Rate of change alarms differ from normal level alarms in that they respond to the change in a variable over a period of time, rather than the absolute value of the variable. See how these alarms work in greater detail.

How do rate of change alarms work?

Rate of change alarms differ from normal (level) alarms in that they respond to a specified rate of signal change rather than to the crossing of an absolute threshold in the measured variable.

For example:

– Normal alarm is set to 850 degrees C.

When t  850°C an alarm is triggered

– Rate of change alarm is set to 10 degrees C per minute

When the temperature changes from 820 to 835 degrees C over one minute, the alarm is triggered (because the temperature changed by 15 degrees per minute – greater than the 10 degree per minute alarm setting).

When temperature changes from 240 to 255 degrees over one minute, the alarm is also triggered.

When temperature changes from 930 to 935 degrees over one minute, the alarm is NOT triggered(the rate of change is only 5 degrees per minute).

We can see that these alarms only respond to changes, and ignore the actual measured value.

Rate-of-change Alarm set points can be positive (signal going UP) or negative (signal going DOWN). The alarm will only be triggered when the rate of change is greater than the rate set as the alarm threshold in the direction of the set point. For example if the rate alarm is set to -10 degrees C per minute, then the measured value must be falling at a rate greater than this for the alarm to be triggered.

NOTE:

Rate of change alarms will only be present while the measured variable is changing. Once the measured variable stops changing, then the alarm will reset.

The rate of change is normally measured over a given sampling period. The reading is effectively averaged over the period of sampling time so instantaneous (quick) changes will be detected but will be suppressed. The measurement can only indicate what the average change over time was. For instance, if a signal changes by 10 degrees in 10 seconds, then the actual rate of change is 60 degrees per minute, but only for that ten second period.

If the rate-of-change alarm is set to a one minute sampling rate, then the rate will be measured as a 10 degree change between the one minute samples (or 10 degrees per minute). If the alarm was set to 15 degrees per minute, then the rate alarm would not have been triggered.

Why are Rate of Change Alarms used?

They are used to predict an event that is behaving abnormally in order to take preventative action in order to avoid a failure. So if a change of process value suddenly increases at a rate beyond the normal, then it can indicate something unusual has changed in the process as shown.

# Omniflex

The company was founded in 1965, formerly known as Conlog, specialising in process control, alarm/event management and instrumentation products and systems. In 1997, the original instrumentation business division of the company changed its name to OMNIFLEX Ltd.

The company was founded in 1965, formerly known as Conlog, specialising in process control, alarm/event management and instrumentation products and systems. In 1997, the original instrumentation business division of the company changed its name to OMNIFLEX Ltd.

Omniflex works across all business sectors including pharmaceuticals, hospitals, oil, gas, nuclear, water, electrical distribution and energy monitoring.

The company has its offices in the UK, South Africa, and Australia, and the company’s products are sold on five continents. Product ranges include well-known brands such as OMNI SIL alarm annunciators, OMNITERM SIL signal conditioning, MAXIFLEX intelligent remote terminal units (RTU) and I/O systems and the MAXILARM/Omni 4000 alarm/event management systems.

We bracket ourselves as a ‘remote monitoring specialist’, using data acquisition systems, with better than 1mS timestamp at the IO point (from several I/O up to thousands of I/O),  alarm event management including multiplexing systems, telemetry systems, Data2Desktop using standard web browsers, over varying forms of media and high-integrity (SIL1) annunciator systems.

A particular area that Omniflex is also very strong in is obsolescence issues on old legacy alarm annunciators systems. We have, in most cases, a solution for replacement of Rochester, Highland and Robinson’s annunciator systems. Our alarm annunciator systems have been substantiated by Sellafield for and on behalf of all the nuclear operators.

An area that we can offer customers significant cost savings is through the use of our CONET networking technology, which is supported by many of our RTU / PLC products. Using our CONET network technology allows existing installed cabling to be used therefore significant cabling/installation costs can be avoided. A typical example of this whereby our customer in the nuclear industry used our CONET technology, retained their existing cabling and infrastructure, saved them in the region of £1m on cabling and scaffolding.

As a worldwide supplier, OMNIFLEX provides technical training, back-up and application engineering support to a variety of industry partners including local distributors, value-adding resellers and systems integrators, to provide superior levels of service to the end user.