8AA2A2006 datasheet(3/20 Pages) MAGNETROL

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P R O B E

S E L E C T I O N

Probe selection is the most critical part of applying an

RF Capacitance device for a given application. The goal

is to select the probe that will give the optimum capaci-

tance change per unit level change (pF/inch). The first

step in selecting an RF probe is determining the correct

configuration for your application. The following guide-

lines will assist you in this selection.

1. Use bare probes for non-conductive liquids.

2. Use insulated probes for conductive liquids. If you are

uncertain about the conductivity value, use an insulated

probe. Teflon® has the widest temperature range and

material

compatibility.

Kynar®

will

maximize

the

capacitance change.

3. Use

probe

with

integral

ground

reference

(Reference Probe) when measuring non-conductive

fluids in horizontal vessels, non-conductive liquids

where the probe will be mounted more than 12" (30 cm)

from the vessel wall, or when measuring any liquid in

non-metallic vessels. The stillwell probe is the most

common. If the application requires “no metal” in the

process; or, if the liquid is too viscous for a stillwell,

use the reference rod probe. The reference wire probe

should

only

used

clean,

conductive,

non-coating applications.

4. Use a flexible (cable) probe when the measurement

range is greater than 10 feet (3 meters). Rigid (rod)

probes are available up to 234 inches (6 meters) in

length, but they are sometimes difficult to handle and

can be damaged during installation.

5. Use

inactive

sheath

probe

when

mounting

horizontally through a nozzle.

R I G I D

P R O B E S

Rigid probes consist of a process connection, probe rod,

and the seal. The rod may take many forms, depending

on the application. The following is a description of some

of the more common rigid probe styles:

Bare probes

Bare rod probes are typically used in non-conductive

process media with a dielectric value less than 10

or a conductivity value less than

µsiemens/in.

Capacitance is measured from the probe through the

process media to the vessel wall.

Insulated probes

Insulated rod probes are used in conduc-

tive

process

media

with a dielectric

value greater than 10 or a conductivity

value greater than

µsiemens/in.

Capacitance

typically measured

from

the probe rod through the insulation to

the process media, which is at the same

potential as the vessel wall for conductive

media. Probes are insulated with Teflon®,

Halar®, or Kynar®. When you are uncertain

about

the

dielectric

constant

your

process media, insulated

probes

are

wise choice.

Inactive sheath probes

An inactive sheath is a metallic tube that is tightly cou-

pled to the insulation on the probe rod and attached

to the process connection. The sheath “deadens” the

portion of the probe covered. It is used when a

false capacitance

could

developed by

interfer-

ence, such as:

• Collection of debris in a nozzle when

the probe is horizontally mounted

• Falling process media entering the vessel

• Use with Retractable Probe Assembly (Hot Tap)

Probe rod

Insulation

Probe rod

Insulation

Sheath

Bare

Probe

Insulated

Probe

Inactive Sheath

Probe

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