PIE Practical Instrument Electronics 820-Elite User Manual

Practical Instrument Electronics

82 E. Main Street Suite 3.14 • Webster, NY 14580 Tel: [phone] • Fax: [phone] • [e-mail] • www.piecal.com

PIE CALIBRATORS

Troubleshooting pH Instruments

PIE Calibrators
help you troubleshoot
& calibrate pH Instruments

Instruments that measure pH are some of the most difficult
devices to maintain in a process plant. While temperature, frequency
& pressure sensors, under the right conditions, measure for years, the pH
sensors require regular maintenance and replacement. Many technicians get
frustrated when there is a problem with pH sensors and instruments not reading
correctly. A PIE calibrator with pH simulation can speed up the troubleshooting and
adjustment of pH transmitters and analyzers. This is the most accurate method of
checking the performance and gives you confidence that the instrument is working
properly.

Why pH instruments require regular probe calibration

A typical pH probe is fragile and subject to damage from rough handling, thermal
shock, dirt, and contamination. In addition the chemistry inside the probe shifts with
time and is accelerated at higher temperatures. All these factors cause the output to
shift and these shifts must be compensated through adjustments of two parameters
to achieve the desired measurement accuracy. The first factor is OFFSET. A
perfect pH probe will produce 0.00 mV when measuring 7.0 pH (deionized water).
Even brand new probes will typically have a few mV of OFFSET. The other factor
is SLOPE. SLOPE is determined by dividing the mV difference by the change in pH
units. The theoretical slope is 59.16 mV. Some instruments indicate slope in percent
which is calculated by dividing the measured slope in mV by 59.16 and multiplying by 100.

Troubleshooting

Some pH instrument manufacturers suggest removing the pH probe and putting a shorting BNC connector or a
short/paper clip across the input terminals and see if the instrument will zero correctly & display 0.00 mV or 7.0 pH.
While this is a help it doesn’t allow you to bring the slope adjustment back to nominal and doesn’t even tell you if the
pH instrument is capable of seeing a mV signal. For instance, an instrument with a defective or shorted preamplifier
may display 0 mV or 7 pH with all input signals!

Calibrating the pH transmitter or analyzer

Disable the automatic temperature compensation (ATC) on the pH measuring instrument or
manually set it to 25°C (77°F). Disconnect the pH probe from the instrument and connect
the PIE Calibrator to the instrument with the calibrator set to output 7.000 pH (refer to the
Operating Instructions for the PIE Calibrator). For a fast check of the pH instrument you may
do a single point CAL or manually adjust the instrument to indicate 7 pH. Some instruments
have a factory reset or reset to nominal. This will reset to nominal values both the OFFSET
to 0.000 mV and the SLOPE of 59.16 mV/1 pH (known as a slope of 100%). If the instrument
has a display of OFFSET mV the reading should be 000.0 mV ±10 mV. If the OFFSET is
greater than 10 mV there may be a problem in the wiring, preamplifier or in the instrument
itself. For a full check of the operation of the instrument perform a two or three point CAL
at the pH points recommended by the manufacturer. Dial the output of the PIE Calibrator to
match the recommended buffer solutions such as 4.0, 7.0 and 10.0 or 4.10, 6.86 and 9.18 pH.

Practical Instrument Electronics, Inc. Copyright © 2016 All rights reserved

Troubleshooting pH Instruments Rev A 6 June 2016

Verifying pH transmitter milliamp output

Practical Instrument Electronics

82 E. Main Street Suite 3.14 • Webster, NY 14580 Tel: [phone] • Fax: [phone] • [e-mail] • www.piecal.com

Example: a pH transmitter is ranged from 4 to 10 pH for a span of 6 pH. After you have completed
verifying the pH input section it is time to verify the output of the transmitter. Set the PIE Calibrator
to simulate 4 pH and verify the transmitter is outputting (or the control system sees) 4.00 mA. Now
simulate 10 pH and verify the transmitter is outputting (or the control system sees) 20.00 mA. If the
transmitter isn’t outputting exactly 4.00 and 20.00 mA adjust the milliamp output per the manufacturer’s instructions.

Reconnecting & compensating for pH probe errors

Once you have verified proper output of the transmitter you may reattach the pH probe, turn automatic temperature
compensation (ATC) back on and proceed with the probe calibration with buffer solutions. This step cannot be skipped –
even new pH probes have some kind of OFFSET and SLOPE errors that must be corrected before the pH transmitter will
measure to within specifications. After the instrument has been adjusted with a PIE Calibrator you MUST do a two or three
point pH buffer calibration of the probe to match it with the instrument. Follow the instrument manufactures calibration
procedure with the proper buffer solutions. If the OFFSET correction is greater than ±50 mV or the SLOPE correction is
not between 85 to 102% the probe will require cleaning or replacement. A typical response time for a pH probe is 30 to
60 seconds. If it takes more than 120 seconds (two minutes) to settle on a reading the probe requires reconditioning or
requires replacement.

Troubleshooting pH probes

There are many things that can wrong with pH probes. Symptoms include slow or incorrect readings, noisy signals or no
change in the reading at all. Some processes will cause a film to build up on the surface of the probe or clog the pores
which requires physical cleaning or submersion in particular solutions. Other processes can poison one or both sides of
the probe which can sometimes be fixed by submersion into solutions or replacement of the electrolyte. Probes that have
dried out for short periods of time can often be brought back to proper operation. How long is a pH probe supposed to
last? In a laboratory environment measuring mild solutions they can last for a year to 18 months. Under harsh process use
they need to be replaced more often. For more information on care and cleaning of pH probes contact the manufacturer of
your pH probe or instrumentation.

PIE Calibrators with pH Simulation

Most calibrator manufacturers don’t have ANY calibrators that simulate pH. Some of the PIE calibrators that include built-in
simulation directly in pH are shown below.

Model 235

Process Voltage Calibrator

Source & Read mV, V dc plus percent
of 1 to 5 volts plus Simulate pH

820-ELITE Multifunction Standard

Process Calibrator

Source & Read mA, V,
Thermocouple, Ohms, RTD &
Frequency plus Simulate pH and
measure pressure with optional
modules

830 Process Calibrator

Dual Display & Loop Diagnostics

All the functions of the 820-ELITE plus
a dual display and loop supply for stand
alone calibration of 2-Wire transmitters.
Advanced diagnostics finds hidden loop
problems.