What is the Custody Transfer Approval?

One definition EASILY obtained through Google:

" ... term custody transfer as the flow measurement of a product between parties where the measurement will be used for financial purposes."

Results 1 - 10 of about 27,600 for how to choose a custody transfer flow meter with Safesearch on. (0.32 seconds)

Accuracy is important in custody transfer - is a clamp on ultrasonic in that class now? İ don't remember all the details but the document will tell you (or certainly should) what class of metering is required. Get the sales document and see what it says. More than likely what you have to do is clearly spelled out.

Custody transfer is required for financial transactions.

Most important of these is when you pay tax on a product. The most familiar of these is the good old petrol (gasoline) pump.

I am a process engineer so you really need an instrument person to fill in the details but from memory the key features for custody transfer are

Accuracy - depending on your service this will be a percentage of the delivered flow. ie for the gasoline pump if it says 10 gallons this should be 10 gal +/- 0.05gal or whatever the figure is

Repeatability - ie if you do several runs of the same flow do you get the same reading (within certain limits) ie for the gasoline pump every driver that delivers 10 gals gets 10+/-0.05 gals

Meter Proving - how is the meter tested to demonstrate it is still reading ok. A govt official turns up with a govt standard 10 gallon can. Fills it from the pump and checks the reading on the pump when they have govt approved 10 gallons in the can. If they don't match there is a problem. Probably the gas station should do their own checks on a regular basis (monthly?)

I think I would be surprised too that a clamp on ultrasonic would meet the repeatability criteria just because I would imagine that it would be easy to misalign the mounting and screw up the reading that way

Those points will be clearly spelled out in whatever standard is being used - AGA is one İ am familar with. What is being measured?

I have never seen clamp on type ultrasonic meters used for custody transfer approval.This again depends on what is changing hands here.

Is it an existing large pipe carrying inexpensive(not true in many contries anymore)water,where both parties agree to use this to settle account.

Ultrasonic flowmeters are extensivly used nowadays for custody transfer of high pressure natural gas.

These meters are flow calibrated using high pressure gas and can achieve an uncertainty of <0.2%(Nothing is certain ).

see AGA#9 for details.

see AGA # 9 for more details