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Yokogawa Electric Corporation

pH Holders

Q1 Please explain the holder types and applications.
A1 A holder is used to mount a pH sensor to a measurement location. It is also used to attach various cleaning devices for cleaning the pH sensor. Selection of the optimum holder and cleaning device for application or conditions of a measurement location is important for ensuring reliable measurements or facilitating maintenance.
YOKOGAWA offers the following holders:

Drop-in Sensor Guide Holder (PH8HG) :
  The simplest holder. No cleaning device can be attached to it.
The holder body is made of polypropylene or PVC resin.
Immersion Type Holder (PH8HS) :
  This holder is for immersing a pH sensor into test water for measurement.
Various cleaning devices can be attached to it, and a flange can also be mounted. The holder body material is either polypropylene or stainless steel (SUS316 or equivalent).
Immersion Type Holder (PH8HSF) :
  (Flameproof Version, available only in Japan)
  This holder is suitable when measuring in an explosion-hazardous area and if ultrasonic cleaning needs to be added.
If measuring in an explosion-hazardous area but ultrasonic cleaning is not required and another cleaning device is desired, the PH8HS should be selected.
Flow-Through Type Holder (PH8HF) :
  This holder is used for measuring mid-way along a pipeline.
Various cleaning devices can be attached. The holder body material is either polypropylene or stainless steel (SUS316 or equivalent).
Flow-Through Type Holder (PH8HFF) :
  (Flameproof Version, available only in Japan)
  This holder is suitable when measuring in an explosion-hazardous area and ultrasonic cleaning is desired.
If measuring in an explosion-hazardous area but ultrasonic cleaning is not needed and another cleaning device is desired, the PH8HF should be selected.
Suspension Type Holder (HH350G) :
  This holder is suitable where the immersion length is large or in an indoor location with a low ceiling because only the sensor needs to be pulled up for calibration with the standard solution or for maintenance. Only a jet cleaning device can be attached to it.
Angled Floating Ball Holder (PB350G) :
  This is suitable for pH measurement in sewage or plant effluent treatment plants where the sensor tends to become contaminated quickly.
It is also suitable for locations where the liquid level varies.
Vertical Floating Ball Holder (PB360G) :
  This is suitable for pH measurement where installation space is limited, in sewage or plant effluent treatment plants where the sensor tends to become contaminated quickly.

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Q2 What materials are used for the O-ring of holders and how are they selected?
A2 O-rings are used for sealing the sections where the pH sensor and cleaning devices are mounted.
The O-ring is usually made of fluoro-rubber, but Dai-El perfluoro rubber can be selected if the test solution is an organic solvent rich solution, alkaline solution, or high-temperature alkaline solution.
However, the performance is not guaranteed.

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Q3 How does the holder material depend on the temperature and pressure of the test solution?
A3 The temperatures and pressures of the test solution at which various holders can be used vary depending on the holder body material and the presence/absence of a cleaning device.
The table below shows these temperatures and pressures for the various holders.
Holder Type Holder Material Cleaning System Process Temp Test solution pressure
Drop-in Sensor
Guide Holder
PP
without -5 to 80℃ Atmospheric pressure
(max. water depth: 3m)
PVC
without -5 to 50℃  
Immersion type
(Include Flameproof Version)
PP
without -5 to 100℃ Atmospheric pressure
(max. water depth: 3m)
SUS
with -5 to 80℃  
Flow-through type
(Include Flameproof Version)
PP
without -5 to 80℃ Atmospheric pressure
to 500 kPa
with -5 to 80℃
SUS
without -5 to 105℃
with -5 to 80℃
Suspension type
SUS
without, with -5 to 80℃ Atmospheric pressure
(max. water depth: 3m)
Floating ball type
PVC
SUS
without 0 to 50℃ Atmospheric pressure
(max. water depth: 3m)
Vertical Floating Ball Holder
PVC
SUS
without 0 to 50℃ Atmospheric pressure
(max. water depth: 3m)
PVC=Rigid Polyvinyl Chloride, PP=Polypropylene, SUS=Stainless Steel

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Q4 What material of ultrasonic cleaning oscillator is best?
A4 We offer three types of oscillator material.
The SUS316 oscillator is for general-purpose use (pH 3 to 14), the titanium oscillator is for salty water, and the Hastelloy C oscillator is for a solution containing a strong acid (pH 0 to 4).

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Q5 What is the maximum length of an immersion-type holder that you manufacture?
A5 For immersion-type holders, we offer pipe lengths of up to 3 m for the general-purpose PH8HS (non-explosionproof type) and lengths of up to 2 m for the explosionproof PH8HSF. For longer lengths, please consult us because it depends on the installation conditions, test solution temperature, etc.

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Q6 What are the considerations for the temperature, pressure, and flow rate of a test solution to be introduced to the holder for high-purity water?
A6 When measuring the pH of deionized water (conductivity: 0.1 to 50 μS/cm), note the following:
The test solution is chemically pure, but its purity will be quickly degraded by air, metallic ions at the connecting section, the ingress of KCl solution, or other factors, causing the pH to change.
Because the test solution has a low conductivity, a large potential is generated by a leakage current or there are effects of external induction, resulting in measurement errors. Moreover, in the flowing condition, a flowing potential occurs at the electrode section, which degrades the signal-to-noise ratio.
Ion exchange on the glass electrode's glass membrane surface takes time, causing a slow response.
To overcome these problems, YOKOGAWA offers the PH8EHP pH Sensor and PH8HH Holder for deionized water measurements.
The temperature of the test solution introduced to the holder for high-purity water must be no more than 50℃ and the pressure must be atmospheric pressure (the outlet open to air).
Moreover, the flow rate of the test solution varies depending on the conductivity of the test solution.
The figure below shows the relationship between the conductivity of a test solution and its flow rate.

the relationship between the conductivity of a test solution and its flow rate

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