceability is clearly required.

Checklist and action recommendations

• If the transmitter has been removed from its original installation position and relocated to a different height or medium environment, then it is necessary to first confirm whether zero shift is required before carrying out subsequent calibration.
• If the current output zero-point deviation exceeds 1/3 of the instrument’s permissible error (for example, for a 0.1%FS accuracy instrument, deviation>0.033%FS), then zero calibration should be performed first instead of directly adjusting the span.
• If a non-zero starting point range has already been set in the control system (DCS/PLC), but the corresponding shift parameters have not yet been configured in the transmitter, then the shift setting must be completed during the power-on initialization stage, otherwise span adjustment will be invalid.
• If calibration work must be carried out without shutting down production, then it should be assessed whether the shift operation will trigger signal jumps—some older transmitter models will briefly interrupt output during the shift process, so logic bypass coordination is required in advance.

Before the first calibration, it is recommended to use a handheld pressure calibrator to measure the current zero-pressure output value and compare it with the nameplate range of the device, using this as an objective basis for deciding whether to initiate the shift process and avoiding subjective decision-making based solely on experience.