Specifications

at each channel in <ch_list> and save those values in RAM as channel Tare
constants.
Important Note for
Thermocouples
·
Do not use CAL:TARE on field wiring that is made up of thermocouple wire.
The voltage that a thermocouple wire pair generates cannot
be removed by
introducing a short anywhere between its junction and its connection to an
isothermal panel (either the VT1419A’s Terminal Module or a remote
isothermal reference block). Thermal voltage is generated along the entire length
of a thermocouple pair where there is any temperature gradient along that length.
To CAL:TARE thermocouple wire this way would introduce an unwanted offset
in the voltage/temperature relationship for that thermocouple. If a thermocouple
wire pair is inadvertently “CAL:TARE'd,” see “Resetting CAL:TARE” on
page 99.
·
Do use CAL:TARE to compensate wiring offsets (copper wire, not
thermocouple wire) between the VT1419A and a remote thermocouple reference
block. Disconnect the thermocouples and introduce copper shorting wires
between each channel’s HI and LO, then execute CAL:TARE for these
channels.
Residual Sensor
Offsets
To remove offsets like those in an unstrained strain gage bridge, execute the
CAL:TARE command on those channels. The module will then measure the offsets
and as in the wiring case above, remove these offsets from future measurements. In
the strain gage case, this “balances the bridge” so all measurements have the initial
unstrained offset removed to allow the most accurate high speed measurements
possible.
Operation After CAL:TARE <ch_list> measures and stores the offset voltages, it then
performs the equivalent of a *CAL? operation. This operation uses the Tare
constants to set a DAC which will remove each channel offset as “seen” by the
module’s A/D converter.
The absolute voltage level that CAL:TARE can remove is dependent on the A/D
range. CAL:TARE will choose the lowest range that can handle the existing offset
voltage. The range that CAL:TARE chooses will become the lowest usable range
(range floor) for that channel. For any channel that has been “CAL:TARE'd”
Autorange will not go below that range floor and selecting a manual range below
the range floor will return an Overload value (see table on page 230).
As an example assume that the system wiring to channel 0 generates a +0.1 volts
offset with 0 volts (a short) applied at the UUT. Before CAL:TARE the module
would return a reading of 0.1 volts for channel 0. After CAL:TARE (@100), the
module will return a reading of 0 volts with a short applied at the UUT and the
system wiring offset will be removed from all measurements of the signal to
channel 0. Think of the signal applied to the instrument’s channel input as the gross
signal value. CAL:TARE removes the tare portion leaving only the net signal
value.
Because of settling times, especially on filtered channels, CAL:TARE can take a
number of minutes to execute.
Programming the VT1419A Multifunction
Plus
Compensating for System Offsets
98 Chapter 3
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