For a differential pressure (DP) transmitter, zero calibration establishes the output corresponding to equal pressure on both sensing sides, while span calibration confirms or adjusts the output across the intended measurement range. I recommend performing both procedures with the transmitter isolated from the process, the high- and low-pressure ports safely equalized, and a traceable pressure source connected to the impulse lines or transmitter ports. For a WIKA DPT-20, the exact menu, button, communication, and adjustment sequence should follow the approved product documentation and the configured transmitter version.
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The practical procedure is to verify the range and output type, stabilize the transmitter, apply a true zero differential pressure, adjust or verify the zero point, apply a known upper test pressure, adjust or verify the span, and then repeat the checks in both directions. A calibration record should include the applied pressure, measured output, error, ambient conditions, equipment identification, and technician. This approach supports commissioning, maintenance, and supplier evaluation without assuming that every DPT-20 installation uses the same configuration.
A DP transmitter measures the difference between pressure at its high-pressure port and pressure at its low-pressure port. The zero point is the output produced when the differential pressure is at the lower range value, commonly zero differential pressure for a gauge-style DP application. The span is the difference between the lower and upper calibrated range values, so it determines how much output changes across the measurement range.
For a traditional 4–20 mA configuration, a transmitter may be configured so that the lower range value corresponds to 4 mA and the upper range value corresponds to 20 mA. That relationship must be confirmed from the device configuration rather than assumed, because DP transmitters can use different output, ranging, square-root, alarm, or digital communication settings. If the transmitter is used for flow measurement, the output may be deliberately configured with a square-root relationship, which changes how the signal should be interpreted during a process check.
Zero and span verification helps determine whether the DPT-20 is accurately representing the pressure difference required by the application. Errors can affect filter monitoring, pump protection, tank level calculation, air-handling control, flow measurement, and process alarms. Calibration cannot correct a blocked impulse line, leaking fitting, liquid seal problem, incorrect manifold position, or unsuitable installation environment, so those conditions must be checked first.
In B2B projects, calibration is also a configuration-control activity. The test should confirm that the transmitter range, engineering units, damping, output behavior, display settings, and alarm limits agree with the project data sheet. I treat calibration as successful only when the instrument response and the installation configuration are both acceptable.
A typical bench or field setup includes a calibrated pressure source, a suitable reference pressure instrument, pressure tubing, isolation or equalizing hardware, a loop power supply, and a current measurement device or calibrator when the output is analog. The reference equipment should cover the intended test range and have accuracy appropriate for the required measurement uncertainty. For example, a test point at 50 psi should not be evaluated with a reference instrument that is unsuitable for that pressure range or accuracy requirement.
Allow the transmitter and reference equipment to stabilize before recording results. A practical stabilization period may be 5 minutes or longer when the installation, ambient temperature, fill fluid, or equipment instructions require it. The correct time depends on the device and test system, so the approved maintenance procedure should control the final value.
Before disconnecting or pressurizing a DP transmitter, identify the process hazards and follow the site isolation procedure. Close the high- and low-side isolation valves, open the equalizing valve where appropriate, and vent or drain trapped pressure using equipment designed for the service. Never open a process connection merely because the displayed differential pressure is zero; equal pressure can still represent hazardous static pressure.
Confirm supply voltage, polarity, grounding, and hazardous-area requirements before connecting test equipment. The DPT-20 configuration may differ by application, so I verify the nameplate, datasheet, wiring diagram, and communication method before making adjustments. If the transmitter is installed in a safety-related or regulated loop, calibration should be performed under the site’s documented approval and record-retention process.
Record the lower range value, upper range value, engineering unit, output mode, and required tolerance. Check whether the transmitter is being used for direct DP, level, or flow, because the process variable and output calculation may not be identical. Also record the serial number and current configuration before changing any parameter.
Check impulse lines for blockage, condensation, leakage, incorrect slope, and trapped gas or liquid. Confirm that the high and low connections are not reversed and that the manifold valves operate correctly. For liquid service, remove unwanted gas where the installation procedure permits; for gas service, manage condensate according to the application design.
With the process safely isolated, use the approved manifold sequence to equalize the high- and low-pressure sides. The pressure at both ports should be effectively equal, not simply disconnected from the process. Allow the output and displayed value to stabilize before recording the as-found zero result.
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If the zero is outside the permitted tolerance, use the DPT-20’s approved zero adjustment method, which may be a local control, display menu, or communication tool depending on the ordered configuration. Do not use a zero adjustment to hide a leak, blocked line, incorrect valve position, or an active static-pressure effect. After adjustment, remove and reapply the equalized condition to confirm repeatability.
Close the equalizing path as required, then apply a stable differential pressure corresponding to the upper range value. For a configured range of 0 to 100 psi, the upper test point would be 100 psi, while a transmitter ranged from 0 to 10 bar should be tested at 10 bar. These values are examples of procedure logic; the actual test pressure must come from the transmitter configuration and approved test plan.
Wait for the pressure and output to stabilize, then record the reference pressure and transmitter response. For a linear 4–20 mA range, the expected output at the upper range value is normally 20 mA, but this must be verified against the actual output configuration. If the span is incorrect, apply the authorized span or upper-range adjustment method and repeat the zero check afterward.
A two-point adjustment alone may not reveal non-linearity, hysteresis, or installation-related problems. I recommend checking at least five points where the quality plan requires it, such as 0%, 25%, 50%, 75%, and 100% of range. A 0–100 psi example would therefore use 0, 25, 50, 75, and 100 psi test points, with increasing and decreasing pressure checks when repeatability is important.
At each point, allow the reading to stabilize and compare the transmitter output with the reference value. For an ideal linear 4–20 mA output, 50% of range corresponds to 12 mA, but square-root extraction, alarm settings, damping, and digital scaling can produce different displayed or transmitted results. The acceptance limit should be taken from the project specification, maintenance standard, or calibration procedure rather than invented during the test.
Zero adjustment changes the response around the lower reference point, while changing the lower and upper range values changes how the transmitter maps pressure into its output. These actions are not interchangeable. If the process design has changed, reconfigure the range according to engineering approval instead of applying a trim that makes the instrument appear correct only at one operating condition.
Use the method supported by the installed DPT-20 version and site policy. Local controls can be useful in the field, while digital communication can make configuration records easier to review, provided the communicator, device description, wiring, and access permissions are compatible. After either method, verify the actual output with independent test equipment rather than relying only on the transmitter display.
Use a controlled calibration form with as-found and as-left results, and record the reference instrument identification and calibration status. Define the acceptance tolerance before testing, including whether it applies to transmitter accuracy, loop accuracy, or total measurement uncertainty. This distinction is important because a transmitter can pass its own specification while the complete loop still contains wiring, scaling, or control-system errors.
For repeat maintenance, establish a risk-based interval rather than using the same interval for every installation. Critical pressure protection, custody-related measurement, and harsh-service applications may require more frequent verification than stable utility service. Trend the recorded errors over time so that the maintenance team can identify drift, impulse-line problems, or installation effects before they cause an operational issue.
When I evaluate a DP transmitter supplier, I request the correct operating manual, wiring information, configuration details, pressure and temperature limits, and recommended adjustment procedure for the supplied version. I also confirm whether the supplier can support range configuration, communication compatibility, replacement documentation, and application review. This reduces the risk of applying a generic procedure to a device with different options.
EMMA can support B2B buyers by reviewing the measurement range, process medium, static pressure, temperature, connection requirements, output signal, installation environment, and documentation needs before supply. For a WIKA DPT-20 application, the final calibration method should remain aligned with the specific product configuration and the customer’s approved procedures. Buyers should provide the data sheet or instrument tag when requesting technical guidance so the recommendation can be checked against the actual order details.
The appropriate procedure for a DP transmitter such as the WIKA DPT-20 is a controlled zero-and-span verification: confirm configuration, isolate the process, equalize both pressure ports, verify or adjust zero, apply the configured upper-range pressure, verify or adjust span, and complete intermediate-point and repeatability checks. The exact local or digital adjustment commands must come from the applicable DPT-20 documentation and ordered configuration. This approach provides a defensible basis for commissioning and maintenance without relying on unsupported assumptions.
As the next step, prepare the transmitter tag, configured range, output mode, process conditions, required tolerance, and available reference equipment. Share these details with EMMA when requesting product or application support, and we can help align the sourcing information with the intended calibration and installation workflow.
If you want to learn more, please visit our website What Zero and Span Calibration Procedures Apply to DP Transmitters.