Suppliers raise test standards
Having visited Endress + Hauser's flow-calibration facility, Process Measurement Technology's Michael Scott believes suppliers' flow rigs could mount a challenge to national standards.
The plant builds an average of more than 200 magnetic flowmeters each day and has to cope with more than 2.4M variations in items like tube flange or coupling size and style, or liner and electrode material.
The shape of the magnetic field is critical to the performance of a magnetic flow-meter. After the liners have been bonded to the tube, a robot positions the electrode holes and coil-mounting brackets. It then checks the roundness of the tube, selects the appropriate points and drills the electrode holes. Next the robot sites and welds the coil mounting spigots. With hard rubber liners, the robot also faces off the liner ends to provide precise flange-to-flange dimensions. Another robot performs the application of the gasket material which bonds the coil-housing shells to the tube. Reductions in cycle times are supported by consistency of sub-assembly.
The flow-calibration hall seems to set new standards. The mounting and removal of flowmeters up to 250mm is labour intensive but, with careful rig design, it can be handled reasonably readily by a single technician. With bigger flowmeters, however, the rig begins to take up a lot of space and mounting can be very time consuming. In the E+H flow lab a carousel has been designed so flowmeters can be mounted during the day and calibration carried out automatically at night. Hydraulic rams are used to clamp the straight lengths up to the flowmeter flanges, removing the need for time-consuming flange bolting.
Cost and reliability were primary considerations in the design of the facility and several networked computers monitor, control and archive all variables. Depending on the size of the flowmeter and the measuring range, flow can be adjusted precisely within seconds. The lab, complete with automatic calibration, certificate generation and pump control is controlled with a Profibus-DP system.
Seven flow calibration rigs cover flowmeters 2mm to 2,000mm using either gravimetric calibration via a diverter valve or volumetric calibration against three precision flowmeters in series. The triplicated reference meters are checked daily against the weigh tanks but can also be checked in the interim.
The flow rigs sit on three sumps containing 1.35M l of water and seven separately controlled 850KVa pumps lift slightly more water than is required to a constant head tank in a 23.6m tower. The constant head can supply a maximum flow rate of 1,666 l/s to the calibration rigs. All seven pumps are needed for calibration of the largest flowmeters.
Gravimetric calibration is achieved by diverting flow into a choice of 400kg, 5t and 50t weigh tanks. The weigh tanks are checked at least once a week against stainless steel reference weights, which can be automatically connected to the bottom of the tank. The reference weights are calibrated by the Swiss Calibration Service and checked annually to the Swiss national reference standard. The eight reference weights for the 50t tank are more than 6t each and each have an uncertainty of ±50gm.
E+H claims an accredited uncertainty of 0.05% of volume or mass rate for the smaller of the rigs and expects to receive an accredited uncertainty of ±0.1% for the largest flow rig in the near future. The company's Dr Richard Furness intends to demonstrate this with a series of private round-robin tests with other laboratories, such as Delft, the NEL, ABB Instrumentation and Danfoss.
Both Danfoss and ABB Instrumentation at Stonehouse claim an uncertainty ±0.1% of volume passed for their flow calibration. Danfoss uses gravimetric techniques, similar to those in the new E+H laboratory and ABB uses proven techniques to establish an audit trial to national standards. NEL at East Kilbride maintains the UK national flow standard and claim a ±0.1% calibration uncertainty of volume passed and as Jane Sattary of NEL, pointed out, you have to be careful to compare like with like when comparing even accredited calibration uncertainties.
Not withstanding the detailed consideration, it could be claimed ABB's and Danfoss' flow-calibration facilities have performance which can compete with the national standard, and the new E+H facility seems to raise manufacturer's standards higher. E+H's facility is a major investment and it raises the interesting question as to whether governments should try to compete? Perhaps all that is necessary is an expert committee and round-robin tests, conducted in a transparent manner, with state-of-the-art products and procedures at the best flow laboratories. Then, perhaps the best-performing laboratories could compete for a grant to service the national standard.