RTD SENSOR Temperature

Typical Assembly of Transmitter and Sensor Head Headmount Transmitter Cable Entry Extension length Head connection Instrument Connection Thread Thermowell In many cases, particularly in Europe, the preferred method of assembly is to mount the transmitter directly to a sensor in a connection head. The head - mounted transmitter in this case is designed such that the head provides most of the protection from the environment. RMD offers a range of head-mount transmitters from simple analogue fixed range (244R) through programmable analogue (244P, 244E) to HART (644). Immersion length (U-length) Stem Mounting arrangement

RTD Sensors What is an RTD ? Resistance Temperature Detector Operation depends on inherent characteristic of metal (Platinum usually): electrical resistance to current flow changes when a metal undergoes a change in temperature. If we can measure the resistance in the metal, we know the temperature! Platinum resistance changes with temperature Have one of the experienced sales people answer this. Thin-film sensing element on ceramic substrate Wire-wound sensing element Rosemount’s Series 65 68, 58 Rosemount’s Series 78, 88 Two common types of RTD elements:

RTD Sensors What is a RTD Element ? Class A RTD Wire Wound Class B RTD Thin Film M. I. Cable (Mineral Insulated Cable), SST or Inconel depending on temperature

Rosemount RTD Construction Sensor Differentiation Explosion Proof Approvals Calibration Services RTD Technology: Proprietary method of packing, O-ring Molded rear house assembly Al2O3 packing for protection against vibration O-ring for protection against moisture

Rosemount RTD Differentiation Sensor Response Time Improved Type of element Platinum Rosemount has externally wound RTD for faster response time Thin-film has slightly faster response time than wirewound Element packaging Rosemount RTD’s are packed in aluminum oxide to provide optimum thermal conduction within the sheath Grounded thermocouples are twice as fast as ungrounded Sheath thickness and material Rosemount uses 316SST and Inconel (for high temperatures) for sheath; both are very good thermal conductors

RTD Sensors Interchangeability Class B Class A Class A = 0.15 + 0.002 |t| Class B = 0.3 + 0.005 |t|

RTD Sensor Why is Platinum used ? It is the most stable & near linear resistance versus temperature function when compared to other metals like thermistor, Nickel & Balco

Question What does it mean Pt100,  = 0.0385? Pt = Platinum  0.0385 = 0 deg. C the probe will read 100 ohms. at 100 deg. C, it will read 138.5 ohms.

RTD Sensors : Wiring 4 - wire to 3 or 2 wire For 3-wire systems use one white and two red leads. Do not common White leads. Insulate or terminate the unused wire in a manner that avoids short circuiting to ground or earth For 2-wire systems common both sets of leads

Vol I - RTD Series Temperature vs  relationships and Tolerances confirm to IEC 751. RED RED WHITE WHITE All Single Element Sensors are supplied as 4 Wire WHITE RED GREEN BLACK All Duplex Element Sensors are supplied as 2 x 3 Wire Customers may remove excess leads as necessary to create 2 and 3-wire configurations as notified with shipping instructions. When converting to 3-wire configuration two leads should not be commoned or twisted together as this will cause a significant measurement error. DUPLEX contains two platinum sensing resistors and are currently only available as 2 x 3-wire output leads. The duplex sensor type is often used where the customer requires redundancy allowing a higher reliability particularly where a second transmitter is also used. The 3244MV hot back-up feature can also be used with this sensor. In environments of high vibration belief in this approach to redundancy may be misplaced 0.25 inches sheath diameter

Vol II - Series 65 RTD Temperature vs  relationships and Tolerances confirm to IEC 751. RED WHITE Note: Wire colors different from Vol.1 All Single Element Sensors are supplied as 4 Wire RED BLACK BLUE GREEN All Duplex Element Sensors are supplied as 2 x 3 Wire Range: -50 to 450°C Standard 6mm sheath diameter 321 SST sheath material Class B Tolerance Customers may remove excess leads as necessary to create 2 and 3-wire configurations as notified with shipping instructions. When converting to 3-wire configuration two leads should not be commoned or twisted together as this will cause a significant measurement error. DUPLEX contains two platinum sensing resistors and are currently only available as 2 x 3-wire output leads. The duplex sensor type is often used where the customer requires redundancy allowing a higher reliability particularly where a second transmitter is also used. The 3244MV hot back-up feature can also be used with this sensor. In environments of high vibration belief in this approach to redundancy may be misplaced

RTD Sensors 2, 3, & 4-Wire RTDs Why use a 2-, 3-, or 4- wire RTD? 2-wire: Lowest cost -- rarely used due to high error from lead wire resistance 3-wire: Good balance of cost and performance. Good lead wire compensation. 4-wire: Theoretically the best lead wire compensation method (fully compensates); the most accurate solution. Highest cost. 4-wire RTD Typically use copper wires for extension from the sensor Find the error in a 2-wire: 18 A.W.G. copper wire, Resistance = 6.385 Ohms/ 1000 ft -Assuming a length of 20 ft (6 meters), this adds 0.33oC error to the measurement. If the gage wire is larger (e.g. 22), the error shoots up to 0.84oC! Red White Green Blue Black Red Sensing Element (I.e. wire-wound, thin film)

Temperature Sensor : Stability, Repeatability & Linearity How does these factors affect Your Measurement ? Inconsistent Quality Introduce downstream variation Waste raw material - rework or disposal ! Lost Production High energy cost Lost profits IEC 751 Curve Temperature (oC) Resistance (W)

Temperature Sensor : Stability Stability is expressed as drift in the temperature reading per unit time, as a drift in resistance per unit time expressed in ohms or as a percentage of resistance. Stability specifications are often define in terms of a temperature exposure history. IEC 751 defines Stability as the limit of drift after 250 hours of exposure to full scale temperature. < 0.005 Deg C/year for laboratory grade <0.1 Deg C of FS/year for thin film Platinum RTD for normal usage <0.05 Deg C per 5-years for temperature ranges -40 to 125 Deg C

Temperature Sensor : Repeatability Repeatability is the range of output that the PT RTD will give when arriving at a target temperature in repetitive cycles from the same direction. Bi-directional repeatability is the output range when a target temperature is approached from two directions. Hysterisis is the difference in the mean values of the two direction dependent output ranges when arriving at a target temperature in repetitive cycles from both directions. IEC 751 defines Repeatability to be the drift observed at 0 Deg C after a sensor experience 10 cycles over its full operating temperature range.

RTD Sensors Resistance changes are repeatable How does a RTD works? Resistance Temperature Detector The resistance changes of the platinum wiring can be approximated by an ideal curve -- the IEC 751 International Resistance vs. Temperature Chart: oC Ohms 0 100.00 10 103.90 20 107.79 30 111.67 IEC 751 Resistance (Ohms) Have one of the experienced sales people answer this. IEC 751 Temperature (oC) Page 3-2: Sensors PDS (V.1)

Temperature Sensor : Linearity Linearity is defined as the deviation of the output-versus- temperature curve, from the best fit linear approximation of the devices behaviour over the operating temperature range. It is expressed as either a percentage of full scale output or as the maximum deviation (either in Ohms or Deg C for an RTD) IEC 751 Curve Temperature (oC) Resistance (W)

RTD Sensor Joining Techniques Soldering Crimping Brazing Spot welding Laser welding Recommended Temperature Range - 30 to 120 Deg C - 50 to 200 Deg C - 50 to 400 Deg C - 50 to 600 Deg C - 50 to 850 Deg C Comments remove flux to prevent corrosion more than 180 Deg C high melting soldering material is used max. allowable tensile strength of the wires used are not exceeded Do not ‘burn’ the RTD chip during blazing process. Remove flux to prevent corrosion. Both spot & laser welding are most secure & temperature resistant. A high level of skill & manufacturing know how with strict adherence to manufacturing parameters.

Temperature : International Standards

What other consideration? The useful range of the thermocouple type. The sheath material maximum operating temperature. The de-rate upper temperature limit due to sensor diameter. The same element with different sheath diameter will affect temperature limit as well. Example : Thermocouple Type K Sheath diameter : 5mm Sheath material : Inconel Measuring range : 0 to 1100deg. C For this combination, the sensor will fail although maximum measuring range is 1372 deg. C. The temperature rating of other components such as connectors, transition joints and wire. Sensor elements for industrial applications must be packaged ruggedly. RMD standard sensors are all of mineral insulated cable design with 6 mm sheath diameter. RTD ( Resistance Temperature Detector) sensors using platinum resistance elements provide the most stable and reliable measurement over the range -200 to 500 C and should always be specified by choice. Sheath material is AISI 321 stainless steel. Model 65, using platinum film type element, has its’ range reduced to meet the requirements of the international specification IEC 751:1983 INDUSTRIAL PLATINUM RESISTANCE THERMOMETER SENSORS. ( including Amendments A1:1986 and A2:1995). Model 75, using a wire-wound element will meet IEC 751 over an extended temperature range. Grade B tolerance class is offered as standard with Grade A as an option. Thermocouples must be offered for temperatures in excess of 500 C. Up to 1100 C type K should be specified. Sheath material must be suitable for these high temperatures and Inconel is available as standard. Thermocouples meet the requirements of IEC 584 :1995 THERMOCOUPLES, and are available to tolerance class 2 as standard. Other tolerance classes, requiring specially selected materials, are available as specials. For temperatures above 1100 C thermocouples , such as types R or S, are available. These are always supplied as specials as the high temperature environment determines the many thermocouple and sheath material options that will be required.

Sensor Lead-Wire Length : RTD Sensor Direct Wiring : Recommended Maximum 250 feet using 18 AWG lead wire For 3-wire RTD, maximum error 0.16 Deg F per 100 feet using 18 AWG lead wire. For best RTD wiring practice to reduce error : use same specification & run-length lead-wires

Temperature Point Response Time Factors Affecting Temperature Point Response Time Thermowell Sensor Thermowell Transmitter Process 75.4 °C Sensor Process Transmitter

Factors Affecting Sensor Response Time Sensor Time Response Factors Affecting Sensor Response Time Type of element Wirewound RTD externally or internally wound Thin-film RTD Thermocouple Element packaging Element coating, potting Contact between element package & sheath Sheath thickness and material OD ceramic bore element sheath Al2O3 packing

Sensor Time Response Industry Average Rosemount (Series 78) wirewound RTD 6.0 - 7.0 s 4.7 s (Series 65 & 68) thin-film RTD 5.0 - 5.5 s 3.38 s Ungrounded thermocouples < 2 s < 2 s Grounded thermocouples < 1 s < 1 s * All results based on standard conditions: time required to reach 63.2% sensor response for water flowing at 3 ft/sec. RTD response times shown are the average + 6 sigma.

Factors Affecting Response Time of Sensors in Thermowells Thermowell design style (thickness at tip) Stepped is the fastest Contact between sensor sheath and thermowell (x and y) Spring loaded sensor ensures contact at the tip (x=0) Industry practice suggests using thermally conductive fill can significantly reduce time lag x y Thermowell Sensor Assembly Thermally Conductive Fill

Time Response for Sensors in Thermowells Tapered thermowell 26 seconds Stepped thermowell 22 seconds Industry data shows stepped t-well with fill = 11 seconds * Based on externally published data for time required to reach 63.2% sensor response for water flowing at 3 ft/sec., with a sensor response time of 5.5 seconds. Time response of assembled point is not additive.

Factors Affecting Transmitter Response Time Time response depends on element (complexity of calculation) 2-wire RTD 440 - 760 ms 3 & 4-wire RTD 520 - 920 ms Thermocouples 300 - 750 ms Above is good provided the analog output changes less than 2% Transmitter update time (output) every 500msec Process Transmitter 75.4 °C

Process Factors in Temperature Response Time Velocity of the material Thermal conductivity of the material Density and viscosity of the material Process time constants can be from seconds to hours: 75.4 °C Water @ 3 fps t = 3.38 s Air at 50 fps, 40-80oC = 38.0 s Oil agitated in a bath: t = 43.0 s Oil not agitated: t = >3 minutes Process

Response Time Sensor < 1 to 4.7 sec Sensor in Thermowell 22 to 26 sec Transmitter .5 to .9 sec Process Seconds to Hours Thermowells and process material/conditions have the greatest effect on temperature point response time We are working on a method to predict time response of a temperature point through modeling of the time response in two known media

Frequently Asked Questions Why should I buy an RTD when TC are more rugged & less expensive ? RTDs are more accurate, drift less over time & resistance to noise. Lower cost copper lead-wires compare the expensive TC-wires. What is RTD & PRT ? RTD is resistance temperature detector. It may use platinum, nickel or copper for its element. A PRT is a platinum resistance thermometer, an RTD that uses platinum for its element. What is the maximum distance between a PRT & a recorder or controller without using a transmitter ? We recommend 250 feet using at least 18 AWG lead wire without a transmitter. Further details can be check with recorder or controller supplier.

Frequency Asked Questions How far can I run the signal with a temperature transmitter ? The only limitation is the transmitter’s minimum voltage (12 VDC) requirement at the terminals. A power supply must overcome the lead wire resistance. A long lead wire can act as an atenna, picking up stray electrical signal & causing RFI & EMI. Twisted shielded wire should be used for long runs or if the wires run next to other wires or electric motors. What is the minimum immersion length for an RTD ? Rule of thumb : At least 10-times greater than the diameter of the sensor or thermowell plus the sensitive length of the PRT. This is to minimise stem conduction errors caused by heat conduction along the sheath or leadwires of a probe or along the length of a thermowell. The heat conduction increases or decreases the measured temperature depending on the applications & environmental conditions.

Frequently Asked Questions How is the calibration of a PRT confirmed ? Simply measure the sensor’s ice point resistance (0 Deg C or 32 Deg F) and compare the measured value to its calibrated or previous value. If the ice-point resistance increase, it is a sign that the probe is being stressed (vibration or shocked) or that the probe is used beyond its rated temperature. A decrease in ice point resistance usually signals a problem with the moisture seal on the element. What is the difference between a .00385 & a .003902 probe ? The only difference is the amount that the resistance changes per degree of temperature. Both probes will read 100 ohm at 0 Deg C, but at 100 Deg C the .00385 probe will read 138.5 ohm and the .003902 probe will read 139.02 ohm. The sensor must be properly matched to the easuring device to obtain an accurate reading

Frequently Asked Questions How can I determine if I have a .00385 or .003902 temperature coefficient ? The colour coding on the wires. White, Red, Red - .00385 Red, White, White - .003902