V Cone and Mag Meter accuracy limits

[Noway2]

A question has come up amongst some people in my department, for which I don't have an informed answer and I am hoping to get some opinions on the subject.
In a chilled water application, we have some buildings that have flow meters used for measuring the BTU usage. The meters are mostly V-Cone, but can also be mag meters. In some of the larger buildings, there is insufficient turn down in the meter to allow reading under largest flow conditions without falling below the zero drop out on the minimum flow condition. In some of the worst locations, we are placing dual meters to cover the range but this is expensive and requires a service outage. My question relates to the ones where we don't put the secondary meters.

If the water flow goes below the meters zero drop out, will the error tend to read, above or below, or bounce between the actual water flow?

 

[VEBill]

The term zero drop out has always (?) meant that the sensor output drops to zero indication when the actual is lower than the cut off (low, but not zero). So the sensor reads low (zero).

Stand by for others that have actual met a V-cone socially.

 

[Noway2]

Thank you for the reply. I think it is more a point of where it can't be read accurately anymore, but it is still analog signal based upon the pressure difference across the cone, so you still get some signal. We call it a zero drop out because when it falls below a fraction of this, we then zero the output. I am curious if it becomes noisy or of it will read biased in one direction or the other.