Flow switch

[Lc85]

I have a flow switch which is used as a fan start permissive it is on the outlet of a lube oil unit. If the flow of oil is greater than 40 l/min then the start can continue. If for some reason the flow goes below 40 l/min then an electric pump needs to be started to bring the flow up. Am I right in thinking that this is only one switch point on 40 l/min falling? It's driving me crazy.

Thanks

 

[dpc]

You might want Google "deadband". The flow switch contact will close at one value and open at a somewhat lower value. The difference between the two is the deadband. So probably one switch contact will be enough unless there are other issues.

 

[DRWeig]

The deadband dpc referred to should be listed in the datasheet for your flow switch.

Best to you,

Goober Dave

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[Lc85]

Thanks guys, the lube oil company don't have a data sheet to give me. Is there a sort of average dead band that can be used?

 

[waross]

The deadband is a property of the switch, not the oil. I would expect a range of 1/2% to 3% of full scale value. It may be more but not much.
Less, more expensive and also too narrow a deadband may result in "chattering" in many applications.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Lc85]

40 l/min is the minimum the four bearings need these are fan DE,NDE and motor DE,NDE. I can't remember off hand how this is split but pump has been sized around 25% larger. I think the single switch point should be fine at 40 falling?

 

[waross]

An exception is the pressure switch on domestic water pressure systems. Typical ON may be 20 PSIG and OFF may be 40 PSIG. Both off point and deadband are adjustable. You may see other pressures but typically the deadband is 50% of the set point.
Whatever the deadband may be, the odds are very good that the designer has chosen a switch with a deadband appropriate for the application.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[danw2]

Manufacturer's pressure switch spec sheets provide a range of deadband values for any given switch because of the way mechanical tolerances add or subtract one another. In the example below, the deadband for the 3 to 100 psi switch (listing in first column) can be anywhere from 1 to 4 psi (listed in 2nd column).

Two identical models can have different deadbands within the spec'd range.

 

[waross]

We realize that you have a flow switch rather than a pressure switch, the pressure switch references are to illustrate dead-band.

Bill
--------------------
"Why not the best?"
Jimmy Carter