B002 – Differential Pressure
Description
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.
Transmitter
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.
Uncertainty
If differential pressure is measured using a transmitter select calculated, otherwise select user entered to enter the estimated expanded uncertainty in pressure.
Signal
- 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
Manufacturer
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.
Model
Select the Model of the differential pressure transmitter.
Range
Select the Range (If Specified by the manufacturer data sheet)
Inputs
The differential pressure transmitter characteristics are inputted from the manufacturer’s data sheets.
Span
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.
Calculations
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.
References
Standards
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
Transmitters
Differential pressure transmitters are available for selection from the following manufacturers: