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National Instruments SCXI-1581 Скачать руководство пользователя страница 35

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National Instruments Corporation

B-1

SCXI-1581 User Manual

B

Measuring Temperature with
Resistive Transducers

This appendix discusses RTDs and thermistors and describes how to
connect resistive transducers to the signal conditioning system.

RTDs

A resistive-temperature detector (RTD) is a temperature-sensing device
whose resistance increases with temperature. An RTD consists of a wire
coil or deposited film of pure metal. RTDs are made of different metals and
have different resistances, but the most popular RTD is made of platinum
and has a nominal resistance of 100

Ω

at 0 °C.

RTDs are known for their excellent accuracy over a wide temperature
range. Some RTDs have accuracies as high as 0.01

Ω

(0.026 °C) at 0 °C.

RTDs are also extremely stable devices. Common industrial RTDs drift less
than 0.1 °C/year, and some models are stable to within 0.0025 °C/year.

RTDs are sometimes difficult to measure because they have relatively low
nominal resistance (commonly 100

Ω

) that changes only slightly with

temperature (less than 0.4

Ω

/°C). To accurately measure these small

changes in resistance, you must use special configurations that minimize
measured errors caused by lead-wire resistance.

Because an RTD is a passive resistive device, you must pass a current
through it to produce a measurable voltage. This current causes the RTD to
heat internally, which appears as an error. Self-heating is typically specified
as the amount of power that raises the RTD temperature by 1 °C, given in
units of mW/°C. You can minimize self-heating by using the smallest
possible excitation current. The amount of self-heating also depends
heavily on the medium into which you immerse the RTD. An RTD can
self-heat up to 100 times higher in still air than in moving water.

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Содержание SCXI-1581

Страница 1: ...SCXI TM SCXI 1581 User Manual SCXI 1581 User Manual March 2003 Edition Part Number 323074B 01...

Страница 2: ...any 49 0 89 741 31 30 Greece 30 2 10 42 96 427 India 91 80 51190000 Israel 972 0 3 6393737 Italy 39 02 413091 Japan 81 3 5472 2970 Korea 82 02 3451 3400 Malaysia 603 9131 0918 Mexico 001 800 010 0793...

Страница 3: ...ation or maintenance instructions owner s modification of the product owner s abuse misuse or negligent acts and power failure or surges fire flood accident actions of third parties or other events ou...

Страница 4: ...with the product for precautions to take bold Bold text denotes items that you must select or click in the software such as menu items and dialog box options Bold text also denotes parameter names it...

Страница 5: ...I Using Traditional NI DAQ 1 7 Manually Adding and Configuring Modules in Traditional NI DAQ 1 9 Verifying and Self Testing the Configuration 1 9 Configuring SCXI Using NI DAQmx 1 10 Manually Adding a...

Страница 6: ...pendix C Common Questions Glossary Index Figures Figure 1 1 Installing the SCXI 1581 Module 1 4 Figure 1 2 Connecting an SCXI Chassis to an E Series DAQ Device 1 6 Figure 1 3 Removing the SCXI 1581 1...

Страница 7: ...nstruments Corporation vii SCXI 1581 User Manual Tables Table 2 1 Front Signal Pin Assignments 2 2 Table 2 2 Signal Descriptions 2 3 Table B 1 Callendar Van Dusen Coefficients B 3 Table C 1 SCXI 1581...

Страница 8: ......

Страница 9: ...s 100 A fixed current excitation The SCXI 1581 is configured using Measurement Automation Explorer MAX What You Need to Get Started To set up and use the SCXI 1581 you need the following items SCXI 15...

Страница 10: ...oad or you can contact a National Instruments sales representative to request it on a CD Installing the SCXI 1581 The following section describes how to install the SCXI 1581 for use with SCXI chassis...

Страница 11: ...the SCXI 1581 module into the SCXI chassis complete the following steps while referring to Figure 1 1 1 Power off the computer that contains the E Series DAQ device or disconnect it from the SCXI cha...

Страница 12: ...anning in an SCXI Chassis In this configuration you can multiplex the input channels of the analog input SCXI modules into a single channel of the E Series DAQ device You need the following items for...

Страница 13: ...multaneous sampling capability you will get an error message each time you run your application Note The SCXI 1581 is not an analog input module To connect the chassis to an E Series DAQ device for mu...

Страница 14: ...eries DAQ Device for Multiplexed Scanning in a PXI Combination Chassis In this configuration you can multiplex the input channels of the SCXI analog input modules into a single channel of the E Series...

Страница 15: ...described in the Connecting the SCXI 1581 to an E Series DAQ Device for Multiplexed Scanning in an SCXI Chassis section 2 Power on the SCXI portion of the PXI combination chassis You should have alrea...

Страница 16: ...sing multiple chassis or one or more of the chassis address DIP switches is not in the OFF position refer to the SCXI Chassis User Manual for more information c If you are using a PXI 1010 or PXI 1011...

Страница 17: ...een instructions to complete the configuration When you have clicked Finish the module configuration is complete 7 Repeat steps 2 through 6 until you have added all the modules The SCXI chassis and SC...

Страница 18: ...ssis go to the Manually Adding and Configuring Modules in NI DAQmx section 3 Add a new chassis to the configured devices and interfaces by holding down the right mouse button on NI DAQmx Devices and s...

Страница 19: ...he Cabling tab 4 When you have finished click OK 5 When you have completed configuring the module click OK The SCXI chassis and SCXI module s should now be configured properly If you need to change th...

Страница 20: ...connected to device x If you get the message Failed to find followed by the module codes and the message Unable to communicate with chassis 1 Make sure the SCXI chassis is powered on 2 Make sure the...

Страница 21: ...section and retest the SCXI chassis If these measures do not successfully configure the SCXI system refer to Appendix C Common Questions for more information Removing the SCXI 1581 This section provi...

Страница 22: ...al instructions and cautions To remove the SCXI 1581 module from an SCXI chassis complete the following steps while referring to Figure 1 3 1 Power off the SCXI chassis Do not remove the SCXI 1581 mod...

Страница 23: ...s by expanding Devices and Interfaces a If the module is installed in a Traditional NI DAQ chassis click the next to the Traditional NI DAQ Devices icon b If the module is installed in a NI DAQmx chas...

Страница 24: ...on information for these modules is also lost 4 You are presented with a confirmation window Click Yes to continue deleting the module or chassis or No to cancel this action The SCXI chassis and or SC...

Страница 25: ...accurate to within 0 05 of the specified value with a temperature drift of no more than 5 ppm C The high accuracy and stability of these current sources makes them especially well suited to measuring...

Страница 26: ...EX9 EX9 22 NC EX10 EX10 21 NC EX11 EX11 20 RSVD EX12 EX12 19 RSVD EX13 EX13 18 NC EX14 EX14 17 NC EX15 EX15 16 NC EX16 EX16 15 NC EX17 EX17 14 NC EX18 EX18 13 NC EX19 EX19 12 NC EX20 EX20 11 NC EX21 E...

Страница 27: ...eries DAQ device and the SCXI system are SERDATIN SERDATOUT DAQD A SLOT0SEL SERCLK and SCANCLK The digital ground DIGGND on pins 24 and 33 provides a separate ground reference for the communication si...

Страница 28: ...signals from other SCXI modules to the E Series DAQ device The SCXI 1581 routes multiplexed signals from other SCXI modules to the E Series DAQ device even though the SCXI 1581 does not have any anal...

Страница 29: ...XI 1581 does not have any input channels to scan multiplexed scanning is the only scanning mode that is possible when the SCXI 1581 is the cabled module If the module you are scanning is directly cabl...

Страница 30: ......

Страница 31: ...herwise noted Stability Recommended warm up time 10 minutes Excitation Channels 32 single ended outputs Current output 100 A Accuracy 0 05 Temperature drift 5 ppm C Output voltage compliance 10 V Maxi...

Страница 32: ...e requirements of the following standards for safety and electrical equipment for measurement control and laboratory use IEC 61010 1 EN 61010 1 UL 3111 1 UL 61010B 1 CAN CSA C22 2 No 1010 1 Note For U...

Страница 33: ...European Directives as amended for CE marking as follows Low Voltage Directive safety 73 23 EEC Electromagnetic Compatibility Directive EMC 89 336 EEC Note Refer to the Declaration of Conformity DoC f...

Страница 34: ......

Страница 35: ...al RTDs drift less than 0 1 C year and some models are stable to within 0 0025 C year RTDs are sometimes difficult to measure because they have relatively low nominal resistance commonly 100 that chan...

Страница 36: ...nt called alpha differs between RTD curves Although various manufacturers specify alpha differently alpha is most commonly defined as the change in RTD resistance from 0 to 100 C divided by the resist...

Страница 37: ...r Van Dusen equation is commonly used to approximate the RTD curve where RT is the resistance of the RTD at temperature T R0 is the resistance of the RTD at 0 C A B and C are the Callendar Van Dusen c...

Страница 38: ...raditional NI DAQ section or the Creating an RTD Virtual Channel Using NI DAQmx section depending on the version of NI DAQ you are using Creating an RTD Virtual Channel Using Traditional NI DAQ To cre...

Страница 39: ...I DAQ Virtual Channels Creating an RTD Virtual Channel Using NI DAQmx To create an RTD virtual channel using traditional NI DAQ complete the following steps 1 Launch MAX 2 Right click Data Neighborhoo...

Страница 40: ...measurement system that also supplies a constant current IEX to excite the RTD As shown in Figure B 2 the voltage drop across the lead resistances labeled RL adds an error voltage to the measured volt...

Страница 41: ...A main advantage of thermistors for temperature measurement is their extremely high sensitivity For example a 2 252 thermistor has a sensitivity of 100 C at room temperature Higher resistance thermis...

Страница 42: ...e is typically limited to around 300 C Figure B 3 shows the resistance temperature curve for a 2 252 thermistor The curve of a 100 RTD is also shown for comparison Figure B 3 Resistance Temperature Cu...

Страница 43: ...or calculated from the resistance versus temperature curve Creating a Thermistor Virtual Channel Using NI DAQmx To creat an RTD virtual channel using NI DAQmx complete the following steps 1 Launch MAX...

Страница 44: ...nique is to use a constant current source and measure the voltage developed across the thermistor As shown in Figure B 4 the measured voltage V0 equals RT IEX Figure B 4 Thermistor Measurement with Co...

Страница 45: ...ion constant minimize the level of the excitation current to minimize self heating errors Connecting Resistive Devices to the SCXI 1581 You can connect resistive devices to the SCXI signal conditionin...

Страница 46: ...pair of wires to sense the voltage across the resistive sensor Because of the high input impedance of the differential amplifier negligible current flows through the sense wires This results in a very...

Страница 47: ...esistances RL1 and RL2 If we assume equal lead resistances RL1 RL2 RL the magnitude of the error voltage is VE 2RL IEX This is the most commonly used configuration for connecting thermistors to a sign...

Страница 48: ...rrent flows through RL3 and therefore RL1 is the only lead resistance that introduces an error into the measurement The resulting measurement error is VE RL1 IEX An advantage of this configuration is...

Страница 49: ...gure B 8 Figure B 8 3 Wire Configuration Using Matched Current Sources In this configuration the lead resistance voltage drop across RL3 is converted into a common mode voltage that is rejected by the...

Страница 50: ...he value of the excitation current In this example the voltage equates to 1 V Converting this voltage into a resistance by dividing by IEX yields a resistance of 0 01 If this error is associated with...

Страница 51: ...pectively the expression for V1 becomes V1 VRL1 VRT The second measurement labeled V2 in Figure B 9 is equal to the voltage drop across the lead resistance RL2 denoted as VRL2 therefore V2 VRL2 If the...

Страница 52: ......

Страница 53: ...ystem Can I use the SCXI 1581 with a version of NI DAQ that works under the Macintosh Operating System Mac OS At the time this manual was published you cannot auto detect or list the SCXI 1581 as a co...

Страница 54: ...ded The accuracy or stability of the resulting current output value is unpredictable Can I connect N current output channels in series to achieve a higher terminal voltage compliance limit No Each cur...

Страница 55: ...ystem Yes E Series DAQ device channels 1 through 7 are available to measure unconditioned signals Use an SCXI 1180 or the 50 pin breakout connector on the SCXI 1346 or SCXI 1349 cable adapter to route...

Страница 56: ...ss the current output and use a voltmeter to measure the voltage drop across it If the current output is working you should measure V 100 10 6 A R where R is given in How does the screw terminal label...

Страница 57: ...ng thermocouples It will work with the SCXI 1581 but only if you remove all resistor packs Refer to the SCXI 1303 32 Channel Isothermal Terminal Block Installation Guide for more information As with t...

Страница 58: ......

Страница 59: ...Glossary Symbol Prefix Value p pico 10 12 n nano 10 9 micro 10 6 m milli 10 3 k kilo 103 M mega 106 G giga 109 T tera 1012 Numbers Symbols percent positive of or plus negative of or minus plus or min...

Страница 60: ...on a type of signal conditioning that improves accuracy in the resulting digitized signal by increasing signal amplitude relative to noise AOGND analog output ground AOGND GUARD analog output ground A...

Страница 61: ...alog D A Data Address DAQ data acquisition 1 collecting and measuring electrical signals from sensors transducers and test probes or fixtures and processing the measurement data using a computer 2 col...

Страница 62: ...ver software software that controls a specific hardware device such as an E Series DAQ device E EMC electromagnetic compliance EMI electromagnetic interference EX positive excitation channel EX negati...

Страница 63: ...that occurs when a signal passes on the SCXIbus from the scanned SCXI module to the cabled SCXI module input bias current the current that flows into the inputs of a circuit input impedance the measur...

Страница 64: ...ignal MOSI master out slave in signal multiplex to route one of many input signals to a single output multiplexed mode an SCXI operating mode in which analog input channels are multiplexed into one mo...

Страница 65: ...ents are made with respect to a common measurement system reference but the voltage at this reference can vary with respect to the measurement system ground O offset error the output of a system with...

Страница 66: ...red to input calculates a specification relative to the input range RTO referred to output S s seconds S samples S s samples per second used to express the rate at which a E Series DAQ device samples...

Страница 67: ...g time the amount of time required for a voltage to reach its final value within specified accuracy limits signal conditioning the manipulation of signals to prepare them for digitizing Slot 0 refers...

Страница 68: ...ware module VI which consists of a front panel user interface and a block diagram program virtual channels channel names that can be defined outside the application and used without having to perform...

Страница 69: ...iguration B 13 3 wire resistive sensor connected in 2 wire configuration B 14 4 wire configuration B 12 lead resistance compensation using 3 wire resistive sensor and two differential amplifiers B 16...

Страница 70: ...ired C 1 O operation of current sources 2 1 P physical specifications A 2 pin assignments front connector table 2 2 rear signal connector figure 2 4 power requirements from SCXI backplane A 1 PXI comb...

Страница 71: ...iance A 3 electromagnetic compatibility A 2 environment A 2 excitation A 1 physical A 2 power requirements from SCXI backplane A 1 safety A 2 stability A 1 stability specifications A 1 T temperature m...

Страница 72: ...rcuits B 10 overview B 7 resistance temperature characteristics B 9 resistance temperature curve figure B 8 troubleshooting self test verification 1 12 U unpacking SCXI 1520 1 2 V verifying and self t...

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