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Compensating for System Offsets
System Wiring Offsets
Important Note for
Thermocouples
Residual Sensor Offsets
Operation
106 Programming the HP E1415 for PID Control
The HP E1415 can compensate for offsets in your system's field wiring.
Apply shorts to channels at the Unit-Under-Test (UUT) end of your field
wiring, and then execute the CAL:TARE (@<ch_list>) command. The
instrument will measure the voltage at each channel in <ch_list> and save
those values in RAM as channel Tare constants.
•
You must not use CAL:TARE on field wiring that is made up of
thermocouple wire. The voltage that a thermocouple wire pair
generates can not be removed by introducing a short anywhere
between its junction and its connection to an isothermal panel (either
the HP E1415'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 you inadvertently CAL:TARE a thermocouple wire
pair, see "Resetting CAL:TARE" on page 107.
•
You should use CAL:TARE to compensate wiring offsets (copper
wire, not thermocouple wire) between the HP E1415 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.
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.
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:TAREd" Autorange will not go below that range floor and
selecting a manual range below the range floor will return an Overload value
(see the table "Maximum CAL:TARE Offsets" on page 107).
As an example assume that the system wiring to channel 0 generates a +0.1
Volt offset with 0 Volts (a short) applied at the UUT. Before CAL:TARE
Chapter 3
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