OPERATING INSTRUCTIONS FP-RTD-122 8-Channel, 3-Wire RTD and Resistance Input Module These operating instructions describe the installation, features, and characteristics of the FP-RTD-122. For details on configuring and accessing the FP-RTD-122 over a network, refer to the user manual or programmer reference manual for the particular FieldPoint network module you are using with the FP-RTD-122. Features The FP-RTD-122 is a FieldPoint RTD and resistance input module with the following features: Measures 100 and 1000 Ω platinum RTDs (Resistance Temperature Detectors) Built-in linearization for six TCR (Temperature Coefficient of Resistance, or alpha) values of RTDs Direct resistance measurements in 400 or 4000 Ω ranges True 3-wire compensation, which is more accurate than typical bridge techniques Filtered against 50 and 60 Hz noise 16-bit resolution Hot plug and play operation 3,000 V input isolation Double insulated for 250 V safe working voltage 40 to +70 C operation RTDs are known for their accuracy, repeatability, linearity, and long term stability. Each input channel of the FP-RTD-122 can be used to measure the temperature of platinum RTDs or to measure FieldPoint is a trademark of National Instruments Corporation. Product and company names mentioned herein are trademarks or trade names of their respective companies. 322033B-01 Copyright 1998, 1999 National Instruments Corp. All rights reserved. June 1999
the resistance of the RTD or other resistive device directly. The inputs are all filtered against 50 and 60 Hz noise, both common mode and normal mode. The 3-wire compensation works with the most common RTDs and is more accurate than typical bridge techniques. Installation The FP-RTD-122 mounts on a FieldPoint terminal base (FP-TB-xx) unit. The hot plug and play operation of the FP-RTD-122 allows you to install it onto a powered terminal base without disturbing the operation of other modules or terminal bases. The FP-RTD-122 receives operating power from the terminal base. To install the FP-RTD-122, refer to Figure 1 and follow these steps: 1. Slide the terminal base key to either position X (used for any module) or position 1 (used for the FP-RTD-122 module). 2. Align the FP-RTD-122 alignment slots with the guide rails on the terminal base. 3. Press firmly to seat the FP-RTD-122 on the terminal base. The terminal base latch locks the FP-RTD-122 into place when it is firmly seated. Key Latch I/O Module Alignment Slot Guide Rails Terminal Base Figure 1. Module Installation Diagram FP-RTD-122 2 www.natinst.com
Field Wiring The terminal base provides connections for each of the three wires of a 3-wire RTD, but may be used with 4-wire and 2-wire RTDs. There are excitation (EXCITE), sense (SENSE) and common (COM) terminals. The wiring connections depend on the number of wires on the RTD you are using, as follows: 3-wire RTDs often have a wire of one color (usually white, sometimes red) for the positive excitation, which should be connected to the EXCITE terminal, and two wires of another color (usually red, sometimes black), which should be connected to the SENSE and the COM terminals. 4-wire RTDs usually have two white wires and two red wires. Leave one of the wires unconnected and connect the remaining three as you would for the 3-wire RTD. 2-wire RTDs should have one wire (either one) connected to the EXCITE terminal and the other wire to the COM terminal, and a short jumper wire connected between the COM and the SENSE terminals. If you are using shielded wiring, you may connect the shield to the COM terminal, but only if the shield is not connected to any of the wires at the RTD end. Figure 2 shows examples of basic wiring connections. The COM terminals of all the channels are internally connected together and are connected to the terminals labeled C on the terminal base. National Instruments Corp. 3 FP-RTD-122
Excite (0) Sense (0) Com (0) 3-wire RTD Excite (1) Sense (1) Com (1) Don't Connect 4-wire RTD Excite (2) Sense (2) Short Jumper Wire 2-wire RTD Com (2) Figure 2. Basic Field Connections with Three Channels Showing 2-, 3-, and 4-Wire Configurations Table 1 lists the terminal assignments for the signals associated with each channel. Channel Table 1. Terminal Assignments Terminal Numbers EXCITE SENSE COM 0 1 2 18 1 3 4 20 2 5 6 22 3 7 8 24 4 9 10 26 5 11 12 28 6 13 14 30 7 15 16 32 FP-RTD-122 4 www.natinst.com
Input Circuit The FP-RTD-122 has eight 3-wire input channels. All eight channels share a common ground that is isolated from other modules of the FieldPoint system. Each of the channels pulses a 0.25 ma current out of the EXCITE terminal, which is returned through the RTD via the COM terminal. The SENSE terminal is used to measure the RTD resistance and to compensate for lead resistance errors. If shielded signal wires are used, the shield should be tied to this COM terminal. Each channel is filtered, then sampled by a 16-bit analog-to-digital converter. Figure 3 shows a diagram of the input circuit for a single channel. FP-RTD-122 Excite 16-bit ADC 3-wire Compensation Pulsed 0.25 ma RTD Sense Com Figure 3. FP-RTD-122 Input Circuit Resistance Measurements You can use the FP-RTD-122 to measure resistance directly in ohms. This capability may be useful either for RTDs that the FP-RTD-122 does not directly support (such as 120 Ω nickel RTDs) or for measuring resistive devices other than RTDs. There are two resistance ranges that may be chosen: 400 and 4000 Ω. Any resistance value outside of the selected range, including an open circuit, results in an Out of range error being reported by the FP-RTD-122 for the affected channels. The FP-RTD-122 ignores any configuration of RTD type when one of these ranges is selected. Temperature Measurements The FP-RTD-122 can directly measure the temperature of a Platinum RTD of either 100 or 1000 Ω nominal resistance, and for 6 TCR (or alpha, α) values. The TCR of an RTD is the average temperature coefficient of resistance of the RTD from 0 to 100 C National Instruments Corp. 5 FP-RTD-122
and is the most common method of specifying the behavior of an RTD. The next section, RTD Types, has more information on the types of RTDs supported by the FP-RTD-122. The FP-RTD-122 linearizes the resistance value and returns a reading in units of temperature. The available ranges are 73 to 1123 K, 200 to +850 C, and 328 to +1562 F. You may configure each channel independently, so you can connect different types of RTDs to each channel. Note You must configure each channel of the FP-RTD-122 for the appropriate RTD type. The FP-RTD-122 cannot identify the types of RTDs connected to it on its own. RTD Types RTDs are generally specified by their material, nominal resistance at 0 C, and TCR. The FP-RTD-122 can directly measure the temperature of Platinum RTDs of either 100 or 1000 Ω nominal resistance. These RTDs are commonly referred to as PT100 or PT1000 RTDs, respectively. Platinum RTDs can be made with a number of different TCRs. The TCRs supported by the FP-RTD-122 are 3.750, 3.851, 3.911, 3.916, 3.920, and 3.928 mω/ω/ C. The TCR of 3.851 mω/ω/ C is the most common and is defined in international standards such as IEC-751, DIN43760, BS1904, and ASTM E1137. The TCR of 3.928 mω/ω/ C is used in the reference function for platinum thermometers in the International Temperature Scale of 1990 (ITS-90) for high-accuracy metrology applications. Unfortunately, the other TCR values are not all as well defined by standards organizations, and the behavior of RTDs of the same TCR value may vary from vendor to vendor. The variations are usually small, and the built-in linearization algorithms of the FP-RTD-122 are appropriate for nearly all applications. The FP-RTD-122 uses a linearization curve known as the Callendar-Van Dusen equation to measure the temperature of RTDs. The equation is as follows: Temperatures below 0 C: R T = R 0 [ 1 + A T + B T 2 + C T 3 ( T 100 C) ] FP-RTD-122 6 www.natinst.com
Temperatures above 0 C: R T = R 0 [ 1 + A T + B T 2 ] T = temperature in C R T = RTD resistance at temperature T R 0 = RTD nominal resistance at 0 C A, B, C are coefficients given in Table 2 Table 2 lists the coefficients used in this equation for each of TCR values. If you have a non-standard RTD that does not match one of these linearization curves, you can still measure its resistance with the FP-RTD-122 and convert it to temperature in the manner suggested by the RTD vendor. Table 2. Callendar-Van Dusen Coefficients Used by the FP-RTD-122 TCR mω/ω/ C A ( C) 1 B ( C) 2 C ( C) 4 3.750 a 3.81 10 3 6.02 10 7 6.0 10 12 3.851 b 3.9083 10 3 5.775 10 7 4.183 10 12 3.911 c 3.9692 10 3 5.8495 10 7 4.233 10 12 3.916 d 3.9739 10 3 5.870 10 7 4.4 10 12 3.920 e 3.9787 10 3 5.8686 10 7 4.167 10 12 3.928 f 3.9888 10 3 5.915 10 7 3.85 10 12 Notes: a. Often used in low-cost PT1000 applications b. Defined in IEC-751, 1995; also used by DIN 43760, BS 1904, and ASTM E1137 c. Sometimes referred to as American or US Industrial Standard d. JIS C 1604 e. Sometimes called US Industrial Standard D100 f. ITS-90 International Temperature Scale. The FP-RTD-122 uses the ITS-90 reference function for this setting. The coefficients in the chart are the best fit of this function to the Callendar-Van Dusen equation. National Instruments Corp. 7 FP-RTD-122
3-Wire Compensation of Lead Resistance Errors The FP-RTD-122 uses a 3-wire compensation technique to compensate for the lead resistances. With this technique, the SENSE lead is used to measure the resistance of the return COM lead. If the EXCITE lead has the same resistance as the COM lead, the FP-RTD-122 corrects for effects of the leads. Using this technique, the only residual errors are those due to the mismatch of the EXCITE and COM leads. Typical RTD specifications are for the lead resistances to be matched to within 5% of each other, so this technique corrects for 95% or more of the errors introduced by lead resistances. This is a more accurate method than the typical bridge completion methods described in many reference books. The bridge methods not only have the same sensitivity to lead resistance mismatch, but also are effective only for temperatures very near those at which the bridge is balanced (usually 0 C). The temperature measurement accuracy specifications for the FP-RTD-122 at the end of these instructions include the effects of a typical application using 10 m of 22 gauge copper wire (approximately 0.5 Ω per lead), with 5% mismatch in the lead resistances. If you are using leads with greater resistances, the additional errors are approximately 3 C per Ω of mismatch in the lead resistances for 100 Ω RTDs, and 0.3 C per Ω of mismatch in the lead resistance for 1000 Ω RTDs. For example, for 2 Ω leads matched to 5% of each other, the lead resistance mismatch is 5% 2 Ω = 0.1 Ω, which would cause 0.3 C of error in measurements of a 100 Ω RTD. If you are using the FP-RTD-122 with 2-wire RTDs, the errors due to lead resistances are much greater, because 3-wire compensation is not used. With 2-wire RTDs, the additional errors are approximately 3 C per Ω of the sum of the lead resistances for 100 Ω RTDs, and 0.3 C per Ω of the sum of lead resistances for 1000 Ω RTDs. For example, a 1000 Ω, 2-wire RTD with 2 Ω leads would have a total lead resistance of 4 Ω (2 Ω per lead) which would cause 1.2 C of error. FP-RTD-122 8 www.natinst.com
Status Indicators Figure 4 shows the module label and status indicators. You can remove the insertable label to see wiring diagrams for the input channels. Figure 4. Status Indicators and Module Label The FP-RTD-122 has two green status LEDs (labeled POWER and READY). After the FP-RTD-122 has been inserted into a terminal base and power has been applied to the connected network module, the green POWER indicator lights and the FP-RTD-122 informs the network module of its presence. When the network module recognizes the FP-RTD-122, it sends initial configuration information to the FP-RTD-122. After receiving this initial information, the green READY indicator lights and the FP-RTD-122 is in its normal operating mode. Isolation and Safety Guidelines Caution Read the following information before attempting to connect any circuits that may contain hazardous voltages to the FP-RTD-122. This section describes the isolation of the FP-RTD-122 and its compliance with international safety standards. The FP-RTD-122 field wiring connections are isolated from the backplane of the terminal base with an optical and galvanic isolation barrier designed and tested to protect against fault voltages of up to 3000 Vrms. In addition, the FP-RTD-122 provides double insulation (compliant to IEC 1010-1) for working common-mode voltages of 250 Vrms. Safety standards (such as those published by UL and IEC) require the use of double insulation between National Instruments Corp. 9 FP-RTD-122
hazardous voltages and any human-accessible parts or circuits. You should never attempt to use any isolation product between human-accessible parts (such as DIN rails or monitoring stations) and circuits that may be at hazardous potentials under normal conditions, unless the product is specifically designed (as the FP-RTD-122 is) for such an application. Even though the FP-RTD-122 is designed to handle applications with hazardous potentials, follow these guidelines to ensure a safe total system. Do not share the external supply voltages (V and C on the terminal base) with other devices including other FieldPoint devices unless those devices are also isolated from human contact. As with any hazardous voltage wiring, ensure that all wiring and connections meet with applicable electrical codes or common sense practices. Mount terminal bases in an area, position, or cabinet that prevents accidental or unauthorized access to wiring with hazardous voltages. The isolation of the FP-RTD-122 is certified as double insulated for normal operating voltages of 250 Vrms. Do not use the FP-RTD-122 as the sole isolating barrier between human contact and working voltages of more than 250 Vrms. Specifications The following specifications are typical for an operating temperature of 40 to +70 C, unless otherwise noted. Input Characteristics Number of channels...8 ADC resolution...16 bits Type of ADC...Delta-sigma Input signal ranges (software-selectable per channel) Temperature...73 to 1123 K 200 to +850 C 328 to +1562 F Resistance...0 to 400 Ohms or 0 to 4000 Ohms FP-RTD-122 10 www.natinst.com
Temperature accuracy (includes 5% matched, 0.5 Ω lead wires 10 m of 22 AWG copper) Measured Value Error 15 to 35 C 40 to 70 C Typical Maximum Typical Maximum 200 to +150 C 0.15 0.30 0.40 1.6 +150 to +850 C 0.25 0.50 0.90 3.0 Resolution...0.016 C Resistance accuracy Offset error, 400 Ω range 15 to 35 C...0.03 Ω typical, 0.08 Ω max. 40 to 70 C...0.08 Ω typical, 0.4 Ω max. Offset error, 4000 Ω range 15 to 35 C...0.2 Ω typical, 0.7 Ω max. 40 to 70 C...0.8 Ω typical, 4.0 Ω max. Gain error 15 to 35 C...0.015% typical, 0.03% max. 40 to 70 C...0.06% typical, 0.13% max. Resolution 400 Ω range...0.0061 Ω 4000 Ω range...0.061 Ω Excitation current...135 ms pulses of 0.25 ma every 1080 ms Input noise...±1 bit pk-pk Input bandwidth...3 Hz Update rate...each channel is updated every 1.08 s Physical Indicators...Green POWER and READY LEDs Weight...140 g (4.8 oz.) Power Requirements Power required from network module...350 mw National Instruments Corp. 11 FP-RTD-122
Environment Operating temperature... 40 to +70 C Storage temperature... 55 to +85 C Relative humidity...5% to 90% noncondensing CE Mark Compliance This product meets applicable EU directive(s) as follows: Safety isolation...en 61010 (double insulation for 250 Vrms working isolation, installation category II) EMC directive Immunity...EN 50082-1:1994 Emissions...EN 55011:1991 Group I Class A at 10 m Mechanical Dimensions Figure 5 shows the mechanical dimensions of the FP-RTD-122 installed onto a terminal base. Dimensions are given in inches [millimeters]. 4.22 [107.19] 4.31 [109.5] 3.60 [91.44] Figure 5. Mechanical Dimensions