401-405 SERIES - 19 -
3.7.1. Zero Shift
The zero offset can occur as the result of natural convection flow through the sensor tube if the
instrument is not mounted in a level orientation with flow horizontal. This natural convection effect
causes a zero shift proportional to the system pressure. The overall effect is more pronounced for gases
with higher density. Normally the shift is within the allowable zero offset range and can be removed by
activating the zero reset at the operating pressure.
3.7.2. Span Shift
The gas properties which form the basis for the flow measurement, such as viscosity and specific heat,
exhibit a slight dependence on the gas pressure. Fortunately, this pressure dependence is predictable
and can be corrected for in cases where it has an impact on accuracy (typically only significant for
pressures in excess of 100 psig). The graph shown in Figure 3-1 shows the expected span shift as a
function of pressure for nitrogen. This behavior is similar for most diatomic gases (O
2
, H
2,
etc), whereas
this effect is insignificant for the monatomic gases (He, Ar, etc). This span shift must be considered and
accounted for as appropriate for accurate flow measurements at high pressure conditions.
3.8. Warnings/Alarms
There are two alarm contacts on the terminal strip connector within the electronics enclosure (See
Section 2.8). These function as isolated semiconductor switches sharing a single, isolated common line.
In its normal state each switch is “open”; when an alarm is activated the switch is “closed”.
The meter’s processor can be configured via the digital interface to establish the internal condition for
activating each alarm. There are many choices for internal alarms and warnings including overflow,
underflow, or various instrument error conditions. Each alarm can also be given a selectable “wait
time”—a period for which it must remain in the alarm condition before the physical alarm is activated.
See the Software Manual for detailed alarm setting and configuration information.
3.9. Multi-gas Calibrations
The Hastings 400 Series flow meters can have up to eight different calibrations stored internally. These
are referred to as gas records. These records are typically used to represent different gases, but they can
also be useful in other ways; for instance reporting the flow in an alternate range, flow unit or reference
temperature. The records are referred to by their number label from #0 – #7. The first six records are,
by default, setup for the same range in the most common six gases as shown in Figure 2-11. If a gas
other than one of these six is specified on the customer order it will be placed in record #6. If a second
different gas is selected, it will be placed in record #7. If multiple different gases or ranges are specified
they will replace some of the standard six gases. Only the gas(es) specified on the order will be verified.
The other records will use nominal gas factors to approximate the gas sensitivity until an actual
calibration is performed to correct for individual instrument variations. Selecting the active gas record
can be done in one of two ways—a hardware setting or a software setting. The hardware setting is done
by accessing a rotary encoder on the upper PC board in the electronics enclosure. When set to a
number position from 0 to 7 it activates the corresponding gas record. When set to a number greater
than 7, the gas record control is passed to software. If the software setting mode is enabled, then the
“S6” digital command can be used to set the active gas record as shown in the example below.
Example: To first determine and then change the active gas record using RS-232,
Computer transmits: {S6}
HFM flow meter replies: {0}
This indicates that gas record #0 is currently active.
Computer transmits: {S6=4}
This changes the active gas record to #4.
See the Software Manual for further information including how to setup a new gas record and how to
reconfigure an existing gas record.
