Flow Forum Q&A Series: Advances in temperature calibration procedures

Flow Control‘s inaugural Flow Forum conference will take place May 7-8 at the Westin O’Hare in Chicago. During the event, Keith Riley will speak about how modern temperature sensor technology is making calibrations easier and less expensive, and often lengthening intervals between calibrations. Riley has more than 22 years of application engineering and product management experience in the process industry, covering the gambit of practical implementation to product development across a variety of industries. Now serving as U.S. temperature and pressure product business team for Endress+Hauser, Riley recently sat down with Flow Control to answer a few questions about some of the key points of discussion he’ll be covering during his Flow Forum presentation.

Q: Why is temperature such a critical measurement for many industrial process applications?

Depending upon the process in question, accurate temperature control can impact product quality, process efficiency, safety or energy consumption.

Certain processes rely on achieving and maintaining a specific temperature (i.e., a Goldilocks zone where temperature isn’t too high or too low) in order for the proper reaction to occur. Similarly, if cleaning between cycles is critical the correct temperature must be achieved and maintained for a specific time period to avoid potential contamination issues.

Of course it would be easy to just say if "X" is good, "Y" must be better. However the generation and distribution of steam is not without cost. Over the course of a year, the ability to control steam temperature between well defined limits can save a facility tens of thousands of dollars.

Proper monitoring of temperature on equipment such as heat exchangers is used for preventive maintenance by detecting operational inefficiencies before they become significant.

Q: How have temperature instrument calibrations typically been handled in process applications?

The two most common methods employed are:

• Removing the temperature probe between process cycles or at regularly scheduled shutdown periods. The RTD is then taken to Metrology where a multiple point calibration is performed to ensure it is still operating within either the IEC 60751 or customer-defined tolerances. If the unit is showing indications of drift, the appropriate adjustment is performed.
• Performing a single point calibration at the process with a mobile Bath or Block heater and reference probe.

Q: What are the common problems that arise from temperature sensors that go "out of spec" between calibrations?

The main issue is you have no way of determining exactly when the RTD went "out of spec." If you have a 12 month calibration cycle, did the RTD go out of specification on month four, six or eight?  How many process batches are at risk?

All RTDs will experience drift over their lifecycle. However, the degree of drift over a given time period is different for each individual RTD.  Consequently most customers end up replacing RTDs that are still viable "just to be safe" or rely on RTDs that are not performing correctly because the calibration cycle is too long. This either results in added/unnecessary cost or increases the risk of product loss.

Q: How is temperature sensor technology evolving to make calibrations easier and less expensive?

New self diagnostics are being developed that allow the temperature transmitter to identify if/when the instrument is at risk of operating outside of a customer defined tolerance. This works to supplement the existing calibration cycle, reduce risk, and determine when preventive versus reactive maintenance is required.

Improvements in the physical components are also being employed. They make removal and insertion of the RTD simple and efficient. It is no longer necessary to disconnect the RTD from your control loop if you perform calibrations with a mobile arrangement at the process connection.  This effectively eliminates human error and reduces down time by half.