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I am using a dp transmitter to meausre the level of water in a tank. This tank always remains 100% full. however, in case of some particular conditions the valve in discharge line which is at bottom of the tank opens to discharge water. My problem is, whenever we open the discharge valve the level value drops down suddenly although the actual level of water in tank is still high. When the valve is close again the level shows correct value?

Confused why it is so?

It sounds as though the pressure transmitter you are measuring the level with is connected to the discharge line so you are getting some pressure drop due to flow.

The transmitter should be installed on a dedicated nozzle for the best results.

Roy

I agree with Roy that a level transmitter must have a dedicated connection on the Tank. and it usually also have a standpipe so that the level being sensed by the transmitter would be quite stable already, more especially so if the Tank's level is being agitated & probably swirling due to inflow. This set-up is a standard practice in water tank level measurement.

dondon

yes, there is dedicated line for the transmitter and the drain is separate.

The transmitter is usually place at the bottom with the high side open to air and low side online, for level measurement.

Is it possible you are creating a partial vacuum on the tank when you open the discharge valve.

If one side of the DP cell were open to atmosphere and the tank pressure drops that would account for a sudden drop in level as the static pressure drops.

Try connecting the LP connection to the open space on top of tank.

Roy

Why you need DP transmitter to measure tank level? I might miss something here since I can't imaging how you tank looks like that demands the use of DP to determine its level.

Normally we can just use level sensor. You don't need DP pressure sensor. Again I might be wrong.

A DP transmitter is one method you can use to measure level in a tank whether opened or closed because you can utilize the fact that the pressure at the bottom and top of fluid are different. The connection will differ due to the condition (hot, volatile, etc) of fluid and in which tank (open/closed) is your fluid.

> A DP transmitter is one method you can use to measure level in a tank whether opened or closed because you can utilize
> the fact that the pressure at the bottom and top of fluid are different. The connection will differ due to the
> condition (hot, volatile, etc) of fluid and in which tank (open/closed) is your fluid.

True. But if you don't have to use DP then don't use it. If you just need to know the level only then why use DP then? Just use level sensor.

You just add one systematic uncertainty for using DP to measure level, i.e. uncertainty related to velocity head component. Velocity head component can be very localized and can be transient too. If you are not lucky enough occasionally the measured level can go very wrong. If the level is used for control then you might have oscillation problem.