This handbook provides a thorough discussion of the considerations for selecting the proper measurement system for a wide variety of applications and conditions. The engineered system selection guidelines are suitable for almost every industrial application.
Temperature is the most widely measured variable in the process industries. Temperature is often a critical factor in industrial processing. If a temperature measurement is not accurate or reliable for any reason, it can have a detrimental effect on such things as process efficiency, energy consumption, and product quality.
Guide to Level Measurement Series
Introduction
Even a small measurement error can be disruptive or very costly in some processes, so it’s extremely important to be certain your temperature measurements are accurate and reliable. Pharmaceutical processing is an example where an inaccurate temperature measurement might ruin a batch of product worth hundreds of thousands of dollars. For this reason, each measurement system needs to be evaluated and carefully engineered to satisfy the requirements of the process.
In a Safety Instrumented Function in a SIS, poor performance could be deadly, costly or both and an error of 2% is considered a dangerous non-diagnosed fault. An example might be a process that can go exothermic and possibly explode if temperature is not measured and controlled accurately.
Another example of where an accurate measurement has enormous cost consequences is custody transfer where the amount of material that is bought and sold (referred to as custody transfer) is based on a measurement of the volumetric flow rate of gas. The amount of material contained in a specific volume of gas decreases with rising temperatures and increases with falling temperatures. Therefore, it is extremely important to know the exact temperature of the gas when determining volumetric flow rate. Inaccurate temperature measurements during custody transfer applications result in over - or undercharging of customers. This can directly impact a customer’s financial performance. A natural gas custody transfer application is one example of where accurate temperature measurements are required.
Measurements are typically made using a sensor (usually a thermocouple or an RTD) and a signal conditioning circuit (either a transmitter or a channel of an input card to a DCS or PLC) to amplify the sensor’s low level (ohm or mV) signal to a more robust 4-20mA current signal.
Combined with a field connection head and thermowell components, the sensor and the signal conditioner are called a temperature system or assembly. Systems are available to meet a variety of measurement accuracy and stability requirements. Some applications need only be within a rather loose ±11 °C (±20 °F) of the actual measurement, some look for trends and accuracy is not as important, while others call for extremely tight measurements of up to ±0.01 °C (±0.025 °F). Long-term measurement stability varies from 5.5 to 11 °C (10 to 20 °F) of span per year for non-critical measurements, to those providing better than 0.044 °C (0.08 °F) of span per year for the most critical applications. In all cases the degree of precision of the measurement is limited by the sensor choice.
This handbook will explore the recommendations, the pitfalls, and the trade-offs for various temperature measurement systems. Guidelines will be presented for selecting the proper sensor and signal conditioner to meet a variety of applications. Design of high reliability systems for use in Safety Instrumented Functions (SIF) within Safety Instrumented Systems (SIS) will also be covered.
Emerson Process Management
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