Safety first for temperature transmitters

An ABB Automation Tech (Instrumentation + Automation) product story

Edited by the Engineeringtalk editorial team Dec 18, 2007

The TTH300 and TTF330 can be used in LNG storage, furnace control, surface measurement and offshore installations.

ABB's new generation of Hart transmitters, the TTH300 and the TTF300, feature SIL2 certification from TUV, making them the safest temperature transmitters on the market.

Ideal for use in a wide range of chemical processing and oil and gas applications, the TTH300 and TTF330 can be used in LNG storage, furnace control, surface measurement and offshore installations.

The TTH300 is the first head-mounted HART temperature transmitter in the world with SIL2 certification, which specifies a "probability of failure on demand" of 0.01 +/-0.001.

This has been achieved through the use of redundancy.

The device will automatically change to a redundant sensor on detection of a failed sensor, giving improved reliability compared with other similar transmitters.

This is particularly important in a safety critical application element, for example, to shut down a process when a set temperature is exceeded.

It also allows the replacement of the failed sensor at the most convenient and cost-effective time.

Both transmitters are intrinsically safe in accordance with ATEX, Ex ia, FM and CSA and are approved for use in ATEX zone 0, allowing operation in explosive atmospheres.

The TTH300 and the TTF300 are the first temperature transmitters with extended diagnostic functionality according to Namur NE107, the German standard governing self-monitoring and diagnosis of field devices.

This provides information such as reports of broken wires or corroded connections, allowing for preventive maintenance.

Fault signalling is via HART, allowing more interrogation of the device for specific data or an ABB-specific "maintenance requirement" pulse, which can be read by ABB control systems such as the 800xA.

With a temperature measuring range of -50 to +85C, both the TTH head mounted and the TTF field mounted versions feature drift detection.

This enables the transmitters to monitor the difference between the values from several sensor elements and output a warning if the difference exceeds the configurable drift limit.

This allows calibration to be based on hard data rather than guesswork and cuts the effort needed in maintenance.

Other onboard diagnostics information contributing to accuracy and reliability include device error, over/under alarm limits, over/under measuring range, drift monitoring, supply voltage undershoot and ambient temperature undershoot and overshoot.

Accuracy is crucial in process temperature measurement as any deviation from set temperature could compromise product quality or risk an explosion.

Excellent accuracy of 0.1K is assured by sensor-transmitter matching and routines for one-point and two-point calibration and is enhanced with ABB's high quality sensors and cables.

Easy to install, the transmitters incorporate easy guide cable entries and large terminal blocks.

They are also designed for robustness with an aluminium or stainless steel housing.

This gives them good resistance to moisture and makes them suitable for harsh environments such as offshore, contributing to their reliability in safety critical applications.

They also have improved resistance to induced voltages.

Featuring two independent inputs, with two-, three- or four-wire sensors, the devices can use either an internal or external reference junction for thermocouple measurement.

Inputs can also be from a resistance-remote signalling unit (0-5kohm) or through voltages from a millivolt transmitter.

This gives operators more flexibility to use both types of input without the need to have two different transmitters, thereby reducing costs.

The transmitter's versatility is highlighted by its ability to use a variety of measuring modes - absolute value, differential value and average value.

Average value allows the transmitter to take the average reading of two sensors, improving accuracy, while differential value allows, for instance, readings to be taken from a sensor inside a tank and one outside.

Any change in the external temperature that would affect the internal temperature of the tank can be taken into account and action taken to cool the tank contents before the temperature rise becomes critical.

Outputs can be either through a two-wire technique, 4-20mA temperature linear signal or Hart signal, which allows more varied data to be sent.

The ABB TTH300 and TTF300 are highly reliable temperature transmitters that will meet the needs of process industries in a wide range of safety critical applications.