Oil & Gas Product News Logo

A Practical Guide to Improving Temperature Measurement Accuracy

A TDZ3 temperature transmitter can provide a good system option.
A TDZ3 temperature transmitter can provide a good system option.

Company info

16650 Schoenborn Street
North Hills, CA
US, 91343-6196


Read more

Some processes do not require temperature measurement accuracy, and others do. However, you may be unsure whether accuracy is important for your particular application, or whether improving accuracy will make enough of a difference in your process results to justify the cost and effort. This article identifies problems that result from inaccurate measurements and outlines ways to solve them that are both effective and economical.

Temperature measurements can be categorized into three groups:

Those which do not require accuracy. You simply need to know if the temperature is stable or going up or going down.

Those which do require accuracy. Strategies in this article will help with those measurements.

Those where there is uncertainty about the accuracy requirement. If the measurements in this category are on a Preventive Maintenance schedule for calibration or verification you may want to take steps to improve accuracy. The same steps we take to achieve higher accuracy also result in reduced drift and that would have a positive impact on your maintenance frequency.

Improving accuracy and reducing measurement drift are often related and can have measureable results. There are a number of steps involved in selecting the best sensor for the application, reducing errors caused by external environment or caused by lead wires, and reducing measurement errors.

Sensors and accuracy

There are times when the temperature sensor is selected based on convenience, what is on the shelf or the “plant standard.” It is not uncommon to see a Type J or K thermocouple measuring a temperature that should be measured with a Platinum RTD (resistance thermometer).

Many process engineers and technicians prefer RTDs. Selecting the best sensor for the application greatly affects the accuracy of the measurement and an RTD is the most accurate sensor to use when the process temperature is within its measuring range. But you will need to use less accurate thermocouples when you need to measure temperatures that are hotter than the RTD’s upper measuring limits. In these instances you will want to take specific steps with thermocouples to improve the accuracy of the measurement results.

You can improve sensor accuracy by using thermocouples constructed with Special Tolerance (also called Premium Grade) wire. The reduced error is achieved by using wire with higher purity alloys. At 500 degrees F (260 degrees C), the uncertainty of a Special Tolerance thermocouple is about ±2.0 degrees F (1.1 degrees C).

Sensor selection is very important to measurement accuracy. As stated above, a Class A Pt RTD uncertainty is about ±1.2 degrees F (±0.67 degrees C) at the same operating temperature. To simplify the process of choosing a sensor, you can operate from the assumption that changing from a Standard Grade thermocouple to a Premium Grade thermocouple cuts the error rate in half; changing from a Premium Grade thermocouple to a Class A RTD cuts the error in half again.

The role of thermocouple extension wire

Thermocouples wired back to a PLC or DCS must use thermocouple extension wire. Unfortunately the extension wire is yet another source of measurement error. Using standard grade J or K extension wire also adds another ±4 degrees F (±2.2 degrees C) error. You can cut the error rate by using premium grade extension wire, which has half the error rate of standard extension wire, just as with premium thermocouples. (These error figures are only true when the wire is new and “pure.”) Over time the error gets worse as the wire gets contaminated from the atmosphere in your plant and the wire is exposed to temperatures greater than or lower than the wire tolerances. There are many instances where contamination causes even more “drift” than the original uncertainty of new thermocouple wire.

If the uncertainty caused by thermocouples was a fixed offset, we could simply calibrate it out and be done with it. But when the error is in the form of drift that changes over time, calibration becomes a Preventive Maintenance program that few want to take on. Most plants prefer to avoid that extra labour whenever possible.

How do you solve these problems? Start by determining how much error is caused by the thermocouple extension wire. Most people overlook this option until Plant Operations declares there is a problem or a catastrophic measurement failure occurs. We all know thermocouples fail, but it is easy to forget that thermocouple extension wire also fails. When it does, it has to be replaced. If you replace the extension wire with new extension wire, you perpetuate the same problems by reintroducing the error and drift it causes.

You may have to live with thermocouples, but you don’t have to live with thermocouple extension wire – you can replace it with other solutions.

Two options for replacing thermocouple extension wire are temperature transmitters and remote I/O hardware. Both use copper wire to transport their signals back to the control system. Unlike thermocouple extension wire, you can expect the copper to last the life of the plant. Modern I/O products have performance characteristics similar to transmitters and can save a lot of money. You still need short sections of extension wire when you use these transmitters or I/O products. Use special grade thermocouple wire instead of standard extension wire in these cases to further minimize the error.

Compensating for lead wire inaccuracies

Copper wire is used for RTD lead wires. If you are familiar with 3-wire RTDs you know one lead is called the compensating lead. Between copper and a compensating lead you might believe that RTD lead wire does not contribute to the measurement error.

Unfortunately, this is not true. Copper wire can cause significant error in an RTD measurement because RTDs are resistors and copper wire is resistance. There are many contaminants in a typical process plant that cause corrosion and this corrosion changes the resistance of the copper lead wires. This resistance change in the lead wire can cause error. To eliminate lead wire error the solution is to use 4-wire RTDs.

Here is why: When a third lead wire is added to the RTD, the measurement is made with today’s electronics by taking two voltage measurements. The important thing to remember is that these are high impedance voltage measurements.

For all practical purposes, there is no current flow through that third lead.

V1 gives the value of the lead wire resistance R1. V2 gives the value of the RTD + R4 lead resistance. Subtract V1 from V2 and as long as the lead resistances R1 = R4, only the value of the RTD remains. This is an accurate measurement. Realize that too many things work against making R1 and R4 identical when accuracy is your primary concern. Wire gauge intolerance and work hardening varies the resistance.

Even if no human error takes place during installation, corrosion constantly works against the measurement and is the main reason R1 never equals R4. So what happens if the lead’s resistances are not equal? If the resistance imbalance is as little as 1 ohm, a 100Ω Pt RTD has an error of about ±4.7 degrees F (±2.6 degrees C). If you are trying to achieve a ±1 degrees F (.55 degrees C) measurement accuracy, this corrosion is standing between you and success. You can spend your life calibrating this error out or eliminate the error totally with a 4-wire RTD.

When using 4-wire RTDs, for all practical purposes, there is no error caused by the lead wire. 4-wire RTDs can have a lead wire of any length and the leads can undergo constant resistance change and still cause no measurement error. It is still important to ensure your total resistance does not exceed the drive capacity of your constant current source. Typically modern day temperature transmitters offer enough current drive to support RTD circuits that have up to 3-4K ohms of total resistance. With lead wire error eliminated you are able to focus on the sensor and measuring device to further reduce error.

The only reasonable objection to using the 4-Wire RTD is that the existing legacy input card only accepts 3-wire RTDs. This is old technology and should be considered for replacement.

There is another option to consider if you are not able to use 4-wire RTDs: switch from 100Ω Pt RTDs to 1000Ω Pt RTDs. Ω of resistance imbalance in the current carrying legs of a 100Ω Pt element produces about ±4.7 degrees F (±2.6 degrees C) error. If you change to a 1000Ω sensor that same 1Ω of imbalance will have one-tenth of the effect. The 1Ω of imbalance error drops to about ±0.47 degrees F (±0.26 degrees C).

While the use of the 1000Ω 3-wire RTD is a big improvement over the use of a 100Ω 3-wire RTD, it is not a panacea. When the lead wire resistance imbalance changes, that causes the measurement accuracy to change. That means you still need a calibration program to temporarily eliminate the error. The 4-wire RTD is still the single best solution because it removes all lead wire error and eliminates the need to calibrate due to the inevitable corrosion.

Plant noise

VFDs, motors and radios create “normal” levels of EMI and RFI which can cause errors on temperature measurements. Thermocouple and RTD signals are very low level mV signals. It does not take much noise to cause significant distortion of the measurements. If you are wiring these low level signals back to the control system please use best practices to keep noise off these signal wires by using drain wires, proper grounding and physical separation.

A better solution is to convert the low level signals to high level signals as close to the temperature sensor as possible. The same amount of noise will affect high signals less than low level signals. Signals like 4-20mA, HART or RS-485 survive most typical levels of noise.

Temperature measurement and remote I/O

When you finally get to the actual temperature measurement device your ability to make significant improvements to accuracy has passed. Modern temperature transmitters and temperature I/O systems from major instrument companies have similar performance specifications. If you are trying to differentiate the finer points you might compare these specifications:

  • The greater the input resolution the measuring circuit can detect, the smaller the changes in the sensor’s temperature can be detected
  • Long term drift spec is a measure of the transmitter’s stability 
  • RTD Excitation current should be low to minimize the self-heating error
  • Seek the highest Input Impedance possible so that the measuring device does not draw current
  • Advanced diagnostics help to predict failures

If you are pursuing the very highest accuracy, you have to deal with the final “as built” error in the RTD. The transmitter can be used to calibrate out that final offset error and match you to the ideal curve. Such a process delivers a typical transmitter and sensor combined accuracy of less than 0.05 percent of span.

Putting a temperature transmitter or remote I/O near your sensor digitizes your temperature measurement. You create two more errors if you then send that signal back to the control or data acquisition system using 4-20mA:

1. D/A error occurs when creating the 4-20mA

2. At the control system, an A/D error occurs when turning the signal back to digital Using the HART digital signal is one way to avoid the conversion errors. MODBUS Serial or MODBUS over Ethernet is another option to keep the measured value digital. 

Temperature sensors often require signal boosts for effective transmission.

More from Instrumentation, Systems & Automation

Rotary encoder from Pepperl+Fuchs allows custom configuration

The newly released ENI58PL series incremental rotary encoder from Pepperl+Fuchs offers incredible efficiency for a wide range of applications. IO-Link enables custom configuration, making it possible to commission machinery and equipment even faster. ENI58PL boasts flexible configuration options and a wide range of use cases-from classic industrial applications to special engineering. 

Low-temperature seal and diaphragm options for Swagelok regulators

Swagelok, a provider of fluid system products, assemblies, and services, has announced the release of a new low-temperature seal option. The new seal, a product expansion for the company's RHPS Series pressure regulators line, is designed to maintain a strong seal in cold climates and applications where significant cooling occurs due to high pressure drop. Comprised of low-temperature-resistant nitrile material, these regulator components are ideal for use even in environments with extensive presence of hydrocarbons, such as at large industrial gas suppliers' facilities where climate control may not be an option.

POSITAL adds new communications interface for tilt sensors

POSITAL has added a new communications interface for its innovative Dynamic TILTIX series - making it even easier to use these smart and rugged instruments in mobile machinery and heavy duty construction vehicles. Now available with either CANopen or SAE J1939 interfaces, these acceleration-compensated tilt sensors are able to deliver accurate measurement under the harshest operating conditions, including rapid motions, shock and vibration loadings. 

Honeywell change management software helps increase control system performance

Honeywell announced the latest release of its change management software, offering users across the process industries significant performance and time savings benefits. Honeywell Trace R130 delivers daily snapshots of control system configuration, changes and engineering anomalies that help amplify users' situational awareness, enabling optimization of system management.

Web-based control system sets new standards in process industry

Siemens sets new standards in process automation and is launching an innovative web-based process control system for all industries at the Hannover Messe.  Simatic PCS neo is a brand-new system software, which offers companies in the process industry unique opportunities in the age of digitalization. Main features include global web-based collaboration in engineering and operation and intuitive handling representing all relevant information in a single workbench.

Liquid level for tough chemical service

Dynatrol CL-10GH Liquid Level Detector is the latest addition to the reliable Dynatrol line of high, intermediate and low point level detectors for liquids or slurries. It has no moving parts, floats, diaphragms or packing glands to deteriorate.  Successful applications in the Petroleum Chemical Industry include liquid hydrocarbons: (1) toluene hexane (2) ethanes (3) glycols (4) butadiene.

AW-Lake introduces PCU series of controllers

AW-Lake Company introduces its new PCU Series of Controllers that connect with Vogtlin or any thermal mass flow meter or controller to provide a local display of flow output.  Serving as a single- or multi-channel mass flow controller with up to four channels, the PCU instruments support multiple unit networked operations.   A large, high-contrast backlit display offers easy-to-read views of process variables and programmed setpoints for each connected device on one screen.

Inductive, contactless linear position sensor from Alliance Sensors Group

H. G. Schaevitz LLC, Alliance Sensors Group has released its LV-45 series inductive, contactless linear position sensor using LVIT Technology. The LV-45 linear position sensors are specifically designed for measuring applications requiring rugged devices, whether measuring position of steam turbine valves, mounted in a paper mill head box or calendar roll stand, or outdoors fastened to a building, bridge or structure. The LV-45 series was designed after identifying the wants and needs of engineers in the industrial world, so LV-45 Series linear position sensors can withstand the high vibration and severe shock environment found in steel, aluminum, and paper mills, as well as extremes of temperature and humidity found in most outdoor applications where many other types of linear position sensors cannot survive.


Get our newsletter

Learn more

Schneider Electric partners with Vericlave to strengthen customers' cyber posture

Schneider Electric, the leader in the digital transformation of energy management and automation, has entered into a global partnership with Vericlave, a leading cybersecurity technology provider. Under the terms of the agreement, Schneider Electric will provide Vericlave's advanced encryption technology to further secure and protect its customers' critical IT and OT systems from the risk of cyberattack.

INOR announces IPAQ C/R330 universal temperature transmitter

INOR Process AB announces the INOR IPAQ C/R330 universal temperature transmitter, a modern, isolated, universal temperature transmitter developed for maximum flexibility, accuracy and reliability. Compatible with RTD and thermocouple sensors, Ω and mV inputs, the IPAQ C/R330's robust design offers high vibration resistance and noise immunity, guaranteeing reliable and accurate operation under the most demanding conditions.

INOR Connect app makes configuring signal converter easy

INOR Process AB announces a whole new way to configure and monitor temperature transmitters without supply voltage and cables. Operators can connect to the transmitter with a smartphone or tablet using near-field communication (NFC) or Bluetooth and configure through the new INOR Connect app. Using wireless communication means operators never need to remove the signal converter from the process. Now available are the INOR 330 series temperature transmitters, and the INOR 530 series, fully compatible with HART 7.

New United Electric Controls gas detector offers carbon monoxide monitoring and flexible mounting

United Electric Controls, a producer of safety, alarm and shutdown technology, has announced that it has added carbon monoxide (CO) to the list of gases detected by the latest Vanguard WirelessHART gas detector.  Vanguard version 1.2 also includes new mounting flexibility and a variety of other features that enhance installation, configurability and maintainability.

Flashing sounders from Pfannenberg ideal for fixed gas detection

Pfannenberg, a global manufacturer of thermal management and signaling technologies, highlights the availability of rugged PATROL Series Flashing Sounders for fixed gas detection alarming. Designed to alert personnel of an evacuation when there is presence of hazardous gas, these combined signaling products increase safety when used with ammonia (NH3), carbon dioxide (CO2), freon (R22), methane (CH4), or chlorine (CL2) gas monitoring systems in energy generation, wastewater treatment, landfill applications, food and beverage manufacturing, refrigeration, gas production, agricultural chemical use and storage, and other industrial applications.


Get Our Magazine

Paper or Digital delivered monthly to you

Subscribe or Renew Learn more

Winters’ PEM now available with vibration dampening solution

The PEM-ZR Economy StabiliZR gauge is now available from Winters. The StabiliZR version of the PEM economy gauges is available in 1.5" (40mm) to 4" (100mm) dial sizes, bottom and back connections from 1/8" to 1/4" NPT and ranges from vacuum to 5,000 psi/kPa. PEM-ZR is a general purpose gauge for plumbing, hydraulic and other applications where pulsation and vibration are present.

Siemens showcases smart solutions for industry-specific implementation of Industrie 4.0

Siemens will be placing smart solutions for the industry-specific implementation of Industrie 4.0 at the heart of its Hannover Messe 2019 presentation under the banner "Digital Enterprise - Thinking industry further!" Over an exhibition space of around 4,000 square meters in Hall 9, Siemens will be showcasing these solutions with an array of new additions to its Digital Enterprise offering designed to enable the digital transformation of the discrete and process industries. 

FLIR launches industry-first high-definition handheld cooled optical gas imaging camera for oil and gas inspection professionals

FLIR Systems, Inc. has introduced what it calls the industry's first high-definition (HD), handheld optical gas imaging (OGI) camera, the FLIR GF620. Designed for oil and gas industry professionals, the GF620 camera sets a new standard for detecting and visualizing invisible leaks of hydrocarbons, such as methane, and common volatile organic compounds (VOCs). With four times the pixels of previous models, the HD resolution GF620 helps inspectors survey for fugitive hydrocarbon emissions from further, safer distances than possible with lower-resolution OGI cameras. 

Digital transformation has tangible benefits for businesses and organizations: Schneider Electric study

Schneider Electric has released the Global Digital Transformation Benefits Report 2019, presenting concrete evidence of the power of digitization across the spectrum of global industry, commerce and the public sector. This evidence takes the form of deep, quantifiable, business benefits derived from a repository of 230 customer projects Schneider Electric completed in the last five years across 41 countries - all employing the company's architecture and platform, EcoStruxure.

FLIR launches Its first uncooled methane gas detection camera

The FLIR GF77 Gas Find IR is FLIR Systems, Inc.'s first uncooled thermal camera designed for detecting methane. This handheld camera offers inspection professionals the features they need to find potentially dangerous, invisible methane leaks at natural gas power plants, renewable energy production facilities, industrial plants, and other locations along a natural gas supply chain. The GF77 provides methane gas detection capability at roughly half the price of cooled gas inspection thermal cameras, to empower the oil and gas industry to reduce emissions and ensure a safer work environment.  

Bedrock Automation and Wood partner to advance Open Secure Automation

Bedrock Automation, the maker of the world's most secure open industrial control system (ICS), has established a partnership agreement with Wood, a globally recognized integration provider.  Under the agreement, Wood's automation and control group will deliver Bedrock Open Secure Automation (OSA) to its clients in energy and industrial markets. Wood has active membership in The Open Process Automation Forum, which is focused on the development of a standards-based, open, secure, interoperable process control architecture. 

Wireless gas detectors gain an edge with rising adoption of internet of things technologies

The rising interconnectivity among devices in industries is opening up the market for wireless gas sensors. These sensors can be installed in smartphones and are designed to support Bluetooth, Wi-Fi, near-field communication, and other wireless connections. Their portability, low power requirements, and high-performance potential make them ideal for Internet of Things (IoT) applications, especially in critical and hard-to-reach areas in mining and oil & gas. With further improvements in battery life, ruggedness, form factor, and standardization, wireless gas sensors will find a wider application and revenue-generation potential.

Turbimax from Endress+Hauser delivers robust turbidity measuring in industrial applications

The new Turbimax CUS50D from Endress+Hauser is a highly reliable absorption style sensor for measuring turbidity and suspended solids measurement in hostile environments. It has a robust polymer measuring surface ideal for aggressive media including salt water. The unique dual path length design (5 & 10 mm) ensures effective measurements through a large range of applications including very dark liquids and sludges. This special measuring surface minimizes the risk of dirt accumulating and any surface contamination can be eliminated by the sensor's optional air cleaning system. 


Get our newsletter

Learn more

Vortex flowmeter from KROHNE designed for advanced energy measurement needs

KROHNE, Inc. announces the availability of the new OPTISWIRL 4200 vortex flowmeter, ideal for advanced energy management systems. Used to measure both conducting and non-conducting liquids, gases and steam, the new OPTISWIRL 4200 vortex flowmeter can be used for internal monitoring of energy flows for saturated and superheated steam or hot water, and heat metering applications. It is also a perfect choice for such applications as steam boiler monitoring, burner consumption measurement, or compressed air network monitoring, including free air delivery (FAD) applications.

KROHNE to manufacture OPTIMASS twin bent tube coriolis mass flowmeter

KROHNE, Inc. announces that it has begun manufacturing the OPTIMASS 6400 twin bent tube Coriolis mass flowmeter at its newly opened state of the art flow and level instrument manufacturing and calibration facility located in Beverly, MA. The OPTIMASS 6400 is ideal for standard liquid and gas applications in the chemical and petrochemical, oil and gas, pharmaceutical, food and beverage, and energy and power industries. The OPTIMASS 6400 is equipped with a new signal converter that features advanced device and process diagnostics, compliant to NAMUR NE 107.

Electro Static Technology appoints Mathew Laufik as global sales manager

Electro Static Technology, the manufacturer of AEGIS Shaft Grounding Rings, and specialist in low- to medium-voltage motor protection technology, has appointed Matthew Laufik as Global Sales Manager.  In this new role, Mr. Laufik will be responsible for the sales of AEGIS products to motor repair shops, motor manufacturers and end users in a range of processing industries worldwide through the company's international sales organizations and distribution network.  He will also manage all sales, marketing, and customer service personnel.

Dynatrol introduces the CL-10GP proportional level detector

The Dynatrol CL-10GP Proportional Level Detector is designed specifically to control liquid levels in pilot plants, processing, small vessels, or anywhere it is necessary to obtain proportional level control over a precise range.  The EC-103C(G) Control Unit is paired with the Detector and can activate electro-pneumatic transducers, valve positioners, indicators, controllers or other DC current devices.

Allweiler announces IN-1000 Pump Condition Monitoring Device

Allweiler, a CIRCOR International brand, and a worldwide supplier of pumping solutions for chemical engineering applications, announces the IN-1000 condition monitoring device, ideal for pumps in operationally critical and environmentally sensitive chemical engineering applications. With one master and up to 10 satellite modules per network, the IN-1000 lets operators monitor as many as 11 pumps at once. Built-in features include a two-level warning and alarm system, logging of sensor data, and remote monitoring with a smartphone app. The IN-1000 is available with ATEX certification for use in EX zones.

Festo’s new EFSD electric stopper delivers upfront, downstream cost saving

Festo's new EFSD electric stopper provides exceptional stop-and-release functionality in the movement of product from station to station in assembly and transfer systems. With benefits such as a lower installation cost, faster commissioning and operational energy savings, it is the ideal choice in stop-and-release wherever an electric actuator is preferred to a pneumatic option.


Get Our Magazine

Paper or Digital delivered monthly to you

Subscribe or Renew Learn more