Know-How sight glasses and flow indicators


Sight glasses are monitoring equipment without indicating mechanisms.

Flow indicators have internal components that are set in motion by the flow.

Flow meters are fitted with a calibrated measuring scale and indicating flag.


Sight glasses and flow indicators are supplied with one or two glasses. Two glasses enable the user to see through the medium. If the installation site is dark or if it is difficult to see the glass, a light may be fixed at the back which allows a better view of the flowing medium.

Glass qualities

Soda lime glass is used for temperatures up to 150 °C (see DIN 8902); in the case of alkaline media (boiler water) only up to 100 °C. Maxos glass is used for temperatures up to 280 °C (see DIN 7080).

Flow direction

Our standard flow indicators are supplied for left-to-right flow; we also supply indicators for right-to-left flow at the customer´s request. Please state the required flow direction when ordering.

Operating limit

Flow indicators and flow meters have an operating limit. Below this limit the flow is insufficient to set the internal components in motion (see Data Sheet).

Indicator flag for one direction

Flow indicators with flag and reset spring for one flow direction are the most popular type and can be installed in any position. Soda lime glasses feature a graduated scale of lines from 1 to 10. This scale allows the flow volume to be gauged easily and quickly. In many simple applications this type of indicator may be used as an alternative to an expensive flow meter. For the smaller DN (up to G 1 / 25 mm) the indicating flag is supplied without a spring to allow it to indicate even small volumes. Vertical installation with flow from top to bottom, however, requires a spring.

Indicator flag for both directions

The centrally mounted flag is deflected towards the flow direction, thus indicating the direction of the flow. The position of the flag changes as the flow increases or decreases.

External indicator

For turbid or opaque liquids without magnetic contamination you should select a flow indicator with external pointer (Type 6.12). What has been said above under item “Indicator flag for one direction” applies to this type of indicator as well.

Ball-type indicator

The flow moves a ball from its rest position (at the bottom of the housing) to the top. This type of flow indicator must be installed vertically for flow from bottom to top.

Indicator with impeller

The flow causes the impeller to rotate. The speed of the impeller allows the flow volume to be gauged. Suitable for all installation positions as well as all pipeline flow speeds below 1 m/sec.


Some media tend to deposit dirt on the internal surface of the glass. For such cases we supply a type fitted with an elastomer wiper. The wiper can be supplied only for single-glass flow indicators.

Flow meters

An internal weighted flag indicates the flow volume in m3/h water at 20 °C on a calibrated scale. Flow meters can be supplied in two measuring ranges for each nominal diameter.

Resistance to media

The standard cover seal is not resistant to all media.


Please note the flow direction if you want to install a flow indicator with fixed direction of flow.


Flow indicators and sight glasses are quite simple units which do not require any special maintenance during operation. Dirty glasses should be cleaned and the cover seals renewed at the same time.


Remove the glass for cleaning. For reassembling the screws should be tightened evenly and diagonally using the specified torque, or the clamp ring seal tightened gently to avoid damaging the glass. We recommend that you keep a supply of replacement glasses.

Valves free of oil and grease or silicone

Please pay attention to order an fit only spares free of oil and grease resp. free of silicone.

Please consult our engineer if extreme operating conditions apply or whenever you are in doubt.

Notes on Safety, Operating Instruction etc. MUST be followed.

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