Reader feedback: September 2018

Readers respond to July's Ask the Experts and the Unfettered blog

I work as a product manager for Emerson and read through the “Ask the experts” installment, “Custody transfer flow measurement.” I was confused by the stated differential pressure (DP) flow turndown of 3:1 to 4:1, and not having the ability to compensate for discharge coefficient. Rosemount’s 3051SMV with Ultra for Flow dynamically compensates for changes in discharge coefficient 22 times per second, and is capable of mass flow measurement ±1% over a 14:1 turndown on flow.

Benjamin Goulet, product manager
Rosemount Measurement

The question was based on standard DP cells that are in use at the particular site. Our editors omitted a footnote to the table that clearly states that smart DP cells provide 14:1 rangeability.

Concerning discharge coefficient compensation, what you provide is correction for gas expansion and thermal expansion factors in the DP mass flow equation, while the value of natural gas is a function of composition and heating value. Pressure and temperature do not detect either of them, no matter how often the calculation is performed.

The DP cell does not measure mass flow (nor volumetric flow); it measures the square of the pressure differential across a flow element. The DP cell is only one component in the flow detection loop, and therefore it's misleading to imply that the DP accuracy and the flow measurement accuracy are the same. In fact, the flow measurement error is the sum of all errors, including installation.

Also, you say that your flow measurement error is ±1% without saying percent of what, and whether that number includes range error, span error or both. If you're claiming 1% of actual reading (AR) accuracy at minimum flow, which is 0.071% full scale (FS) in terms of flow and in terms of DP, it is 0.005% FS. I would like to see the test report of an independent testing laboratory the supports that claim. If all you are claiming is 1% FS accuracy, at minimum flow that means an error of 14%, which is useless.

Béla Lipták

I read with interest Joe Weiss’ recent blog post about the cybersecurity concerns of moving control and communications capabilities down into field devices. Current thought at the FieldComm Group/Fieldbus Foundation and in the Open Process Automation Forum (OPAF) calls for moving function blocks to field instruments, and providing Internet connectivity to those field instruments/actuators. At least in OPAF, we plan to base all connectivity on OPC UA, which has Transport Layer Security (TLS) for all devices, certificate-based authentication, and AES encryption for all transmissions.

We appreciate Joe’s caution on lockdown or elimination of USB ports, but since we use Ethernet for network communications, my input to the OPAF effort is that routine diagnostics include at least a checksum to prevent unauthorized code changes. We must provide for patching/updates through the network, but even this must be done securely.

Richard Caro

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