KROHNE BM 90 EN - Manual (Page 34)

34
and connect a meter across terminals 25 and 26 and
repeat Pr.34 test.
If the output is now stable check wiring and meters
G) Analogue has no Output:
1. Check programme value at Pr.30 - Value 1-6.
2. Insert a test meter in series with the output. Under
Pr.34 enter a fixed output.
If still no output, connect a test meter directly across ter-
minals 25 and 26, repeat test under Pr.34.
If no value is read at terminals 25 to 26 contact Krohne.
H) Analogue Output is less than 20 at maximum dis-
play reading:
1. The load attached to the output may be too high. To
check this disconnect all the external wiring and see if it
know reads 20. The output is capable of driving 20mA
into 750 Ohms.
I) Analogue Output does not correspond to applica-
tion:
1. Checked that the correct options (Pr.30 to Pr.33) have
been selected.
2. Check that the correct span (Pr.4) has been input, this
is the value over which the analogue will be spanned
unless a separate entry has been made at Pr.32 or Pr.33.
J) Reading on display and outputs stay high:
* This is usually caused by return echoes from close-in
obstructions.
1. Check for obstructions. If the transducer is mounted
on a standpipe, check for rough edges at the connection
with the vessel, refer to the figure Page 12.
2. If there are no close-in obstructions ensure that the iso-
lation kit is fitted on the transducer and the transducer is
mounted correctly. The isolation kit should enable the
transducer to move slightly, it should not be solid. ( Not
applicable to flanged transducers.)
3. Check the entry at P5, Blanking distance, and return to
0.5m if reduced from factory setting
4. May be caused by rate of change, Pr.6 being too small.
K) Reading is lower than expected:
* This only occurs when the system is locked on to a mul-
tiple of the true echo.
1. Check that Pr.3 and Pr.4 are correctfor the application
2. It can be caused when the level rises into the blanking
zone. The system can then lock on to a multiple echo,
and may continue tracking the multiple when the level
decreases. Using check search Pr. 69 should rectify this
situation, but preventing the level entering the blanking
zone is the preferred solution.
3. It can also be caused by the level moving at a much
faster rate than is allowed for by the defined rate of
change (Pr.6) . To solve the problem the rate of change
value should be increased to more closely match the real
rate.
L) Reading is unstable :
1. If a high rate of change (Pr.6) is defined the display will
be more unstable. Therefore, an unnecessarily high value
of Pr.6 should be avoid.
M) Reading changes in steps:
* This is usually caused by the rate of change value (Pr.6)
being too small to keep up with the process.
1. To rectify, increase the value of Pr.6 to match the rate
of change of level.
N) The display is inaccurate:
1. The empty distance (Pr.3) of the vessel may be incor-
rectly set.
2. The dimensions of the vessel or flume may be incor-
rect, as may the values of maximum flow, volume or
mass conversion.
3. The system may require temperature compensation.
4. The application may include vapours that significantly
change the speed of sound. Provided these are constant
over the range the speed of sound can be adjusted
through Pr.70.
O) Temperature is inaccurate :
1. The position of the transducer/temperature sen-
sor is important to prevent heating by sunlight and con-
vection currents. Also the sensor should be in a free-air
vented position if possible to prevent hot-spots.
2. Check that temperature compensation is enabled at
Pr.37.
3. Check the resistance of the temperature probe when
disconnected against the value in Pr.39 when connected.
If using a temperature compensated transducer, check
this resistance value across the shield and black core
when disconnected.