B001 – Pressure


This block calculates the uncertainty in pressure for a specified 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 transmitter used.


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


This is the type of pressure transmitter, gauge or absolute.


  • Analogue

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

  • Digital

Select this option where the output of the transmitter is digital

For example: HART


Select the Manufacturer of the Pressure Transmitter from the given list.

Note: if the manufacturer of the transmitter is not shown select unspecified and enter values as per manufacturer data sheet.


Select the Model of the Pressure Transmitter.


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


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


Enter the span which the pressure transmitter is set to.

Note: the span plus the LRV must be lower than the URL of the transmitter and higher than the input pressure value otherwise calculation will not run and return NaN.

Lower range value

Enter the Lower range value (LRV) which the pressure transmitter is set to. By default this is 0

Voltage difference

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

ΔT ambient

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

Calibration uncertainty

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

e.g Dead weight tester, Beamex etc.

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 Pressure Transmitter.

Detector resistance tolerance

The tolerance in the flow computer resistor used for ADC conversion.

Note: this input is only applied for analogue transmitters.

ADC resolution

The maximum tolerance allowed during ADC signal checks.

Note: this input is only applied for analogue transmitters.

Ambient pressure uncertainty

Uncertainty due to variations in atmospheric pressure that are not accounted for in a flow computer.

Note: this input is only applied for gauge pressure transmitters.


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

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

Pressure transmitters are available for selection from the following manufacturers:



Endress + Hauser



GE Druck





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