This document contains information about calibrating National
Instruments 9235/9236 modulesusing NI-DAQmx. This calibration
procedure is intended for metrology labs.
This document does not discuss programming techniques or compiler
configuration. The NI-DAQmx driver contains online help files that have
compiler-specific instructions and detailed function explanations. You can
install these help files when you install NI-DAQmx on the calibration
computer.
Where to Go for Support......................................................................... 20
Page 2
Conventions
The following conventions appear in this manual:
»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.
boldBold 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 and hardware labels.
italicItalic text denotes variables, emphasis, a cross-reference, or an introduction
to a key concept. Italic text also denotes text that is a placeholder for a word
or value that you must supply.
monospaceMonospace text 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.
monospace italic
Software
Note NI recommends that you install the NI-DAQmx driver software before physically
installing the NI 9235/9236. NI-DAQmx, available at
and controls the NI 9235/9236.
Italic text in this font denotes text that is a placeholder for a word or value
that you must supply.
Install NI-DAQmx 8.8 or later on the calibration computer. NI-DAQmx
includes high-level function calls to simplify the task of writing software to
calibrate devices. You must have the proper device driver installed on the
calibration system before calibrating the device.
ni.com/downloads, configures
NI-DAQmx supports a number of programming languages, including
LabVIEW, LabWindows
Visual Basic 6.0, Microsoft .NET, and Borland C++.
You can access the NI-DAQmx header file,
library. You can find examples of how to use the NI-DAQmx driver in the
Program Files\National Instruments\NI-DAQ\Examples
directory.
™
/CVI™, Microsoft Visual C++ 6.0, Microsoft
NIDAQmx.h, like any standard
NI 9235/9236 Calibration Procedure2ni.com
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Documentation
Note The documents above are installed with NI-DAQmx. You can also download the
latest versions from the NI Web site at
You might find the following documentation helpful as you write the
calibration procedure:
•NI-DAQmx Help—This help file contains general information about
measurement concepts, key NI-DAQmx concepts, and common
applications that apply to all programming environments. To access
this help file, select Start»All Programs»National Instruments»NI-DAQ»NI-DAQmx Help.
•NI-DAQmx C Reference Help—This help file contains C reference and
general information about measurement concepts. To access this help
file, select Start»All Programs»National Instruments»NI-DAQ»NI-DAQmx C Reference Help.
•DAQ Getting Started guides for NI-DAQ 8.0 or later—This guide
describes how to install NI-DAQmx for Windows software and
NI-DAQmx-supported DAQ devices, and how to confirm that your
device is operating properly. To access this guide, select Start»
All Programs»National Instruments»NI-DAQ»DAQ Getting
Started Guide.
ni.com/manuals.
•NI 9235/9236 Operating Instructions and Specifications—This
document describes how to use the NI 9235/9236 and includes
specifications and terminal assignments for the NI 9235/9236. The
limits you use to verify the accuracy of the devices are based on the
specifications found in this document. You can download the latest
version of this document from the NI Web site at
ni.com/manuals.
Calibration Interval
The NI 9235/9236 should be calibrated at a regular interval as defined by
the measurement accuracy requirements of your application. National
Instruments recommends that you routinely perform a complete calibration
at least once every year. You can shorten this interval based on the accuracy
demands of your application or requirements of your processes.
National Instruments recommends that you use the following equipment
for calibrating the NI 9235/9236.
Table 1. Recommended Equipment
EquipmentRecommended ModelRequirements
CalibratorFluke 5500AIf this instrument is unavailable, use a calibrator with
generation of resistances in the ranges of 108 Ω – 132 Ω
and 315 Ω –385Ω with 0.01 Ω resolution, an accuracy
of at least 90 ppm, and automatic lead wire
compensation.
DMMNI 4070If this instrument is unavailable, use a multi-ranging
6 1/2 digit DMM with an accuracy of at least 40 ppm.
ChassisNI cDAQ chassis—
Connecting
wires
Test Conditions
—Connections between the module and the calibrator are
insensitive to connection resistance except between the
calibrator LO terminal and the module RC terminal. The
resistance of this connection must be less than 200 mΩ to
limit bridge linearity errors. The verification test limits in
Tables 5 and 6 assume this requirement is met.
Follow these guidelines to optimize the connections and the environment:
•Keep connections to the device as short as possible. 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.
•Maintain an ambient temperature of 23 ±5 °C. The device temperature
will be greater than the ambient temperature.
•Keep relative humidity below 80%.
•Allow a warm-up time of at least 10 minutes to ensure that the
measurement circuitry is at a stable operating temperature.
NI 9235/9236 Calibration Procedure4ni.com
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Calibration Procedure
This section provides instructions for verifying the performance of the
NI 9235/9236.
Calibration Process Overview
The calibration process consists of the following steps:
1.Initial Setup—Configure the device in NI-DAQmx.
2.Verification Procedures—Verify the existing operation of the device.
This step confirms whether the device is operating within its specified
range and whether it needs adjustment.
3.Adjustment—If the device does not fall within the desired
specifications, submit the device to NI for a factory calibration to
adjust the calibration constants.
4.Verification Procedures—Perform another verification to ensure that
the device operates within its specifications after adjustment.
The first two steps are explained in the following sections.
Initial Setup
You must configure the device in Measurement & Automation Explorer
(MAX) to communicate with NI-DAQmx.
Complete the following steps to configure a device in MAX:
1.Install the NI-DAQmx driver software.
2.Make sure that no power is connected to the module terminals. If the
system is in a nonhazardous location, the chassis power can be on
when you install the module.
3.Insert the module into an available slot in the cDAQ chassis.
4.Launch MAX.
5.Right-click the device name and select Self-Test to ensure that the
device is working properly.
Note When a device is configured with MAX, it is assigned a device name. Each function
call uses this device name to determine which DAQ device to calibrate. This document uses
dev1 to refer to the device name. In the following procedures, use the device name as it
Complete the following steps to connect the calibrator to the module for
both the measurement accuracy verification and the shunt calibration
verification:
1.Set the calibrator to standby mode (STBY).
2.Connect the AUX LO terminal of the calibrator to the LO terminal of
the calibrator.
3.Using discrete connections, connect the LO terminal of the calibrator
to the AI and RC terminals of the module channel you want to verify.
Leave all other channels unconnected.
Note The three connections to the calibrator LO terminal must be connected at the
calibrator output jack.
Note Ensure that the connection resistance between the calibrator LO terminal and the
module RC terminal meets the requirement listed in Table 1.
4.Connect the HI terminal of the calibrator to the EXC0 terminal of the
module.
5.Connect the AUX HI terminal of the calibrator to the EXC1 terminal
of the module.
Note You do not have to change the EXC terminal connections for each channel you
verify, because the EXC terminals all connect to a common excitation supply inside the
NI 9235/9236.
NI 9235/9236 Calibration Procedure6ni.com
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Refer to Figure 1 for an illustration of the calibrator to module connections.
Fluke
5500A
AUX HIEXC1
EXC0
AI0
RC0AUX LO
HI
LO
NI 9235/9236
Figure 1. Calibrator to Module Connections
Refer to Figure 2 for the terminal assignments of the NI 9235/9236.
6.Set the calibrator resistance output to 120 Ω for the NI 9235 or 350 Ω
7.Set the calibrator to operate mode (OPR).
8.Wait one minute to allow the bridge elements formed by the calibrator
Verification Procedures
Verification determines how well the device is meeting its specifications.
By completing this procedure, you can see how the device has drifted over
time, which helps you determine the appropriate calibration interval for
your application. The NI 9235/9236 has eight independent analog input
channels. You can conduct verification on any or all of them, depending
upon your desired test coverage.
You can verify the following for NI 9235/9236 modules:
•measurement accuracy
•shunt calibration accuracy
•excitation voltage
Measurement Accuracy Verification
Complete the following steps to test the measurement accuracy of the
device:
1.Zero the calibrator with the ohms-only zero (OHMS ZERO) operation.
2.Set the calibrator output for two-wire compensation (COMP 2-wire).
3.Connect the calibrator to the channel you want to verify, as explained
4.Set the calibrator resistance output to a Calibrator Output value
for the NI 9236.
and the module completion resistor to thermally stabilize.
Refer to the calibrator documentation for more information about
zeroing the calibrator.
This automatically compensates for the lead wire resistance between
the HI terminal of the calibrator and the EXC terminal of the module.
in the Connecting the Calibrator to the Module section.
indicated in Table 5 for the NI 9235 and Table 6 for the NI 9236,
beginning with the 0 V/V nominal value.
NI 9235/9236 Calibration Procedure8ni.com
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5.If you use C function calls, create a task using DAQmxCreateTask,
as shown in the following table. If you use LabVIEW, skip this step.
The task is created in step 6 in LabVIEW.
LabVIEW Block DiagramNI-DAQmx Function Call
Call DAQmxCreateTask with
the following parameters:
LabVIEW does not require this step.
taskName:
AIVerificationTask
taskHandle: &taskHandle
6.Create and configure an AI Custom Voltage with Excitation channel using
the DAQmx Create Virtual Channel VI, as shown in the following table.
Note Throughout the procedure, refer to the NI-DAQmx function parameters for the
LabVIEW input values. Refer to the block diagram images for the correct instance to use
for polymorphic VIs.
7.Configure the timing properties for the acquisition using the DAQmx
Timing VI, as shown in the following table.
LabVIEW Block DiagramNI-DAQmx Function Call
Call DAQmxCfgSampClkTiming
with the following parameters:
taskHandle:
taskHandle
source: NULL
rate: 5000
sampleMode:
DAQmx_Val_FiniteSamps
sampsPerChan: 5000
8.Turn off the shunt calibration for the channel using the DAQmx
Channel Property Node, as shown in the following table.
LabVIEW Block DiagramNI-DAQmx Function Call
Call
DAQmxSetAIBridgeShuntCal
Enable with the following
parameters:
taskHandle:
taskHandle
channel: NULL
data: FALSE
9.Start the acquisition using the DAQmx Start Task VI, as shown in the
following table.
LabVIEW Block DiagramNI-DAQmx Function Call
Call DAQmxStartTask with the
following parameter:
taskHandle:
NI 9235/9236 Calibration Procedure10ni.com
taskHandle
Page 11
10. Acquire 5,000 points of data using the DAQmx Read VI, as shown in
the following table.
LabVIEW Block DiagramNI-DAQmx Function Call
Call DAQmxReadAnalogF64 with
the following parameters:
taskHandle:
taskHandle
numSampsPerChan: –1
timeout: 10.0
fillMode:
DAQmx_Val_GroupByChannel
readArray: data
arraySizeInSamples: 5000
sampsPerChanRead: &read
reserved: NULL
11. Average the readings that you acquired and record the result.
12. Clear the acquisition using the DAQmx Clear Task VI, as shown in the
following table.
LabVIEW Block DiagramNI-DAQmx Function Call
Call DAQmxClearTask with the
following parameter:
taskHandle:
taskHandle
13. Repeat steps 4 through 12 for all calibrator output values.
NI recommends that you verify all values, although you can save time
by verifying only the values used in your application.
14. Perform the following calculation for each test result value other than
0 V/V, using the results you recorded in step 11.
15. Compare the compensated result for each value to the Upper Limit
strain ε()
4–V
r
×
GF 12V
r
×()+()
-------------------------------------------=
(V/V) and Lower Limit (V/V) values in Table 5 for the NI 9235 and
Table 6 for the NI 9236. If the result is between these values, the device
passes the test.
Note You can analyze data in V/V form or convert it to strain using the standard
quarter-bridge equation:
where V
is the compensated reading from the NI 9235/9236 and GF is a gage factor of 2.
r
16. Repeat steps 3 through 15 for all channels.
17. Set the calibrator to standby mode (STBY).
18. Disconnect the calibrator from the device.
Shunt Calibration Accuracy Verification
Complete the following steps to test the shunt calibration accuracy of the
device:
1.Using the DMM and 4-wire resistance measurement connections,
measure the resistance of the wire to be connected between the
calibrator LO terminal and the module RC terminal, and record the
result for later use. You must do this because this connection to the
module creates a lead wire desensitization error. This results in module
readings that are systematically lower than the true shunt calibration
output. Knowing the actual resistance value, however, allows for
compensation of this effect.
2.Set the calibrator output for two-wire compensation (COMP 2-wire).
This automatically compensates for the lead wire resistance between
the HI terminal of the calibrator and the EXC terminal of the module.
3.Connect the calibrator to the channel you want to verify, as explained
in the Connecting the Calibrator to the Module section.
4.Set the calibrator resistance output to 120 Ω
for the NI 9236.
for the NI 9235 or 350 Ω
NI 9235/9236 Calibration Procedure12ni.com
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5.If you use C function calls, create a task using DAQmxCreateTask,
as shown in the following table. If you use LabVIEW, skip this step.
The task is created in step 6 in LabVIEW.
LabVIEW Block DiagramNI-DAQmx Function Call
Call DAQmxCreateTask with the following parameters:
LabVIEW does not require this step.
taskName:
SCVerificationTask
taskHandle: &taskHandle
6.Create and configure an AI Custom Voltage with Excitation channel
using the DAQmx Create Virtual Channel VI, as shown in the
following table.
Note Throughout the procedure, refer to the NI-DAQmx function parameters for the
LabVIEW input values. Refer to the block diagram images for the correct instance to use
for polymorphic VIs.
15. Compare the final result to the Upper Limit (μV/V) and Lower Limit
strain ε()
4–V
r
×
GF 12V
r
×()+()
-------------------------------------------=
(μV/V) values in Table 7. If the result is between these values, the
device passes the test.
Note You can analyze data in V/V form or convert it to strain using the standard
quarter-bridge equation:
where V
is the compensated reading from the NI 9235/9236 and GF is a gage factor of 2.
r
16. Repeat steps 3 through 15 for all channels.
17. Set the calibrator to standby mode (STBY).
18. Disconnect the calibrator from the device.
Excitation Voltage Verification
The excitation source of the NI 9235/9236 is common to all eight input
channels so you need to verify excitation voltage on only one channel.
Complete the following steps to verify the excitation voltage:
1.Connect the positive input of the DMM to any EXC terminal of the
module and the negative input of the DMM to any RC terminal of the
module. Leave all other terminals unconnected.
2.Measure the DC voltage and compare it to the Lower Limit and Upper
Limit values in Table 8. If the result is between these values, the device
passes the test.
3.Disconnect the DMM from the device.
NI 9235/9236 Calibration Procedure16ni.com
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Specifications
The values in the following table are based on calibrated scaling
coefficients, which are stored in the onboard EEPROM. The following
calibration specifications are for 23 ±5 °C.
Table 5. NI 9235 Measurement Accuracy Verification Test Limits
The following definitions describe how to use the information from
Tables 5 through 8.
The Calibrator Output is the calibrator resistance output entered for
verification.
The Nominal Value is the approximate value that the module should read,
given the corresponding calibrator output.
The 1-Year Limits column contains the Upper Limits and Lower Limits for
the test results. That is, when the device is within its 1-year calibration
interval, the test results should fall between these upper and lower limit
values.
For accuracy verification, Upper Limits and Lower Limits are given in units
of V/V for verification and units of με for reference.
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