CALIBRATION OPTIONS AND STRATEGIES
REFERENCE > ADVANCED TECHNIQUES >
Calibration Overview
Calibration Options Dialog
Working with Thermocouples
Advanced Techniquess
Time Stability and Temperature Drift
Calibration Overview
There are two kinds of calibration: Factory Calibration and Software Calibration.
Factory calibration is done at the factory when the product is manufactured; and software calibration
is done in the field to remove further errors due to time ("stability") and temperature ("drift") change.
Software calibration is divided into several components:
- Full internal software calibration
- i4xx 3.3Vref readback with A/D
- Thermocouple cjc screw terminal calibration
- Check for i4xx power cycle
- i100 offset temperature drift calibration
Full calibration involves everything that could possibly be calibrated, 3.3Vref readback
is used to read back the i4xx 3.3V reference voltage (i.e. which is used with some sensors), Cjc
calibration involves measuring screw terminal temperature (i.e. for thermocouple sensors), checking for
i4xx power cycle is used to see if the i4xx lost power and show an alert accordingly (which means it
needs to be reconfigured), and i100 offset temperature drift involves calibrating out i100 offset
temperature drift.
The Full software calibration 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 calibrate every X minutes). The rate of software auto-calibration is set in the
Calibration Options dialog, as described below. Full calibration includes all items in the above list.
Full calibration does not run if digitizing at any sample rate.
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 non-full items in the above list (e.g. 3.3Vref, Cjc,
i4xx power, i100 temp drift) continue to operate.
Calibration Options Dialog
instruNet software Version >=1.40.1.1 contains a Calibration Options dialog that provides several calibration
options, as described below. Calibration is helpful at reducing Time Stability
and Temperature Drift errors. 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 instruNet Basic, please see BASIC.
CAL BUTTON
This popup controls the function of the Calibration button in the Network page. It is set to one of:
- Measurement - instruNet hardware is 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.
CAL ON DIGI
If set to On, the instruNet hardware is automatically calibrated when the user presses the Start button and begins to
digitize.
CAL ON WARM
If set to On, the instruNet hardware is automatically calibrated 10 minutes and 25 minutes after the instruNet
software is first run.
AUTO CAL RATE
This field (to access, click 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. A full calibration occurs if not digitizing; and a less intensive Cjc
only calibration, described below, occur if digitizing at a slow sample rate. For details on i100 auto-calibration while digitizing, click here.
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 to the thermocouple wire. This is sometimes referred to as "Cold Junction
Compenstation", "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).
CJC CAL RATE
This field (to access, click Record tab, Setup button, Calibrate button) sets the rate that instruNet hardware
automatically performs Cjc Calibration, in units of minutes, which is used for thermocouple
measurements, as described above. If 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 thermcouple measurement error by that amount (since the software subtracts the screw terminal temperature when
calculating the thermocouple temperature).
THERMOCOUPLE DRIFT ERRORS
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 perscribed 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. Note that DasyLab and instruNet World always run in high speed mode when recording from voltage input channels as noted here. See Troubleshooting Thermocouples, for more information.
Advanced Techniques
AUTO-CALIBRATE UPON COMMAND
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.
ENABLING A BEEP WHEN CALIBRATION OCCURS
With instruNet >= v1.31.6, if you have a text file named "iNetLoad.ibs" in directory "c:\windows\system\" for Windows 95/98/Me and "c:\windows\System32\" for Windows >= Nt/2k/Xp and the file contains "Calibrate CjcBeepOn" text, a beep will sound when the cjc calibation is done; and if it contains "Calibrate BeepOn" on a separate line, a beep occurs when regular instruNet device calibration is performed.
GETTING ERRADIC READINGS INDEPENDENT OF CALIBRATION SETTINGS?
Please see Noise Reduction Techniques.
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