Liquid level switch BM 34

© KROHNE 09/2001 7.02370.21.00 

GR/PRINTO 

Liquid level switch 

BM 34 

Status: 10/99 

Variable area flowmeters 

Vortex flowmeters 

Flow controllers 

Electromagnetic flowmeters 

Ultrasonic flowmeters 

Mass flowmeters 

Level measuring instruments 

Communications engineering 

Engineering systems & solutions

Liquid level switch 


For use in potentially hazardous environments 

Operating principle 

A float guided on a non-magnetic tube 

follows the level of the liquid surface,thereby 

actuating up to 6 reed contacts, monostable 

or bistable, inside the tube via a built-in 

magnet system. 

Types of contacts 

All contacts are fitted to a rod with a clamping 

device. Where switching points are not 

known or the contact has to be adjustable 

the clamps are loosely attached. 

For overfill protection systems, the uppermost 

contact is non-detachable. 

The length of the guide tube and installation 

height of the reed contacts are specified 

from the lower edge of the screw connection 

or mounting flange. 

Approved for 

hazardous duty 

and as part of an 

overfill protection 

system 

Up to 6 freely 

adjustable or preset 

switching elements 

Temperature range 

–200 … +200°C 

(–328 … +392°F) 

and operating pressure 

max. 40 bar (580 psig) 

0.3 – 6 m 

(1 – 20 ft) 

tube length 

Monostable 

reed contacts 

These switches are only 

actuated as long as the 

float and magnetic system 

are at an equal height. 

If the float moves away, 

the contact returns to its 

original setting. 

The achieved switching 

state can be maintained 

with an auxiliary relay. 

Floats with a ring magnet 

system are used. 

Bistable 

reed contacts 

These switches are 

actuated by the float’s 

magnetic system and remain 

in the switching state. 

Therefore, they provide a 

clear indication of the float’s 

location, i.e. above or below 

the switch, without the need 

for an auxiliary relay. The 

switch setting is retained 

until the float moves past 

the switch in the opposite 

direction. Floats with a 

radial magnetic system are 

used. 

A variety of materials 

permits use also 

in aggressive media 

Float 

with ring 

magnet 

system 

Float 

with radial 

magnet 

system 

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Range of application 

BM 34 is ideal wherever liquid levels need to be monitored in 

process and storage tanks. 

Typical liquids are: 

● Water and aqueous fluids 

● Acids / alkalis 

● Organic and inorganic solvents 

Special versions 

Interface detection, BM 34 TS 

If two liquids with different densities are stored in one tank, the level 

of the interface can be measured using a special interface-type float. 

The weight and buoyancy of the float are such that it floats on the 

heavier and sinks in the lighter liquid. The minimum difference in density 

between both liquids should be greater than 100 g/l (6.24 lb/ft 3 ), 

and the float must be fully immersed in the light liquid. 

If the lighter liquid is not always stored in the tank, this must be 

specified on ordering to ensure appropriate float design. 

For ships’tank with test device, BM 34/T 

In the BM 34/T version, an additionally installed rod magnet in the 

tube can be raised with a test knob. By means of a magnetic coupling, 

a sleeve on the outside of the tube follows this motion. It is thus possible 

to raise the float to contact point, thus permitting a check of the 

instrument’s switching functions. If the float has developed a leak and 

filled with liquid, the magnetic coupling breaks off due to the heavier 

weight and no contact is produced. 

Guide tube centering aid 

For agitated liquids, a centering device welded to the tank bottom is 

recommended to avoid buckling of the guide tube. 

For approved devices, a bottom centering device is mandatory where 

the length of the guide tube is > 3000 mm (> 9.8 ft). 

Product 

Overview 

Vibration 

Continuous, non-contact Switches 

Ultrasonic Radar Ultrasonic Buoyancy Capacitance 

Materials /coatings 

For use in aggressive products, all wetted parts can be coated with PVC, 

PP or PTFE. Other materials are titanium or Hastelloy-C4. 

Continuous, contact 

Ultrasonic Buoyancy TDR 

Dimensions in mm (inches) 


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Responsibility for suitability and intended use of our instruments rests solely with the purchaser. 

Technical data 

Length (guide tube) 

0.3 – 6 m (1 – 19.7 ft) 

Switching accuracy ± 3 mm (± 0.12’’) 

Max. allowable operating pressure 

Standard 


BM 34/F/WB, BM 34/F/WN max. 3 bar (43.5 psig) 

BM 34/Ex/Z0, BM 34/F/Ex/Z0 


depending on flange pressure rating and float pressure resistance. 

Process temperature 

Standard 

–55 … +150°C ( –67 … +302°F) 

Option 

–200 … +200°C (–328 … +392°F) 

BM 34/Ex/Z0 

–20 … +150°C ( –4 … +302°F) 

BM 34/F/WB 

–20 … +60°C ( –4 … +140°F) 

BM 34/F/WN 

–20 … +200°C ( –4 … +392°F) 

BM 34/F/Ex/Z0 

–20 … +75°C ( –4 … +167°F) 

Operating data 

Min. density 

Standard, BM 34/Ex/Z0 0.4 kg/l (25 lb/ft 3 ) 

Option, BM 34/F/WB, BM 34/F/WN, BM 34/F/Ex/Z0 0.5 kg/l (31 lb/ft 3 ) 

Viscosity* 

Standard 

≤ 5000 m Pa·s 

For approved instruments 

≤ 1000 m Pa·s 

Solids content* 

≤ 100 g/l (6.24 lb/ft 3 ) =^ 10% dry substance 

Particle size* ≤ 200 µm 

Reed contacts 

Type/number 

max. 6 monostable or 6 bistable changeover contacts 

(bistable contact not possible with F/WN, F/WB or F/Ex/Z0) 

Version Standard F/WN, F/WB Ex/Z0, F/Ex/Z0 

Operating voltage 65 V 30 V 20 V 

Max. contact rating 40 W / 60 VA 0.9 W (VA) 200 mW (VA) 

Max. current switched 1 A 30 mA 40 mA 

Contact spacing 

monostable changeover contact > 50 mm (1.97’’) 

bistable changeover contact > 70 mm (2.76’’) 

Switching hysteresis 5 mm (0.2’’) 

Holding range 17 mm (0.67’’) 

Lowermost switching point < guide tube length – 130 mm (5.12’’) 

Uppermost switching point 

> 100 mm (3.94’’) distance from bottom edge of flange 

Connection 

Standard DIN 2501, Form C DN 25, PN 40 

Option DIN 2501, Form C DN 40 – DN 150/PN 6 – PN 40 

ANSI B 16.5 1’’– 6’’ 150/300 lb/RF 

DIN 2999 

G 11/2’’ A (SW 50 mm wrench) 

Plastic-coated flanges DIN 2501, Form C ≥ DN 50/PN 6 – PN 40 

ANSI B 16.5 ≥ 2’’ 150/300 lb/RF 

BM 34/T DIN 2501, Form C DN 50/PN 40 

ANSI B 16.5 2’’ 150/300 lb/RF 

Information on higher pressure ratings and other standards available on request 

Protection category to EN 60529 / IEC 529 IP 56, equivalent to NEMA 3 

Materials 

Flanges 

Screw connection 

Guide tube 

Float 

Housing 

Stainless steel 1.4571 (316 Ti) 

Stainless steel 1.4571 (316 Ti)/titanium plated 

Stainless steel 1.4571 coated with PVC, PP or PTFE 


See instrument versions 

See Selection of float type 


* Relevant for instruments approved as part of an overfill protection system. 

36 

BM 34


Selection of float type 

Shape Dimensions Materials Min. density Max. allowable Product temperature 

(diameter x height x wall-thickness) operating pressure min. max. 

in mm in inches kg/l lb/ft 3 bar psig °C °F °C °F 

Standard versions without explosion protection 

Cylinder 97 x 145 x 1.0 3.82 x 5.71 x 0.04 Stainless steel 1.4571 (316 Ti) 0.85 53.07 40 580 -200 -328 200 392 


Ball 142 x 145 x 0.7 5.59 x 5.71 x 0.03 Stainless steel 1.4571 (316 Ti) 0.40 24.97 25 363 -200 -328 200 392 


Cylinder 97 x 145 x 1.0 3.82 x 5.71 x 0.04 Titanium 0.45 28.09 35 508 -160 -256 200 392 

Ball 142 x 145 x 1.0 5.59 x 5.71 x 0.04 Titanium 0.40 24.97 35 508 -160 -256 200 392 

Cylinder 94 x 100 3.70 x 3.94 PVC 0.60 37.46 4 58 -40 -40 40 104 

Cylinder 94 x 100 3.70 x 3.94 PP 0.40 24.97 4 58 10 50 60 140 

Cylinder 130 x 165 5.12 x 6.50 Hard glass 0.70 43.70 16 232 -50 -58 200 392 

Cylinder 94 x 100 3.70 x 3.94 PVDF 0.70 43.70 4 58 -40 -40 100 212 

Cylinder 140 x 175 x 1.0 5.51 x 6.89 x 0.04 Hastelloy-C4 0.70 43.70 2 29 -200 -328 200 392 

BM 34/T 


Instrument approved for use in Zone 0 (EC) 

BM 34/Ex/Z0 







Instruments approved as part of an overfill protection system 

BM 34/F/Ex/Z0 



Cylinder 97 x 145 x 1.0 3.82 x 5.71 x 0.04 Titanium 0.60 37.46 23 334 -20 -4 75 167 

Ball 142 x 145 x 1.0 5.59 x 5.71 x 0.04 Titanium 0.50 31.21 23 334 -20 -4 75 167 

BM 34/F/WB 





BM 34/F/WN 



Cylinder 140 x 170 x 1.0 5.51 x 6.69 x 0.04 Hastelloy-C4 0.70 43.70 2 29 -20 -4 200 392 



Product 

Overview 

Vibration 





Important ! 

The specified max. allowable operating pressure applies to 20°C or 68°F only! 

Float test pressure = 1.3 times the max. allowable operating pressure. 

Hazardous-duty units = 1.5 times the max. allowable operating pressure. 


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Instrument versions 

Version BM 34/RR BM 34/R* BM 34/R-PVC BM 24 R-PP 

(not approved) 


Flange Stainless steel 1.4571 Screw connection Stainless steel 1.4571 Stainless steel 1.4571 

(316 Ti) Stainless steel 1.4571 (316 Ti) (316 Ti) 

(316 Ti) with PVC face with PP face 

Guide tube Stainless steel 1.4571 Stainless steel 1.4571 Stainless steel 1.4571 Stainless steel 1.4571 

material (316 Ti) (316 Ti) (316 Ti) / with PVC tube (316 Ti) / with PP tube 

Guide tube 

dimensions 

mm 21.30 x 2.00 21.30 x 2.00 20.00 x 1.50 / 25.00 x 1.80 20.00 x 1.50 / 25.00 x 1,80 

inches 0.84 x 0.08 0.84 x 0.08 0.79 x 0.06 / 0.98 x 0.07 0.79 x 0.06 / 0.98 x 0.07 

Weight ** 

approx. kg 4 3 5 5 

approx. lb 8.8 6.6 11 11 

Version BM 34/R-PTFE BM 34/R-Titan BM 34/R-Hastelloy-C4 


Flange Stainless steel 1.4571 Stainless steel 1.4571 Stainless steel 1.4571 

(316 Ti) (316 Ti) (316 Ti) 

with PTFE face titanium plated Hastelloy-C4 plated 

Guide tube 

material Stainless steel 1.4571 Titanium Hastelloy-C4 

(316 Ti) / with PTFE tube 

Guide tube 

dimensions 

mm 21.30 x 2.00 /25.00 x 2.00 21.30 x 2.00 25.00 x 1.60 

inches 0.84 x 0.08 / 0.98 x 0.08 0.84 x 0.08 0.98 x 0.06 

Weight ** 

approx. kg 5 4.5 4.5 

approx. lb 11 9.9 9.9 

** Only for standard instruments and those approved for use in Zone 0 

** Weight = Instrument with 1 m (3.3 ft) guide tube length plus 1 kg (2.2 lb) for every additional meter (3.3 ft) tube length 

Guide tube test pressure = 1.5 times the max. allowable operating pressure at 20°C (68°F). 

38 



Dimensions 

in mm (inches) 

BM 34/RR 

Flange version 

BM 34/R 

Version with screw connection 

BM 34/T 

For ships’ tank with test device 

Product 

Overview 

1 Cap 

2 Test knob 

3 Wire 

4 Reed contact 

5 Float 

6 Rod magnet 

7 Sleeve 

Vibration 






39


Approvals 

Application Instrument version Certification mark 

With explosion protection BM 34 /– / K .. / Ex / Z0 PTB No. Ex-85 / 2156 X 

In stationary storage tanks for flammable 

water pollutants in classes AI, AII and B (excl. CS 2 ), 

float and guide tube in Zone 0 

Overfill protection with explosion protection BM 34 /– / K ..- F / Ex / Z0 PTB No. III B / E 26390 F 

In stationary storage tanks for flammable, 

water pollutants in classes AI, AII and B (excl. CS 2 ), 

float and guide tube in Zone 0 

Overfill protection without explosion protection BM 34 /– / K ..- F / WB PTB No. III B / S 1806 F 

In stationary tanks [max. operating pressure 

3 bar (43.5 psig)] for flammable water pollutants 

in class AIII 

Overfill protection without explosion protection BM 34 /– / K ..- F / WN DIBT Approval Notice Z-65.11-223 

In stationary tanks [max. operating pressure 

3 bar (43.5 psig)] for non-flammable water pollutants 

PTB = Physikalisch-Technische Bundesanstalt 

Type code 

Instrument 

BM 34 Liquid level switch 

Material 

RR 

Stainless steel 

R 

Stainless steel, screw connection 

R-PVC PVC version 

R-PP PP version 

R-PTFE PTFE version 

R-Titan Titanium version 

R-Hastelloy-C4 Hastelloy-C4 version 

Number of contacts 

K1 1 contact K4 4 contacts 

K2 2 contacts K5 5 conatcts 

K3 3 contacts K6 6 contacts 

Type of contact 

W monostable 

Bi bistable (not approved as part of an overfill protection system) 

Range of application 

N normal range (not for approved instruments) 

Ex/Z0 electrical equipment for use in hazardous areas 

F/Ex/Z0 hazardous-area electrical equipment as part of an 

overfill protection system for flammable 

water pollutants, classes AI, AII, B 

F/WB part of an overfill protection system for flammable, 

water pollutants, class AIII 

F/WN part of an overfill protection system for non-flammable, 

water pollutants 

Special versions 

T Instrument with test device for non-hazardous areas 

and for ships’ tanks 

TS Interface detection (when used in Zone 0, the 

max. guide tube length is dependent on the float weight). 

Not approved as part of an overfill protection system. 


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Liquid level switch BM 34

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