National Instruments SCC-TC Series Manual de usuario

USER GUIDE

SCC-TC Series Thermocouple

Input Modules

The SCC-TC Series thermocouple input modules, SCC-TC01 and

SCC-TC02, accept input signals from B-, E-, J-, K-, N-, R-, S-, and T-type

thermocouples. Each module contains thermistor circuitry powered by

2.5 V, to compensate for cold-junction effects and can detect open

thermocouple circuits. Each module has one thermocouple input channel,

which consists of a gain of 100 differential amplifier and a dual-pole 2 Hz

filter.

Note Alternatively, you can use the SCC-TC02 as a 2 Hz lowpass bandwidth ±100 mV

input module.

Conventions

The following conventions are used in this guide:

»The »symbol leads you through nested menu items and dialog box options

to a final action. The sequence File»Page Setup»Options directs you to

pull down the File menu, select the Page Setup item, and select Options

from the last dialog box.

This icon denotes a note, which alerts you to important information.

This icon denotes a caution, which advises you of precautions to take to

avoid injury, data loss, or a system crash. When this symbol is marked on

the product, refer to the Read Me First: Safety and Radio-Frequency

Interference document, shipped with the product, for precautions to take.

When symbol is marked on a product, it denotes a warning advising you to

take precautions to avoid electrical shock.

When symbol is marked on a product, it denotes a component that may be

hot. Touching this component may result in bodily injury.

SCC-TC Series Thermocouple Input Modules User Guide 2 ni.com

bold Bold text denotes items that you must select in the software, such as menu

items and dialog box options. Bold text also denotes parameter names.

italic Italic text denotes variables, emphasis, a cross reference, or an introduction

to a key concept. This font also denotes text that is a placeholder for a word

or value that you must supply.

monospace Text in this font denotes text or characters that you should enter from the

keyboard, sections of code, programming examples, and syntax examples.

This font is also used for the proper names of disk drives, paths, directories,

programs, subprograms, subroutines, device names, functions, operations,

variables, filenames, and extensions.

SC-2345 SC-2345 refers to both the SC-2345 connector block and the SC-2345

configurable connector.

SCC SCC refers to any SCC series signal conditioning module.

©National Instruments Corporation 3 SCC-TC Series Thermocouple Input Modules User Guide

What You Need to Get Started

To set up and use the SCC-TC0X, you need the following items:

❑SC-2345/2350 with one of the following:

– SCC-PWR01

– SCC-PWR02 and the PS01 power supply

– SCC-PWR03 (requires a 7 to 42 VDC power supply, not included)

❑One or more SCC-TC0Xmodules

❑SCC-TC Series Thermocouple Input Modules User Guide

❑Read Me First: Safety and Radio-Frequency Interference

❑SC-2345/2350 User Manual, available at ni.com

❑SCC Quick Start Guide, available at ni.com

❑SC-2345 Quick Reference Label

❑68-pin E Series DAQ device, documentation, and 68-pin cable

❑1/8 in. flathead screwdriver

❑Numbers 1 and 2 Phillips screwdrivers

❑Wire insulation strippers

❑NI-DAQ (current version) for Windows 2000/NT/XP/Me

Note Software scaling of measurements is not supported on the Macintosh operating

system. Refer to the Scaling Voltage Measurements section for more information about

measurement scaling.

SCC-TC Series Thermocouple Input Modules User Guide 4 ni.com

Device Specific Information

Note For general SCC module installation and signal connection information, and

information about the SC-2350 carrier, refer to the SCC Quick Start Guide, available

for download at ni.com/manuals.

Installing the Module

Caution Refer to the Read Me First: Safety and Radio-Frequency Interference document

before removing equipment covers or connecting/disconnecting any signal wires.

You can plug the SCC-TC0Xinto any analog input socket on the SC-2345.

The socket you select determines which E Series DAQ device channels

receive the SCC-TC0Xsignals.

For single-stage input conditioning, plug the SCC-TC0Xinto any socket

J(X+1), where Xis 0 to 7, and connect the input signals to the module as

described in the Connecting the Input Signals section.

If you use the SCC-TC0Xin a dual-stage configuration, the SCC-TC0X

must be the first-stage module. Plug it into any socket J(X+9) and plug the

second-stage SCC into socket J(X+1), where Xis 0 to 7. Connect the input

signals to SCC-TC0Xas described in the Connecting the Input Signals

section. The SC-2345 connects the output signals of the first-stage SCC to

the inputs of the second-stage SCC. An example of dual-stage conditioning

in an SCC-TC0Xfollowed by an SCC-LP01 lowpass filter module.

Sockets J9 to J16 also are available for digital input/output (DIO)

conditioning or control. Refer to the SC-2345 User Manual for more

information on configuring, connecting, and installing SCC modules

Note SCC-TC0Xdual-stage configuration is only available in NI-DAQ 7.1 or later.

©National Instruments Corporation 5 SCC-TC Series Thermocouple Input Modules User Guide

Connecting the Input Signals

Note The signal names have changed. Refer to ni.com/info and enter rdtntg to

confirm the signal names.

The SCC-TC01 has a two-prong uncompensated thermocouple

miniconnector that accepts any miniature or subminiature two-prong

male thermocouple connector. The SCC-TC02 has a three-position

screw-terminal connector that accepts 28 to 16 AWG thermocouple wires.

The screw-terminal connector provides a ground connection for shielded

thermocouples. Otherwise, the two modules function identically.

To use an SCC-TC01, plug the male thermocouple connector into the

module.

The SCC-TC02 accepts up to three signals: TC+, TC–, and GND. TC+is

the positive thermocouple lead and TC– is the negative thermocouple lead.1

The GND terminal connects to AI GND on the E Series DAQ device.

The SCC-TC0Xhas a 10 MΩbias resistor connected from the negative

thermocouple input to ground. This resistor allows the thermocouple to be

ground-referenced or floating without requiring external bias resistors

connected to ground.

1 For ANSI color-coded J-type thermocouples, the red wire is negative and the white wire is positive. Refer to the thermocouple

data sheet if possible. You can find information about other color-coding schemes in the NI KnowledgeBase at ni.com/

support.

SCC-TC Series Thermocouple Input Modules User Guide 6 ni.com

The amplified thermocouple signal and cold-junction sensor signal

are measured by E Series DAQ device channel Xand channel X+8

respectively, where Xis 0 to 7 depending on where you plug the

SCC-TC0X. Refer to Figure 1 for SCC-TC0Xsignal connections.

Figure 1. SCC-TC0XSignal Connections

For more information about how to configure the SCC-TC module using

NI-DAQmx, refer to the SCC Quick Start Guide.

Thermocouple

Source

AI (

)

AI (

+8)

SCC-TC0

E Series

DAQ Device

–

4.75 kΩ

5 kΩ

0.1%

10 kΩ

2.5 kΩ

5 kΩ

at 25oC

10 MΩ

10 µF

16 V 0.1 µF

0.1 µF

-to

Ref 5 V

LM 4040

2.5 V

0.1%

2-Pole Filter/

Buffer Stage

Offset

Calibrator

AI SENSE

TC+

GND

(SCC-TC02

only)

TC-

AI GND

–

©National Instruments Corporation 7 SCC-TC Series Thermocouple Input Modules User Guide

Using the TC0X

Scaling Voltage Measurements

Note NI-DAQ includes thermocouple and thermistor conversion utilities that implement

the conversions required in steps 3 and 5 of the following procedure. Refer to your

software documentation for more information on these utilities.

Your software environment may return only voltage measurements from

E Series DAQ devices. If so, you must convert voltage measurements to

temperature measurements. To make this conversion, complete the

following steps:

1. Measure the thermocouple voltage.

a. Read the thermocouple channel on the E Series DAQ device

VESERIES [CH(X)].

b. Calculate the thermocouple voltage by using the following

formula:

where

VTC is thermocouple voltage.

VESERIES is E Series DAQ device voltage.

This step provides proper scaling for the thermocouple amplifier in the

SCC-TC0X.

2. Measure the reference-junction (cold-junction) temperature.

a. Read the thermistor voltage [AI (X+8)].

b. Convert the thermistor voltage to cold-junction temperature using

the formula in the Cold-Junction Sensor section.

3. Calculate the cold-junction compensation voltage by converting the

cold-junction temperature from step 2 to a thermocouple voltage. Use

the polynomial expressions applicable to the type of thermocouple you

are using.

4. Apply the cold-junction compensation to the thermocouple reading

by adding the cold-junction compensation voltage from step 3 to VTC.

5. Calculate the thermocouple temperature by converting the voltage

result from step 4 to a temperature. Use the polynomial expressions

applicable to the type of thermocouple you are using. This calculation

gives you a linearized temperature measurement.

Note Polynomials are from NIST Monograph 175.

VTC

VESERIES

100

---------------------=

SCC-TC Series Thermocouple Input Modules User Guide 8 ni.com

Uncompensated Connectors and Accuracy

If you are using an SCC-TC02 with an uncompensated SCC panelette,

temperature gradients between the module and the junctions on the

panelette affect the accuracy of your measurements.

Detecting Open Thermocouples

The SCC-TC0Xcontains a 10 MΩpull-up resistor that connects to

+5 V to detect open thermocouples. To determine if you have an open

thermocouple, check whether the corresponding E Series DAQ device

channel is saturated. The pull-up and bias resistors saturate the channel by

applying +2.5 V at the input of an open channel. This results in saturation

to either the positive or negative rails of the E Series DAQ device (±10 V).

Errors Due to Open-Thermocouple Detection Circuitry

The open-thermocouple detection circuitry can cause measurement errors.

These errors are the results of common-mode voltage at the input of the

SCC and current leakage into the signal leads. The 10 MΩbias resistor in

the SCC-TC0Xcauses this error to be negligible. With the 10 MΩbias

resistor connected to ground and the 10 MΩpull-up resistor connected to

+5 VDC, a current leakage of approximately 0.25 µA (5 V/20 MΩ) flows

into the unbroken floating thermocouple. Long thermocouple leads result

in larger voltage drops due to lead resistance.

For example, if you have a 24 AWG J-type thermocouple that is 20 ft long,

a voltage drop of approximately

4 µV = (0.145 Ω/ft +0.658 Ω/ft) ×20 ft ×0.25 µA

can develop in the thermocouple, which corresponds to an error of 0.09 °C.

With 10 MΩpull-up and bias resistors, a common-mode voltage of

+2.5 VDC develops if the thermocouple is floating. The common-mode

rejection of the SCC-TC0Xis sufficiently high, which results in the offset

voltage being negligible in most applications.

If your application demands extremely high accuracy, you can eliminate

these errors by calibrating the system. Refer to the Calibrating System

Offsets section for more information.

©National Instruments Corporation 9 SCC-TC Series Thermocouple Input Modules User Guide

Cold-Junction Sensor

The cold-junction sensor voltage output varies from 1.91 to 0.58 V over a

0 to 55 °C temperature range.

Note NI-DAQ includes thermistor conversion utilities that implement the equations listed

below. Refer to your software documentation for more information on these utilities.

You can use the following formulas to convert the cold-junction sensor

voltage to cold-junction temperature:

where TKis the temperature in kelvin.

where

a= 1.295361 ×10–3

b= 2.343159 ×10–4

c= 1.018703 ×10–7

RTis the resistance of the thermistor in ohms.

where VTEMPOUT is the output voltage of the cold-junction sensor.

where T(°F) and T(°C) are the temperature readings in degrees Fahrenheit

and Celsius, respectively.

Note VTEMPOUT varies from 1.91 V (at 0 °C) to 0.58 V (at 55 °C). For best resolution, use

the maximum gain for this range on the analog input channel.

For the cold-junction sensor measurement accuracy, refer to the

Specifications section.

For more information about how to configure the SCC-TC using

NI-DAQmx, refer to the SCC Quick Start Guide.

T°C() TK273.15–=

ab Rln T

()cRln T

()

++[]

--------------------------------------------------------------=

RT5000

VTEMPOUT

2.5 VTEMPOUT

–

--------------------------------------





T°F() T°C()]9[

------------------------32+=

SCC-TC Series Thermocouple Input Modules User Guide 10 ni.com

Calibrating System Offsets

To calibrate the offset voltage of the SCC-TC0Xin the system using the

E Series DAQ device, complete the following steps. The E Series DAQ

device must be on for 30 minutes before you calibrate the offset voltage.

1. Select the desired channel and gain on the E Series DAQ device.

2. Short the inputs on the SCC-TC0Xscrew terminals or miniplug.

3. While acquiring data on the selected channel, use a screwdriver to

adjust the potentiometer protruding through the top of the module until

you read 0 VDC.

Calibrating the SCC-TC0XUsing a Thermocouple

Calibrator

To calibrate the system using a thermocouple calibrator, complete the

following steps:

1. Make sure that the thermocouple connections at both the SCC and the

thermocouple calibrator are at the same temperature.

2. Connect the thermocouple calibrator to the SCC-TC0X. For best

results, use thermocouple wire of the same length and type that you

use in the thermocouple.

3. Set the thermocouple calibrator to the required calibration

temperature.

4. Measure the calibrator voltage using the SCC-TC0X.

5. Convert the measured calibrator voltage to a temperature measurement

as described in the Scaling Voltage Measurements section.

6. Adjust the potentiometer on the top of the SCC-TC0Xso that the

measured temperature is equal to the calibration temperature.

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