VXI VT1422A, User Manual

Page: 272 / 529

270 VT1422A Command Reference Chapter 7
CALibration
Command
Sequence
Storing A/D cal constants
perform complete A/D calibration, then...
CAL:STORE ADC
Storing channel tare (offset) values
CAL:TARE <ch_list>
to correct channel offsets
CAL:STORE TARE
Optional depending on necessity of long
term storage
CALibration:TARE
CALibration:TARE (@<
ch_list
>)
measures offset (or tare) voltage present on the
channels specified and stores the value in on-board RAM as a calibration constant
for those channels. Future measurements made with these channels will be
compensated by the amount of the tare value. Use CAL:TARE to compensate for
voltage offsets in system wiring and residual sensor offsets. Where tare values
need to be retained for long periods, they can be stored in the module's Flash Memory
(Electrically Erasable Programmable Read Only Memory) by executing the
CAL:STORe TARE command. For more information, See “Compensating for
System Offsets” on page 153.
Note for RSC Units
CAL:TARE does not remove offsets in an RSCs field wiring. For the VT1529A/B,
the SENS:STR:UNSTrained value is the equivalent of the tare offset. Any offset in
the analog signal line between the VT1529A/B SCP and an RSC is accounted for
during the CAL:REMote? operation.
Notes For
Thermocouples
1. CAL:TARE must not be used on field wiring that is made up of thermocouple
wire. The voltage 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 VT1422A'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 channel. If a
thermocouple wire pair is inadvertently CAL:TARE’d, use
CAL:TARE:RESET to reset all tare constants to zero.
2. CAL:TARE should be used to compensate wiring offsets (copper wire, not
thermocouple wire) between the VT1422A 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.