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This presentation is a systematic
procedure to isolate problems in your ORP measuring system.
Troubleshooting
Equipment
The following tools are
recommended to assist you in problem identification:
- Portable pH/mV meter with interconnect
cable
The meter and mV standards can verify operational status of an
ORP electrode.
- DVM (digital volt meter)
If you do not have a portable pH/ORP meter, you can read ORP electrode
output on a DVM in millivolt scale.
- pH/ORP Simulator
Simulators similar to Sensorex
C110 pH/ORP Checker can quickly identify or eliminate the
meter, preamplifier, or controller as the cause of the problem.
- ORP/mV
solutions
Use 7 and either 4 or 10 buffer depending upon at which end of
the pH scale you normally operate in.
Meter, Preamplifier, Controller Troubleshooting
The meter or controller
is the easiest component to eliminate as a possible cause of your
problem. Follow the stepwise instructions below for quick
problem solving.
- Disconnect the ORP electrode from the meter, preamplifier (if
you have an external preamp), or controller.
- Connect a pH/ORP Simulator (like the Sensorex C110 pH/ORP Checker)
in the position where the electrode was connected.
- Set the C110 simulator to +mV(+700mV) and verify the
meter, preamplifier, or controller output .
- Reset the C110 simulator to -mV and verify output.
If you did not get +700 and -700mV respectively, recalibrate or
troubleshoot the meter, preamplifier, or controller in accordance
with its instruction manual. Note
that miswired preamps usually give negative readings or constant
zero reading regardless of electrode or simulator input
ORP Electrode Troubleshooting
Remove the electrode from the sample or process and disconnect it
from the meter or controller. Connect to the portable pH/mV meter
or DVM and test electrode in ORP/mV solutions of your choice. We
suggest using 2 standards so that a span/slope** number can be established.
Compare results to the following table:
| 0 mV in all mV standards |
a) Bad connection
b) Internal short circuit |
a) check/fix connection
b) Contact Sensorex for Return Authorization |
| Buffers read close to expected value but speed of response*is
slow (>30 seconds) |
dirty electrode pH glass and/or reference junction |
Clean electrode per instructions included with it when shipped |
| Large offset in ORP/mV solutions** |
a) reference poisoned
b) Ground loop**** |
a) Contact Sensorex about special references
b) Ground solution for tank or line to known earth ground or
buy Sensorex Ground
Loop Interrupt ORP electrode |
Short span***
(Typically less than 70%) |
a) Dirty platinum or reference junction or scratched platinum
b) aged electrode |
a) clean electrode per electrode's care and use instructions
b) replace electrode (too old) |
| Unstable or drifting reading |
reference dirty or plugged |
clean electrode per electrode's care and use instructions
(do not use abrasives to clean reference
junctions!) |
If you are still having
problems after trying the above diagnostics, fill out our Application
Questionnaire and e-mail it to us for assistance.
Speed
of Response*
New ORP electrodes from Sensorex will respond to within 95% of their
final value in less than 30 seconds in ORP solutions/mV standards.
Response in the solution being measured may be slow and could
take hours to equilibrate. As electrodes age, Speed of Response
will become slower. Coatings or scratches on the ORP electrode's
platinum surface will also give a slower response time.
Offset**
A perfect electrode will read within the range of the respective
ORP/mV solution as shown on the label. Offset is defined as
the difference between the electrode's reading in an ORP/mV solution
and the expected range. For example, when using Sensorex's ORP Calibration
Kit, pH 7 buffer + quinhydrone should give 70-105mV (90mV being
a perfect reading) and pH4 + quinhydrone should give 245-280mV (260mV
being a perfect reading). Based on the ranges, an electrode
that reads outside of the range should be evaluated further.
Cleaning the electrode or replacing should rectify the offset.
Span***
A perfect electrode will have 100% of the theoretical Span which
means that when an electrode is measured in 2 different ORP/mV solutions
and the difference is calculated, if the difference is within 10%
of the ideal span then the electrode can continue to be used. Normal
electrode aging will reduce the Span.
Ground
Loops****
When an ORP system is unstable, erratic, or the
offset drifts, the most common problem is an electrical ground loop
in the system, particularly if the tank and/or pipes are plastic.
To verify this problem, remove the electrode and calibrate it in
a known buffer in a beaker. If the electrode measures within specification
(stable and adjustable offset) when calibrated, place a copper wire
in the beaker and the other end in your system. This will permit
voltage in the system to be transferred through the wire into the
solution in the beaker. If the reading becomes unstable or
shifts, a ground loop is your problem. To fix the ground loop,
ground the solution to a known earth ground source or use a Sensorex
Ground
Loop Interrupt ORP electrode.
ELECTRODE CALIBRATES
THEN
ELECTRODE READS HIGH
IN BUFFER
OR OUT OF RANGE IN PROCESS
 
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