Accurate measurement: 4-20ma vs Modbus

Measurement devices in the Oil & Gas Industry deliver crucial information by communicating across multiple devices. This is made possible by protocols – a system of rules that allow two or more parts of a system to transmit data. Today, Modbus is the most popular industrial protocol, but the Oil & Gas Industry still often uses an older analogue convention – 4-20ma.

Out with the old

During oil and gas processes, sensors relay information such as gas flow and temperature. Often these sensors need to be mounted far from a controlling station, requiring lengthy wires that transmit information across a current. Longer wires cause drops in voltage, affecting the current and severely reducing the accuracy of the sensor.

Industrial process engineers discovered a solution by using current loops for sensing and control – known as the 4-20 mA convention. Born in the 1950s, it became the common standard analogue current loop and is still used today. The loop allowed a current to stay constant regardless of voltage fluctuations and deliver data accurately. However – being analogue – it has an accuracy range, meaning there is a point when the distance between the control and the sensor is too great to maintain accurate meter readings.

In with the new

Most devices today are digital, meaning 4-20 mA signals have to add a digital to analogue conversion, complicating the meter reading process while losing the benefits of increased accuracy.

Modbus is a digital protocol. It is free and open to use by manufacturers, transmitting information digitally between electronic devices. It has become the most widely used network protocol in the industrial manufacturing environment today. As it can run over virtually all communications media – from wireless to microwave – a Modbus connection can be established in new or existing plants with ease.

Modbus’ main advantage is accuracy. Transmitted data is 100 per cent accurate and can be sent in large volumes.

An adaptable device

The main barrier preventing oil and gas operators from switching to Modbus is the need for an extra cable at a refinery or platform. This can be costly and raises the question of whether accuracy is worth the price of installation.

Fluenta’s 160 Flare Gas Meter is capable of delivering information through analogue 4-20 mA outputs and Modbus protocols. The meter measures the volume of natural gas being flared on oil and gas sites – the result of remote oil and gas locations not having the infrastructure to capture and safely transport natural gas. In these cases flaring is essential for ensuring safety as burning excess natural gas protects against the dangers of over-pressuring industrial equipment. Fluenta’s meter ensures natural gas is being flared appropriately, safely and only when necessary.

For more information on our meters, click here.

April 10, 2018 | News

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