National Instruments CB-27T Manual

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370344A-01 Copyright ©2001 National Instruments Corp. All rights reserved. May 2001
CALIBRATION PROCEDURE
CB-27T TERMINAL BLOCK
Introduction
This document contains information and step-by-step instructions for
verifying the performance of the temperature sensor on the
National Instruments CB-27T terminal block. This temperature sensor
is for cold-junction compensation of thermocouples on the terminal block.
What Is Calibration?
Calibration consists of verifying the measurement accuracy of adevice
and adjusting for any measurement error. Verification is measuring
performance of a device and comparing the results to the factory
specifications. Because the CB-27T terminal block is not user-adjustable,
calibrating this device consists of verifying the performance only, without
correcting for any error.
Why Should You Verify?
The accuracy of electronic components drifts with time and temperature,
which can affect measurement accuracy as the device ages. Verification
ensures that your CB-27T terminal block still meets NI standards. If the
results of the procedure indicate that the temperature sensor on
your terminal block is out of specification, return the terminal block to
National Instruments for repair or replacement.
How Often Should You Verify?
The measurement accuracy requirements of your application determine
how often you should verify the performance of your CB-27T terminal
block. National Instruments recommends you verify your terminal block
at least once every year. You can shorten this interval to six months or
90 days based on the demands of your application.

CB-27T Calibration Procedure 2 ni.com
Equipment and Other Test Requirements
This section describes the equipment, software, documentation, and test
conditions required for verifying the performance of your CB-27T terminal
block.
Test Equipment
Verification requires a multiranging 5 1/2 digit digital multimeter (DMM)
with 120 ppm accuracy that can perform a 5 to 10 kΩ 4-wire resistance
measurement with an excitation current of 100 µA or less.
NI recommends that you use the following equipment for verifying the
performance of your CB-27T terminal block:
• DMM—HP 34401A
• 0.76 mm pin connection adapters—Pomona Electronics 4690
• A thermometer that is accurate to within 0.01 °C
If an HP 34401A is not available, use the accuracy requirements listed
above to select a substitute calibration standard.
Software and Documentation
You can find all the necessary information to verify the performance
of the CB-27T in this verification procedure. No other software or
documentation is required. If you would like more information on the
CB-27T, refer to the CB-27T Terminal Block Installation Guide,which
you can download free of charge from the NI Web site at
ni.com/manuals
Test Conditions
Follow these guidelines to optimize the connections and the environment
during verification:
• Keep connections to the CB-27T terminal block short. Long cables and
wires act as antennae, picking up extra noise that can affect
measurements.
• Use shielded copper wire for all cable connections to the device.
Use twisted-pair wire to eliminate noise and thermal offsets.
• Keep relative humidity below 80%.
• Maintain a temperature between 15 and 35 °C.

©National Instruments Corporation 3 CB-27T Calibration Procedure
Verifying Temperature Sensor Performance
Complete the following steps to verify the performance of the temperature
sensor on your CB-27T terminal block:
1. Move the IEX switch to the OFF position. Figure 1 shows the location
of the IEX switch.
Figure 1. CB-27T IEX Switch
2. Connect the excitation leads of the DMM to the terminal block.
a. Hold the terminal block horizontally and view the connector so
that pin 1 is in the upper-right corner. Figure 1 shows the location
of pin 1 on the terminal block. Figure 2 shows the connector pin
assignments.
b. Attach a wire to pin 18 of the connector block. Attach the other
end of this wire to the positive terminal of the 25 µAcurrent
source of the DMM.
c. Attach a wire to pin 19 of the connector block. Attach the other
end of this wire to the negative terminal of the 25 µAcurrent
source of the DMM.
1Pin1 2 IEX Switch
2
1

CB-27T Calibration Procedure 4 ni.com
Figure 2. CB-27T Connector Block
3. Connect the sense leads of the DMM to the temperature-sensor output
of the terminal block.
a. Attach a wire to pin 2 of the connector block. Attach the other end
of this wire to the positive input terminal of the DMM.
b. Attach a wire to pin 3 of the connector block. Attach the other end
of this wire to the negative input terminal of the DMM.
4. Place the terminal block in a temperature-controlled environment
where the temperature is between 15 and 35 °C.
5. When the terminal block temperature stabilizes with its surroundings,
measure the resistance of the temperature sensor Rmeas using a
calibrated DMM.
6. Measure the actual temperature Tact in the temperature-controlled
environment using a calibrated thermometer.
7. Convert Rmeas (in ohms) to measured temperature Tmeas (in degrees
Celsius) by performing the following calculations:
a. Calculate:
b. Calculate:
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
Pin 1
CJS-
CJS+
IEX+
IEX-
yln Rmeas()=
15.273
)(
1
2−






++
=cybya
Tmeas

©National Instruments Corporation 5 CB-27T Calibration Procedure
Tmeas is in degrees Celsius and
a= 1.295361 ×10-3
b= 2.343159 ×10-4
c= 1.018793 ×10-7
8. Compare Tact to Tmeas
•If (Tmeas –0.06 °C) ≤Tact ≤(Tmeas +0.06°C), you have verified
the performance of the terminal block temperature sensor.
•If Tact <(T
meas –0.06 °C), the terminal block temperature sensor
is nonfunctional. Return the terminal block to NI for repair or
replacement.
•If Tact >(T
meas +0.06°C), the terminal block temperature sensor
is nonfunctional. Return the terminal block to NI for repair or
replacement.
You have completed verifying the performance of the temperature sensor
on your CB-27T terminal block.