Orifice Meter - Causes of Bias

[Zoobie]

I've got AGA3 in front of me and a bunch of other reference material but the answer I need is not jumping out at me. Could anyone out there give me the Readers Digest version of what factors contribute to bias errors as opposed to random +/- errors. Are there particular items that will consistently make the flow measurement higher than actual? The installations specifically are field gas gathering orifice meters.

Thanks in advance.

 

[zdas04]

The only things that I've ever seen that shift field measurement high are incorrect parameter entry (e.g., wrong plate size, wrong pressure ranges), a shifted zero in a transmitter, or salt plugging in the plate bore. Everything else tends to shift it low. Some examples:
- backwards plate
- plugged sensing lines
- swirl in the flow
- bowed plate

I've attached a cartoon I use in my Primary Element talk to illustrate that the things we check in a calibration are just the tip of the iceberg.

David

 

[Zoobie]

Thanks...and I really like the cartoon too.