Best Practice Maintenance and Troubleshooting

| pH Calibration Best Practice Maintenance and Troubleshooting | November 19, 2020 |
© Yokogawa Corporation of America
Best Practice
Maintenance and
Troubleshooting
pH Calibration Webinar
Cherlyn Marlow & Tim Roth
November 19, 2020

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Presenters
3
Cherlyn Marlow
Process Analyzer Marketing Manager
Yokogawa
Cherlyn has been with Yokogawa for over 12 years as the Liquid Analyzer Product Manager
while the roles and responsibility change and adapt to meet the company’s growing needs.
This past year she has moved to become the Process Analyzer Marketing Manager. She has
built a reputation for herself within her industry as someone that end users can call to get
help and information to solve difficult problems, even when it is not her equipment.
Tim Roth
Senior Analytical Application Specialist
Yokogawa
Tim has played an integral role in the promotion, application, and sale of process
instrumentation having successfully held various positions in direct sales and sales
management over the past 40 years. Tim is a veteran of the United States Air Force and
earned his Associate degree in computer engineering from the Milwaukee School of
Engineering and Bachelor of Science degree from Marquette University. In Tim’s current
role, he is actively involved with the application and sale of both liquid and gas phase
analytical analyzers used across a broad spectrum of markets and applications.
Presenters

Agenda
4
1. Theory
 What is pH?
 Nernst
2. Maintenance
 Cleaning
 Calibration
3. Troubleshooting
4. Summary

5
pH Theory

pH Working Principle
6
E = E0+0.059 Ln (H+) volt
 Slope @ 20°C = 58.16 mV/pH
 Slope @ 25°C = 59.16 mV/pH
E = E0+ R·T/F Ln [Mn+] volt
– R = gas constant
– F = Faraday
– N = loading of the metal
– [Mn+] = concentration metal ions
– T = Absolute temperature (Kelvin)
– E0 = normal potential (1mol/l + metal)

pH – Acid or Base?
7
1.0
0.1
0.01
0.001
0.0001
0.00001
0.000001
0.0000001
0.00000001
0.000000001
0.0000000001
0.00000000001
0.000000000001
0.0000000000001
0.00000000000001
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
.00000000000001
.0000000000001
.000000000001
.00000000001
.0000000001
.000000001
.00000001
.0000001
.000001
.00001
.0001
.001
.01
0.1
1.0
[OH-
][H+
]
Acid
Base
Neutral
pH
pH = -log [H+
(Hydrogen Ion Concentration)
Moles/Liter

8
pH Electrode- How Does it Work?
The following are the most
significant potentials:
• E1 = Potential between pH Glass
and Process
• E2 = Potential between pH Glass
and Fill Solution (7 pH buffer)
• E3 = Potential between Measuring
Pin and Fill Solution
• E4 = Potential between Reference
Pin and Electrolyte (KCl)
• E5 = Potential between Electrolyte
and Process (Diffusion Potential)
Liquid
E2
Membrane
ElectrolyteE4
Electrolyte
E5
Diaphragm
E1 E2
E3
E5 E4
E1
E3

9
Process Temperature Compensation
5
6
7
8
9
10
11
12
13
14
15
5 6 7 8 9 10 11 12 13 14
PH AT PROCESS
TEMP.
PH AT REFERENCE TEMPERATURE
pH as a Function of Temperature
10°C
25°
C40°C
80°C
pH 11 at 25°C
becomes 9.8 pH at 80°C

Electrode Life – Temperature Effect
10
0
2
4
6
8
10
12
14
25°C 50°C 75°C 100°C
SensorLife(Months)
Process Temperature

11
Maintenance

12
pH Glass - New
What does sensor look like when it is new?
 pH glass is “porous”
 Special recipe  defines “hole size”
 pH glass only selective for H+
pH Glass
H+
H+
H+
H+
H+
H+
H+
H+
H+
H+

13
pH Glass - Aged
What happens to the glass with age/usage?
 The glass layer get thinner
 Not equal over the whole glass
 Gives less response to pH changes
 Thinner glass > faster > not in this case
pH Glass

14
pH Glass
How does it work?
 Low concentration of H+
 High concentration Na+
 Causing “Sodium error”
 pH will read lower than it really is
 ± 0.17 pH from pH > 12
pH Glass
H+
H+
H+
H+
Na+ Na+
Na+
Na+
Na+
Na+
Na+
Na+
Na+

Maintenance versus Calibration/Validation
15
 Do not confuse maintenance with calibration
Bad cleaning / good calibration
good calibration/ bad installation
 Usefulness of maintenance
To change changeable effects
 Usefulness of Calibration
To take into account the un-changeable effects like aging
 To guarantee accuracy and reliability

Calibration / Validation Interval
16
 How often :
 Determined by a combination of type of sensor/ process
 Process:
 Temperature
 Pressure
 Fouling
 Chemistry / process composition
 Electrode:
 Specifications of the electrode
 Aging:
 pH sensors ages over time, glass deteriorates
 Slope will decrease
 Two point calibration will show this
 Analyzer will predict life time expectancy

Cleaning, Cleaning, Cleaning
 Clean the electrode before any maintenance or
calibrations are performed.
17
 Alkaline or Scale
 Acidic Coatings
 Oil, Grease, Organics
 2, 5 or 10% hydrochloric acid (no more
then 10%)
 Weak caustic (less than 4% NaOH)
 Typically a hot, soapy detergent (like
Dawn) or if coating is difficult an organic
solvent compatible with sensor materials.
Contaminant Cleaning Solution

Types of Calibration: One Point Calibration
18

Types of Calibration: Two Point Calibration
19

Maintenance – Calibration / Validation Procedure
20
 DO’s
 CLEAN THE pH ELECTRODES FIRST
 Use FRESH Buffer Solutions
 2-Point Calibration (the very best technique)
 Zero point: pH 7
 Slope: pH 4
 Allow Stabilization Time
 Rinse the between calibration measurements
 DO NOT’s
 Do Not - Calibrate to Grab Samples.
 Do Not - Use Caustic Buffers (9 or 10 buffer).
 Do Not - Wipe across Glass Membrane.
The Frequency of Calibration Depends On:
Process Solution • Maintenance • Desired Accuracy
For More about Maintaining pH loop refer to Technical Note TNA0918
Temperature Traceable pH Value
°C °F pH 7 pH 4 pH 10
0 32 7.12 3.98 10.31
5 41 7.09 3.99 10.24
10 50 7.06 3.99 10.17
15 59 7.04 3.99 10.11
20 68 7.02 4.01 10.06
25 77 7.00 4.00 10.00
30 86 6.99 4.00 9.96
35 95 6.98 4.01 9.92
40 104 6.98 4.02 9.88
45 113 6.97 4.03 9.85
50 122 6.97 4.05 9.82
Temperature NIST pH Value
°C °F pH 7 pH 4 pH 9
0 32 6.98 4.00 9.46
5 41 6.95 4.00 9.40
10 50 6.92 4.00 9.33
15 59 6.90 4.00 9.28
20 68 6.88 4.00 9.23
25 77 6.86 4.01 9.18
30 86 6.85 4.02 9.14
35 95 6.84 4.02 9.10
40 104 6.84 4.04 9.07
45 113 6.83 4.05 9.04
50 122 6.83 4.06 9.01

21
Troubleshooting

Typical pH Electrode Problems
22
 Poisoning of Internal Element
 Rapid Depletion of Electrolyte
 Plugging of the Junction
 Ground Loop Effects
MEASURE
(GLASS) Electrodes
 Coating of the pH Membrane
 Slow Response (High Impedance)
 Abrasion and/or Breakage
 Temperature Shock
REFERENCE
Electrode

Troubleshooting Quick Tips
23
Problem Causes Solutions
Slow Response
Coating of measuring electrode or
plugging of reference electrode;
aged electrodes.
Clean and calibrate; replace
electrodes.
Noisy Response
Reference electrode poisoned or
depleted; faulty cabling or solution
ground connection.
Check cabling; Clean, calibrate
and replace electrodes as needed.
Measurement Drifts
Coated glass electrode; Plugged
reference junction; faulty cabling or
connections; aged electrodes.
Check cabling; Clean, calibrate
and replace electrodes as needed.
Incorrect Measurement
Plugged Reference junction;
electrodes out of tolerance.
Clean, calibrate and replace
reference electrode as needed.
Will Not Buffer Calibrate
Bad buffer solution; unclean
electrodes; faulty electrodes or
cabling.
Make sure buffer tables match
buffers used; check cabling; clean,
calibrate and replace electrodes as
needed.

Where to Start?
24
 The best troubleshooting tool is your eyes.
 Some of the problems you can see:
 Poisoning
 Dirty/Coated
 Broken sensors
 Wet/damage cables
 Bad connections
 Poor/Improper installation
 Incorrect wiring
 Dry sensors
 Old sensors

Real-time Sensor Diagnostics
25
Process Guard
Real-time online impedance
checking to detect pH glass
breakage, when the sensor is out
of solution or when the reference
junction is starting to become
coated.
Maintenance Manager
Sensor wellness function shows
the condition of the pH glass and
reference electrode, predicts
when the next calibration is due
and when the sensor should be
replaced. The calibration logbook
stores past and present
calibration values to help
determine the present condition
and the remaining life of the
electrode.

Step-by-Step: Troubleshooting the Analyzer
26
Check data in the
analyzer
Simulate pH 7 and 4
Short inputs 13, 14 and
15

Troubleshooting pH: The Analyzer Video
27

28
Troubleshooting pH: Before and After Readings

29
Troubleshooting pH: The Analyzer Wiring Video
• Short inputs
• pH = 7
Liquid earth
Reference
Measuring
Stable measurement pH 7

Step-by-Step: Cables and Liquid Earth
30
Connector dry?
Check Wiring
Cable Coloring
Coding
Visual Check
Resistance of the
cable

Troubleshooting pH: Cables & Liquid Earth (2/4)
31
DON’T FORGET
THE JUMPERS
Cable Color Coding
Measuring Reference Temperature

32
Visual Check
• Mechanically
• (outside mantel)
• Is the cable shorter ?
• Check Isolation
• Wet cable ?

33
Cable Resistance
• Shield (blue) and outside gold plated bushing
= low resistance
• Measuring lead – inside pin = low
resistance
• Shield – measuring = high resistance

Troubleshooting pH Video: Sensors
34
VALIDATION /
CALIBRATION
Response: Slow, Fluctuating,
Fast
Cleaning
Visual Check /
Poisoning
Fresh Buffers

35
Troubleshooting pH: Sensors - Visual Check
Look closely…..
Empty chamber No longer a gel

36

37
Empty
chamber

 Compare specifications sensor with real process
• Pressure
• Temperature
• pH range
• Process composition (HF / NaOH, sulfite) etc.
 Installation
 Any recent changes?
38
Troubleshooting pH: Installation/Process

 Installation
 Before dosing
 “Dead” time
 Air bubbles
 Flow
 0.1 ~ 10 l/min
 Max. 6 m/s
39
Troubleshooting pH: Process
 Low conductivity
 50μS/cm
 Process composition
 Fluoric Acid
 Caustic

40
Process Installation Heavy buildup

41

42

43
 Musts when mounting horizontal
 Ensure proper over pressurized sensor is used
Normal pH sensors have air
bubble to accomidate expansion
with temp increases

44
What’s
wrong
with this
installation?

45

46

47
Conclusion and Wrap-up

Summary
 Theory: What you need to know
 Best practice maintenance
 What to look for when troubleshooting
 So What Now?
 pH Quick Reference Poster
 FAQ #ThursdayTip Video Playlist
 pH Calibration Best Practice
 Contact us:
 webinars@us.yokogawa.com
 Talk to our experts:
 support@us.yokogawa.com
 1-800-888-6400

49
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Best Practice Maintenance and Troubleshooting

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