Bently Nevada 991 Specifications

991 Thrust Transmitter

Datasheet

Bently Nevada Machinery Condition Monitoring

Description

The 991 Thrust Transmitter is intended primarily for the original
equipment manufacturers (OEMs) of centrifugal air
compressors or small pumps, motors, or fans who prefer to
provide a simple 4 to 20 mA proportional axial displacement
(thrust) signal as the input to their machinery control system.
The transmitter is a 2-wire, loop-powered device that accepts
input from our 3300 NSv proximity probe and its matching
extension cable (available in 5 m and 7 m system length
options). The transmitter conditions the signal into
appropriate engineering units proportional to the shaft’s axial
position1, and provides it as a 4 to 20 mA industry-standard
signal for input to the control system where machinery
protection alarming and logic occurs2.

The 991 transmitter provides the following notable features:

l Integrated Proximitor Sensor requires no external unit
l Non-isolated “PROX OUT " and "COM" terminals plus a

coaxial connector provide a dynamic vibration and
gap voltage signal output for diagnostics3.

l Non-interacting zero and span potentiometers under

the Transmitter label supports loop adjustment.

l Test Input pin allows quick verification of loop signal

output, using a variable DC voltage source as the input.

l A Power-up Inhibit circuit eliminates signal errors due to

line voltage transients.

l A Not OK/Signal Defeat circuit prevents high outputs or

false alarms due to a faulty proximity probe or loose
connection.

l Choice of DIN-rail clips or bulkhead mounting screws as

standard options simplifies mounting.

l Potted construction for high humidity (up to 100%

condensing) environments.

141618 Rev. K

991 Thrust Transmitter
Datasheet 141618 Rev. K

l Compatibility with 3300 NSv proximity

probe allows transducer installation in
small areas with minimal clearance,
typical of centrifugal air compressors.





Notes:

1. Probe adjustment and range are critical

in thrust position measurements.
Incorrect probe gap settings may
prevent the transmitter from reaching
full-scale in either the normal or counter
directions (no machinery monitoring).
For proper adjustment, follow the
instructions in the manual.

2. Thrust transmitters have many

limitations when compared to a
continuous monitoring system. They are
a practical solution in some applications
for measuring rotor axial position and are
a valuable tool for trending thrust
readings. While the transmitter is capable
of alarming on thrust position and non­OK checking, monitor functions such as
Timed OK channel defeat, Danger Bypass,
and Trip Multiply cannot be used. In
addition, PLCs attached to the thrust
transmitter are not suitable for plant­wide diagnostic systems such as System
1 or Rule Paks.

with ac-powered test equipment. The
adapter also inverts and isolates the
990's transducer signal, making it
suitable for equipment such as
oscilloscopes and analyzers, and
preserving industry-standard
conventions for signal polarity. We
strongly recommend that you use this
test adapter for all applications to
maintain isolation between test
equipment and the loop signal, and to
maintain machinery protection integrity.



3. The 991 Vibration Transmitter's "Prox Out"

coaxial connector provides a non­isolated dynamic transducer signal for
machinery diagnostics. You can
connect this signal directly to battery­powered or isolated test equipment to
diagnose machinery problems. However,
since the "PROX OUT" signal is not isolated
from the 4 to 20 mA loop signal, an
interface is available (and strongly
recommended) for signal isolation. The
990/991 Test Adapter conditions the 990
Transmitter's "PROX OUT" signal for use

2/17

991 Thrust Transmitter
Datasheet 141618 Rev. K

to +158 °F).

Specifications

Unless otherwise noted, the following
specifications apply at +22 °C (+72 °F) using a
3300 NSv Probe and Extension Cable, and an
AISI 4140 steel target.

Electrical

Input

Power

4 to 20 mA
Signal Output

4 to 20 mA
Loop
Accuracy

Maximum
Loop
Resistance

Current
Limiting

Zero and
Span

Not OK/Signal
Defeat

Proximitor
Sensor
Output

Output
Impedance

Linear Range

Prox Out
Incremental
Scale Factor

Temperature
Stability

Accepts 1 non-contacting 3300 NSv
Proximity Probe and extension cable.

Requires +12 to +35 Vdc input at the
transmitter terminal.

4 to 20 mAdc over specified full­scale range in 2-wire configuration.

Within ±1.5% over specified full scale
range (typical). Accuracy is rated
from the TEST signal input to the
voltage measured across a 250 Ω
loop resistance.

1,000 Ω including cable at 35 Vdc.

23 mA typical.

Non-interacting external
adjustments.

Signal output will go to less than 3.6
mA within 100 µs after a Not OK
condition occurs. Signal output is
restored within 0.1 seconds after the
Not OK condition is removed.

Compatible with ungrounded,
portable test equipment. When
using grounded, ac-powered test
equipment, use the 122115-01 Test
Adapter for signal isolation.

Prox Out has a 10 kΩ output
impedance calibrated for a 10 MΩ
load.

0.25 to 1.65 mm (10 to 65 mils) for
Prox Out. Suitable for 0.6-0-0.6 mm
or 25-0-25 mils current loop linear
ranges.

7.87 mV/µm (200 mV/mil) ± 6.5%
typical including interchangeability
errors when measured in increments
of 0.25 mm (10 mils) over the linear
range using a flat 30 mm (1.2 inch)
target.

Incremental scale factor remains
within ±10% of 7.87 mV/µm (200
mV/mil) from 0 °C to +70 °C (+32 °F

Minimum
target size

Leadwire
Length

Non­Hazardous,
Zone 2 or Div
2 Hazardous
area
locations

Intrinsically
Safe
Hazardous
area
locations

Electrical
Classification

Maritime Approvals

American Bureau of Shipping (ABS) Type Approval

Certification
Number

Environmental Limits

Transmitter Temperature

Operating
Temperature

Storage
Temperature

Probe Temperature

Operating
Temperature

Storage
Temperature

Relative
Humidity

9.5 mm (0.375 in) diameter.

Proximitor Sensor Output (BNC
connector), maximum cable
distance is 3 metres (10 feet).

13 km (8 miles) maximum between
transmitter and receiving device for
signal output.

See Hazardous Area Approvals on
page6.

General Purpose Approval by
Canadian Standards Association
(CSA/NRTL/C) in North America and
by VDE in Europe. The 991 has the CE
mark for Europe.

06-HS177078-3-PDA

-35 °C to +85 °C (-31 °F to +185 °F)

-51 °C to +100 °C (-60 °F to +212 °F).

-35 °C to +177 °C (-31 °F to +350 °F).

-51 °C to +177 °C (-60 °F to +350 °F).

100% condensing, non-submerged,
with protection of coaxial
connectors.

3/17

991 Thrust Transmitter
Datasheet 141618 Rev. K

Mechanical

Transducer
Tip Material

Transducer
Case Material

Probe Cable

Cable Armor
(optional)

Tensile
Strength

Transmitter
Weight

Total System
Weight

Polyphenylene sulfide (PPS).

AISI 303 or 304 Stainless Steel (SST).

75Ω coaxial, fluoroethylene
propylene (FEP) insulated.

Flexible AISI 302 SST with optional FEP
outer jacket.

222 N (50 lbf) probe case to probe
lead, maximum.

0.43 kg (0.9 lbm).

0.82 kg (1.8 lbm) typical.

4/17

991 Thrust Transmitter
Datasheet 141618 Rev. K

Compliance and Certifications

FCC

This device complies with part 15 of the
FCC Rules. Operation is subject to the
following two conditions:

l This device may not cause harmful

interference.

l This device must accept any

interference received, including
interference that may cause
undesired operation.

EMC

EN 61000-6-2

EN 61000-6-4

EMC Directive 2014/30/EU

ATEX

ATEX Directive 2014/34/EU

RoHS

RoHS Directive 2011/65/EU

Maritime

ABS 2009 Steel Vessels Rules

1-1-4/7.7,4-8-3/1.11.1,4-9-7/13



5/17

991 Thrust Transmitter
Datasheet 141618 Rev. K

Hazardous Area Approvals

For the detailed listing of country and product
specific approvals, refer to the Approvals Quick
Reference Guide (108M1756) available from

Bently.com.

CSA/NRTL/C

Class I, Div 2
Groups A, B, C, D

T5@ Ta [+85°C, Type 4

Installed per Drawing 128838

ATEX/IECEx

II 1 G

Ex ia IIC T4 Ga

T4 @ Ta = -35°C to +85°C

II 3 G

Ex nA IIC T4 Gc
T4 @ Ta= -35 ˚C to + 85˚C



Entity Parameters

Zone 2

Power Supply: 28V

Zone 0/1

Terminal blocks
E1-E2 “power
supply 4-20
mA”

Ui [28 V Uo[28 V Uo[ 28 V
Ii [120 mA Io [6 mA Io [ 100 mA
Pi[ 0.84 W Po [ 0.17 W Po [ 0.8 W
Ci [20 nF Co [ 80 nF Co[ 27.3 nF
Li [10 μH Lo [1H Lo [5.3 mH

Terminal blocks
E3-E4 and
connector J2
"Proximitor"

Connector J3
"Probe"

6/17