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Calibration Overview
Calibration while Digitizing
Calibration Options Dialog
Zero "Calibrate" BUTTON, for Strain Gage & Load Cell Sensors
Working with Thermocouples
Advanced Techniques
Time Stability and Temperature Drift

Calibration Overview

Calibration involves two different systems:

  1. instruNet Internal Calibration - This is done automatically to improve the accuracy of the instruNet voltage measurement. It is done using two different techniques:
    • Factory calibration occurs at the factory at the time of manufacture.
    • Software calibration occurs in the field to remove further errors due to time ("stability") and temperature ("drift") change.
  2. Sensor Calibration - This is done by the end user to improve sensor measurement accuracy. Here, the end user provides a known stimulus to a sensor, records the measured value, and then has the software map this value to the expected value. This is done either with one point (e.g. at 0 stimulus) or two points (e.g. 0 and maximum input).


instruNet Software Calibration

Software calibration improves the accuracy of the instruNet voltage measurement and is divided into several components:

  • Internal measurement circuitry software calibration - Measure internal errors that are automatically subtracted when later voltage measurements are performed.
  • i4xx/i60x 3.3Vref read back with A/D - Read the 3.3V reference voltage with the A/D. This reference is used with several sensors such as RTDs, Thermistors and strain gages.
  • Thermocouple CJC screw terminal calibration - Measure i100 or i51x screw terminal temperature, which is needed when doing thermocouple measurements.
  • Check for i4xx/i60x power cycle - Looks for device loosing power and shows an alert accordingly. If hardware looses power, then it will need to be reconfigured by the software.
  • i100 offset temperature drift calibration - Calibrate out a small errors (e.g. 10μV) that occurs when the i100 box changes temperature.

The Full software calibration, which includes all of the above components, occurs when the system is reset (i.e. press RESET button, load .prf configuration file, or start instruNet software), and when the system is Software Calibrated (i.e. press CALIBRATE button, issue software calibrate command, or set up software to auto-calibrate every X minutes). The rate of software auto-calibration is set in the Calibration Options dialog, as described below. Full calibration does not run if digitizing at any sample rate.

Calibration While Digitizing

If digitizing at a fast rate (e.g. > 1Ks/s/sec/ch), then no calibration of any kind occurs. However, if digitizing slowly (e.g. ≤ 1Ks/sec/ch), then the 4 lower items in the above list (e.g. 3.3Vref, Cjc, i100 temp drift) continue to operate.

Calibration Options Dialog

instruNet software contains a Calibration Options dialog that provides several calibration options, as described below. Calibration is helpful at reducing errors that occur when the measurement circuitry internal temperature changes. This dialog, shown below, is accessed by pressing the Setup button in the Record page and then pressing the Calibration button.

Programmatically, one accesses these fields via {netNum=0, devNum =0, modNum=1, chanNum=3, settingGroup = -21 = sgt_CalOptions, fieldNum = 1 to 8 (e.g. 5 = fldNum_CalOptions_CjcCalRate)}. For information on setting these fields programmatically, please see Calibration Routines. For information on setting these with Visual Basic, please click here.

Zero "Calibrate" BUTTON, for Strain Gage & Load Cell Sensors
One can remove fixed offset errors from strain gage and load cell sensors using a technique called "balancing". There are two ways to do this -- the easy method described here, and the less easy method described in the following paragraphs.

There is a button in the Network page called "Calibrate", as illustrated above. And there is a popup menu in the Calibrate dialog box called "Calibrate Button", shown above, that controls the function of this Calibration button at the top of the Network page. The popup menu is set to one of:

  • Measurement - instruNet hardware is fully software calibrated when the Network Calibrate button is pressed.
  • Bal Gages - instruNet hardware is calibrated and all strain gages and load cells are balanced when the Network Calibrate button is pressed.
  • Bal Bridges - instruNet hardware is calibrated and all bridges (i.e. strain gages, load cells, and pots) are balanced when the Network Calibrate
  • Bal VDividers - instruNet hardware is calibrated and all voltage dividers are balanced when the Network Calibrate button is pressed.

"Balancing" involves reading the voltage across a sensor and placing it into the "Vinit" field within the Constants settings area. This is done to establish a zero point from the sensor, and should be done when the sensor is not receiving a stimulus (e.g. Load Cell with 0 Kg applied).

If this popup menu is set to On, the instruNet hardware is automatically calibrated when one begins to digitize (e.g. user presses Start button in Record page).

If this popup menu is set to On, the instruNet hardware is automatically calibrated 10 minutes and 25 minutes after the instruNet software is first run.

This field, which is accessed by clicking {Record tab, Setup button, Calibrate button}, sets the rate that instruNet measurement electronics is automatically calibrated, in units of minutes. For example, if this field is set to 5, then instruNet hardware is automatically calibrated every 5 minutes. If set to 0, the Auto-Calibrate feature is disabled. The minimum rate is .016 minutes. During calibration, as much as possible is calibrated (e.g. full calibration if not digitizing, no calibration if digitizing at fast rate (>1Ks/s/ch) and several items are calibrated if digitizing at a slow rate. For details on i100 auto-calibration while digitizing, click here.

This field sets the rate that the instruNet screw terminal temperature is measured, and is used with thermocouple sensors, as described below.

Working With Thermocouple Sensors

In order to calculate the temperature of a thermocouple tip, one must know the temperature of the screw terminals that attach the thermocouple wire to the measurement system (e.g. i51x or i100 screw terminals). This is sometimes referred to as "Cold Junction Compensation", "Cjc Calibration", or "Cjc". If at least 1 thermocouple is connected to the system, this Cjc Calibration (i.e. measuring the temperature of the screw terminals) occurs when a Full Calibration occurs (e.g. at the Auto Cal Rate), or when a Cjc Only Calibration occurs (e.g. at the Cjc Cal Rate).

This field, which is accessed by clicking {Record tab, Setup button, Calibrate button}, sets the rate that instruNet hardware automatically performs Cjc Calibration, in units of minutes. CJC, as noted above, is used only with thermocouple sensors. If the CJC CAL RATE is set to 0, this feature is disabled. The minimum rate is 0.016 minutes. Reading the CJC sensor can occur while digitizing at slow <1KHs/sec speeds and when the sum of the integration time is less than 40% of the sample period. If the iNet hardware reads the screw temperature, and the screw then drifts by X degrees before the next CJC cal, one will incur a thermocouple measurement error by that amount (since the software subtracts the screw terminal temperature when calculating the thermocouple temperature).

When working with thermocouples, it is recommended that AUTO CAL RATE be set to 5 minutes (i.e. to do a full calibration every 5 minutes) and CJC CAL RATE also be set to 3 minutes (i.e. to calibrate errors from temperature drift of the iNet screw terminals). If reading temperatures via the single value read/write GetChannelValue() routines, both AUTO CAL and CJC CAL will function at their prescribed rates. For details on AUTO CAL and CJC CAL, please see the above discussions. For differences between single value read/write and high speed digitize, please click here. See Troubleshooting Thermocouples, for more information.

Advanced Techniques

For information on forcing the calibration with DasyLab, please see SET PARAMETER ICON. If programming, see Calibration Routines. One line of program code can tell the instruNet hardware to auto-calibrate.

With instruNet ≥ v3.4, if you have a text file named "iNetLoad.ibs" in directory "..MyDocuments\instrunet\end user\" and the file contains "Calibrate CjcBeepOn" text, a beep will sound when the CJC calibration is done; and if it contains "Calibrate BeepOn" on a separate line, a beep occurs when regular instruNet device calibration is performed.

Attaching a little wire between instruNet GND and the Vin- screw terminal often fixes thermocouple problems. If that does not fix it, please see Noise Reduction Techniques.

See Also