National Instruments 9235 Manual

NI 9235/9236 Calibration Procedure

This document contains information about calibrating National

Instruments 9235/9236 modules using 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.

Contents

Conventions ............................................................................................ 2

Software .................................................................................................. 2

Documentation ........................................................................................ 3

Calibration Interval ................................................................................. 3

Test Equipment ....................................................................................... 4

Test Conditions ....................................................................................... 4

Calibration Procedure ............................................................................. 5

Calibration Process Overview ......................................................... 5

Initial Setup...................................................................................... 5

Connecting the Calibrator to the Module ........................................ 6

Verification Procedures ................................................................... 8

Measurement Accuracy Verification........................................ 8

Shunt Calibration Accuracy Verification ................................. 12

Excitation Voltage Verification................................................ 16

Specifications .......................................................................................... 17

Test Limits .............................................................................................. 18

Calibrator Output ............................................................................. 18

Nominal Value ................................................................................. 18

1-Year Limits ................................................................................... 18

Where to Go for Support......................................................................... 20

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NI 9235/9236 Calibration Procedure 2 ni.com

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.

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 and hardware labels.

italic Italic 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.

monospace Monospace 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

Italic text in this font denotes text that is a placeholder for a word or value

that you must supply.

Software

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.

Note NI recommends that you install the NI-DAQmx driver software before physically

installing the NI 9235/9236. NI-DAQmx, available at ni.com/downloads, configures

and controls the NI 9235/9236.

NI-DAQmx supports a number of programming languages, including

LabVIEW, LabWindows™/CVI™, Microsoft Visual C++ 6.0, Microsoft

Visual Basic 6.0, Microsoft .NET, and Borland C++.

You can access the NI-DAQmx header file, NIDAQmx.h, like any standard

library. You can find examples of how to use the NI-DAQmx driver in the

Program Files\National Instruments\NI-DAQ\Examples

directory.

©National Instruments Corporation 3 NI 9235/9236 Calibration Procedure

Documentation

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.

Note The documents above are installed with NI-DAQmx. You can also download the

latest versions from the NI Web site at 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.

NI 9235/9236 Calibration Procedure 4 ni.com

Test Equipment

National Instruments recommends that you use the following equipment

for calibrating the NI 9235/9236.

Test Conditions

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.

Table 1. Recommended Equipment

Equipment Recommended Model Requirements

Calibrator Fluke 5500A If this instrument is unavailable, use a calibrator with

generation of resistances in the ranges of 108 Ω– 132 Ω

and 315 Ω–385Ωwith 0.01 Ωresolution, an accuracy

of at least 90 ppm, and automatic lead wire

compensation.

DMM NI 4070 If this instrument is unavailable, use a multi-ranging

6 1/2 digit DMM with an accuracy of at least 40 ppm.

Chassis NI cDAQ chassis —

Connecting

wires

—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 mΩto

limit bridge linearity errors. The verification test limits in

Tables 5 and 6 assume this requirement is met.

©National Instruments Corporation 5 NI 9235/9236 Calibration Procedure

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

appears in MAX.

NI 9235/9236 Calibration Procedure 6 ni.com

Connecting the Calibrator to the Module

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.

©National Instruments Corporation 7 NI 9235/9236 Calibration Procedure

Refer to Figure 1 for an illustration of the calibrator to module connections.

Figure 1. Calibrator to Module Connections

Refer to Figure 2 for the terminal assignments of the NI 9235/9236.

Figure 2. NI 9235/9236 Terminal Assignments

Fluke

5500A

AUX HI EXC1

EXC0

AI0

RC0AUX LO

NI 9235/9236

EXC0

AI0

RC0

EXC2

AI2

RC2

EXC4

AI4

RC4

EXC6

AI6

RC6

EXC1

AI1

RC1

EXC3

AI3

RC3

EXC5

AI5

RC5

EXC7

AI7

RC7

NI 9235/9236 Calibration Procedure 8 ni.com

6. Set the calibrator resistance output to 120 Ωfor the NI 9235 or 350 Ω

for the NI 9236.

7. Set the calibrator to operate mode (OPR).

8. Wait one minute to allow the bridge elements formed by the calibrator

and the module completion resistor to thermally stabilize.

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.

Refer to the calibrator documentation for more information about

zeroing the calibrator.

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 a Calibrator Output value

indicated in Table 5 for the NI 9235 and Table 6 for the NI 9236,

beginning with the 0 V/V nominal value.

©National Instruments Corporation 9 NI 9235/9236 Calibration Procedure

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.

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.

LabVIEW Block Diagram NI-DAQmx Function Call

LabVIEW does not require this step.

Call DAQmxCreateTaskwith

the following parameters:

taskName:

AIVerificationTask

taskHandle: &taskHandle

LabVIEW Block Diagram NI-DAQmx Function Call

Call

DAQmxCreateAIVoltageChan

WithExcit with the following

parameters:

taskHandle: taskHandle

physicalChannel: dev1/ai

nameToAssignToChannel:

myVoltageChannel

terminalConfig:

DAQmx_Val_Cfg_Default

minVal: –0.027

maxVal: 0.027

units: DAQmx_Val_Volts

bridgeConfig:

DAQmx_Val_QuarterBridge

voltageExcitSource:

DAQmx_Val_Internal

voltageExcitVal:

2.0 for the NI 9235

3.3 for the NI 9236

useExcitforScaling: TRUE

customScaleName: NULL

* xrefers to the channel number.

NI 9235/9236 Calibration Procedure 10 ni.com

7. Configure the timing properties for the acquisition using the DAQmx

Timing VI, as shown in the following table.

8. Turn off the shunt calibration for the channel using the DAQmx

Channel Property Node, as shown in the following table.

9. Start the acquisition using the DAQmx Start Task VI, as shown in the

following table.

LabVIEW Block Diagram NI-DAQmx Function Call

Call DAQmxCfgSampClkTiming

with the following parameters:

taskHandle: taskHandle

source: NULL

rate: 5000

sampleMode:

DAQmx_Val_FiniteSamps

sampsPerChan: 5000

LabVIEW Block Diagram NI-DAQmx Function Call

Call

DAQmxSetAIBridgeShuntCal

Enable with the following

parameters:

taskHandle: taskHandle

channel: NULL

data: FALSE

LabVIEW Block Diagram NI-DAQmx Function Call

Call DAQmxStartTaskwith the

following parameter:

taskHandle: taskHandle

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