18
The parameters for a third-party 4-20 mA transducer must often be
calculated. As an example, if a 4-20 mA flow transducer specifies an
output range of 10 to 100 GPM (gallons per minute), calculate the
parameters as
Linearity
= 0 (will most often be the case)
Scale factor = 100 - 10 = 90 GPM full scale
Offset
= 10 GPM
You can have readings in any other units of flow by multiplying all
three parameters by the appropriate conversion factor. The warmup
delay will have to be determined by experiment if it is not identified
by the manufacturer. Use the shortest warmup delay possible that
gives consistent and stable readings.
Note:
To help you keep track of input configurations such as the
flow meter example above, you might want to enter an I.D. descrip-
tion such as “FLOW GPM.”
Programming Level Types
The level type input derives level
readings from head values
taken with a pressure trans-
ducer. There are several
programmable parameters that
allow you to solve a wide variety
of level measurement problems.
The parameters for a level type in-
put are presented in three menus. Use
the
more
option to access a deeper
menu. Use
CLEAR
to back up to the
previous menu level.
Three of the menu param-
eters are used to convert the
4-20 mA transducer output
to a pressure in PSI units.
These may be found on the
cable reel, or on a data tag
attached to the cable. Use
the column labeled “Qua-
dratic Coefficients.”
• LIN
Linearity — Set to zero if not specified by the transducer
manufacturer.
• SCALE
Scale factor — Set as specified by the manufacturer; this
value should not be zero for a pressure transducer.
• OFFS
Offset — Set to zero if not specified.
• DELAY
Warmup delay — For an In-Situ transducer, set to 50
mSEC unless specified otherwise. You will probably have to de-
termine this value by experiment for third-party transducers.
Re-
minder
: During the first ten minutes of log sampling, the HER-
MIT 2000 uses a warmup delay of 50 mSEC, regardless of the
delay programmed. If you’ve selected the log sample mode, make
certain that your transducers are compatible with this require-
ment.
The unit will check that the sum of
the delays does not exceed the se-
lected sample rate. (This is only likely
to be a problem with third-party XDs
and a very fast LIN2 sample rate.) This error message means you
need to use shorter delays if possible, or a longer sample rate.
The intermediate pressure reading is calculated using the quadratic
formula
P = LX
2
+ SX + O
where
P =
Pessure in PSI
X =
Normalized* transducer value (0-1)
L =
Linearity
S =
Scale factor in PSI full scale
O =
Offset in PSI
*
Transducer reading (in mA) minus 4 divided by 16
Units Selection
Head and level may be displayed as
feet or meters of water. Select the
UNITS
option from the menus.
Use the special function keys to se-
lect feet or meters. The current se-
lection is shown in brackets.
Your choice of units is also displayed
with the type description in all of the
menus.
The intermediate transducer pressure is converted to a head value
as follows:
Section 5: Setting Up Transducers
Test 0: DELAYS>RATE!
(cant start/step)
Input 2: Level (F)
DELAY SG UNITS MODE
Input 2: Level (F)
DELAY SG UNITS MODE
Units [F]?
FEET METERS
Input 2: Level (F)
READ I.D. REF. more
Input 2: Level (F)
LIN SCALE OFFS more
Input 2: Level (F)
DELAY SG UNITS MODE