B002 – Differential Pressure


This block calculates the uncertainty in pressure for a specified differential pressure transmitter. The pressure transmitter characteristics should be selected from the drop down boxes and other inputs should be entered depending on the type of differential pressure transmitter used.


The differential pressure transmitter signal, manufacturer, model and range are selected from the drop down boxes.

In this section, a wide variety of inputs are given based on KELTON’s experiences.


If differential pressure is measured using a transmitter select calculated, otherwise select user entered to enter the estimated expanded uncertainty in pressure.


  • Analogue

If analogue is selected the uncertainty in Detector resistance tolerance and ADC resolution will be included.

  • Digital

Select this option where the output of the differential pressure transmitter is digital

For example: HART


Select the Manufacturer of the differential pressure transmitter from the given list.

Note: if the manufacturer of the transmitter is not shown select unspecified and contact KELTON for further information.


Select the Model of the differential pressure transmitter.


Select the Range (If Specified by the manufacturer data sheet)


The differential pressure transmitter characteristics are inputted from the manufacturer’s data sheets.


Enter the span over which the differential pressure transmitter is to be calibrated.

Note: this must be lower than the URL of the transmitter and higher than the input differential pressure value otherwise calculation will return NaN.

Calibration Uncertainty

Enter the uncertainty percentage in the calibration reference to the differential pressure transmitter.

Calibration tolerance

Enter the Maximum tolerance allowed during calibration process. (Taken from calibration records where available, otherwise assumed).

Typical value is between ± 0.15% of span to ±0.25% of span.

Calibration interval

Number of months between each calibration of the differential pressure transmitter.

Calibration pressure

Enter the calibration pressure for the differential pressure transmitter.

Temperature difference

Difference in ambient temperature between calibration and operation of the transmitter.

Voltage difference

Difference in supply voltage between calibration and operation of the transmitter.


When a transmitter is selected, the relevant uncertainty components from that transmitter’s specification sheet are used for the uncertainty calculation.

The differential pressure value is taken from the Input/Output tab to allow the uncertainty to be calculated for the relevant differential pressure.

Uncertainty Budget

The uncertainty budget table shows a break down of the different components that contribute to the overall calculated uncertainty.

The values input into the uncertainty budget are derived from the measured differential pressure and the transmitter specific values relating to its calibration and specification. These values are taken in as the expanded uncertainties and are divided by a coverage factor to gain the standard uncertainty.

The coverage factor is determined by the probability distribution that best suits that uncertainty component. The standard uncertainty is then multiplied by the sensitivity value then squared. This is done for each component that contributes to the overall uncertainty in differential pressure. The standard absolute uncertainty in differential pressure is the square root of the sum of each component variance as shown in the following equation:

Calculated Uncertainty

The Expanded Uncertainty is the Standard Uncertainty multiplied by the coverage factor (k). The coverage factor is defaulted to k = 2 (equivalent to a confidence level of approximately 95%).

The Relative Uncertainty is the Expanded Uncertainty divided by the absolute Differential Pressure.



ISO 5168:2005 – Measurement of fluid flow – Procedures for the evaluation of uncertainties

ISO Guide 98-3 – Uncertainty of measurement – Part 3: Guide to the expression of uncertainty in measurement (GUM:1995) (2008)

Manufacturer documentation

See DP transmitters?


Differential pressure transmitters are available for selection from the following manufacturers:





Hartmann & Braun



Solartron ISA

Solartron Mobrey




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