Endress+Hauser gammapilot FTG 671 Technical Information

Technical
Information
TI 177F/00/en

Radiometric Level Measurement

gammapilot FTG 671

Non-invasive, radiometric limit detection
Intrinsically safe signal circuit [EEx ib] IIC /
ATEX II (2) GD
For scintillation or Geiger-Müller detector

Application

The Gammapilot FTG 671 transmitter is
designed for

•

non-invasive, level limit detection

in containers - mixers, reactors,
hoppers, silos, tanks - with inflammable,
abrasive, poisonous and aggressive
bulk material and liquids, e.g. in acid
tanks, boilers, cement silos, ballast
hoppers, cyclones, cupolas, rotary
furnaces etc.. Thanks to the fact that the
measuring system is fitted externally, it
also finds application in the food
processing industry.

Gammapilot transmitters may also be
used for applications in explosion
hazardous areas, and possess
intrinsically-safe sensor circuits
conforming to EEx ib IIC, or EEx d IIC,
EEx de IIC depending on the detector.

The Gammapilot
FTG 671 transmitter is a
19" plug-in card, shown
here in Monorack II
housing

Features and Benefits

• Suitable for scintillation detector

DG 57 or Geiger-Müller counters
DG 17 or DG 27

• Uses lowest possible source activity

with DG 57/100 mm or longer

•

Digital signal transmission

- Interference-free function with
two-core installation cable up to
1000 m

•

Intrinsically safe sensor circuits

- Ex-protection withoutZener barriers

approved as overspill protection
(VBF, WHG Germany) and for use
in explosion hazardous areas

•

Simple operation with parameter
matrix

- Uniform handling as for all
Commutec transmitters

•

Practical background functions

- Automatic compensation of source
decay

- Automatic adjustment of switch
point

•

Commutec transmitter

- Simple connection to automatic
systems via Rackbus

- Up- and download of data

The Power of Know How

Hauser+Endress

Measuring System

Gammapilot
FTG 671
transmitter

Standard level detection
application showing the
arrangement of the
system components
source container,
detector and transmitter

Source

Measuring System

A working system for level limit detection
comprises:

Gammapilot FTG 671 transmitter,

•

• QG 020 or QG 100 source container

with Co 60 or Cs137 gamma source

• DG 17, DG 27 Geiger-Müller counter

or DG 57 scintillation counter.

Detector

Min./max.
relays

Signal Input Circuit

The Gammapilot supplies the power to
the DG… detector and, depending on
type, the detector returns an
interference-free pulse frequency or
pulse code modulated countrate signal.
The intrinsically safe signal input is
electrically isolated from the transmitter
supply and the outputs.

Conveying limestone
Gamma radiation
barriers control the
drying process

Automatic filling of beer
barrels
The level is checked as
the barrels move past
the barrier

Scintillation Counter DG 57

Most measurement systems use the DG
57 scintillation counter, thus ensuring:

• highest sensitivity

•

with lowest source strength.

Endress+Hauser have many years of
experience with rod scintillation counters
and the new DG 57/100 mm counter has
been specially developed for limit
detection. Sensitivity can be further
increased by using the DG 57/
400 mm or longer.

Minimum
level
detection

Vibration
channel

Maximum
level
detection

FTG 671

Output Signals

The Gammapilot FTG 671 has two limit
relays which can be operated with freely
adjustable hysteresis independently of
each other in minimum or maximum
fail-safe mode.

Self-Monitoring Function

The Gammapilot FTG 671 continuously
monitors all signal lines. On fault
condition:

•

An alarm relay with potential-free
changeover contact de-energises

•

In default condition the limit relays also
de-energise.

FTG 671

Feed rate 1
barrel/2s

Filling
station

Full move
forward
Half full are
rejected

2

Measurement Principle

Principle limit detection
by gamma radiography

Nmax

Countrate

Nmin

Change in countrate
as product moves
through barrier

1000

Statistical fluctuation
in countrate

Countrate due to residual
and background
radiation

time

Gamma-Ray Absorption

Radiometric level measurement relies on
the fact that gamma rays are absorbed
as they penetrate a material. The
attenuation of the radiation is dependent
upon the density ρ, the thickness d of
the material as well as the linear
absorption factor µ. This in turn is
dependent on the material and gamma
source. The attenuation is calculated
from the formula

µ . ρ . d

= e

F

s

The radiation also decreases with the
square of the distance between source
and detector.

Limit Detection

For limit detection µ, and d are
constants and the detected radiation
level depends upon the presence or not
of the penetrated material:

•

The countrate is at a maximum when
the path to the detector is completely
free and

•

at a minimum when all the radiation
has to travel through the material and
is attenuated.

Radiation Statistics

The emission of gamma rays by a
radioactive source is governed by the
laws of statistics. For this reason the
measurement precision of gamma
systems is dependent on the statistical
variation in countrate. The variance, i.e.
the degree of variation between
measurements, depends upon the time
interval chosen. The precision of the
measurement depends on the chosen
confidence level - 1σ, 2σ or 3σ. For a
countrate of N pulses per second:

•

σ =N;±

68.28 % confidence

level

•

2= 2N;σ±

•

3= 3N;σ±

95.48 % confidence level

99.74 % confidence level

Normally, a confidence level of 2σ is
used. The statistical variation can be
reduced by increasing the sampling or
integration time τ:

Statistical variation

2N±

=

τ

For limit switching the integration time
corresponds to the response time of the
relay.

The maximum and minimum countrates

and N

N

max

are obtained when the

min

transmitter is calibrated, whereby their
dependency is given by:

max

−µ . ρ . d

e

= N

N

min

The switch points can be set anywhere
within these two countrates, which are
normalised to produce the end values 0
and 100.

3

Background Functions

The Gammapilot FTG 671 is equipped
with a number of background functions
which ensure accurate and reliable limit
switching. Thus, for example, the
statistical fluctuations in countrate and
decay of the gamma source described
above are accounted for without any
action on the part of the operator.

After calibration, the FTG automatically
sets the switch points to preset values. It
warns the operator if incorrect limits are
entered, and when the activity of the
source is no longer sufficient for reliable
switching.