Heating up competition: redesigned temperature compensator extends flowmeter capabilities.

In the US, liquefied propane is bought by weight but dispensed by volume, making athermalization, an aspect of instrument design that prevents temperature changes from affecting what is being measured, is mandatory on flowmeters. By National Weight and Measures Bureau regulations, the standard unit of trade for propane is the gallon at 60[degrees]F.

Most scientists and instrument designers know that the most accurate athermalization device available today is a liquid-filled metal bellows. Incompressibility of the liquid ensures a positive linear displacement for the bellows with ambient temperature change. Kinematically through a linkage, this displacement adjusts the gear ratio between the sensing element inside the meter (usually a piston) and the readout on the register.

The athermalization assembly works as follows: As the volume of the fluid within the thermostat bellows assembly expands and contracts with temperature changes, the change in volume is translated by the bellows into a linear movement. This linear movement is transferred by a Linkage to the flowmeter's register. The entire device acts as a "transmission" to offset the output of the measuring element to the register in a manner appropriate to the temperature/density of the fluid being measured.

Originally, the LPM series of flowmeters made by Liqua-Tech Corporation, Ukiah, CA, was qualified for propane monitoring only. By switching to a nickel bellows in their design the series now can handle many other gases, including ammonia and the more contaminant-laden propanes found in overseas commerce. The bellows that the company had been using was mechanically formed of beryllium copper and encased in a brass housing filled with glycol. However, deliveries from the original bellows vendor were continually hitting delays, so Liqua-Tech's director of operations, Ed Bruce, shopped for a drop-in replacement from another vendor.

Design engineers at Servometer, Cedar Grove, NJ, suggested several changes to the athermalization assembly that would improve the Liqua-Tech product without changing the size envelope. "While brass and beryllium copper may be fine for pure propane service, ammonia gas corrodes these 'red metals', as do many contaminants found in all but the purest propane," says Paul Hazlitt of Servometer. Hazlitt replaced the beryllium copper bellows with nickel and the brass cover with stainless steel. He also proposed that the bellows metal be formed by electrodeposition rather than by mechanical press forming. Electrodeposition made possible thinner walls and finer convolutions, leading to a more sensitive athermalization structure and a more reliably accurate meter.

For safety, Servometer also recommended switching to a silicone-based fill liquid in place of the original glycol compound. "Glycol is flammable, and a known carcinogen; the silicone liquid is neither of these," notes Hazlitt. "This change makes their product safer for us to manufacture, and safer to use in the field. Where there's propane, there's always risk for fire."

The athermalization assembly measures 3.581-in. long by 0.930-in. in diameter. The push rod extends 1.544 in. out of one end (at 60[degrees]F). The bellows itself measures 1.759-in. long by 0.775-in. in diameter. The new assembly fits into the same space in the flowmeter bodies as before. With minimal additional changes, the model LPM 102 flowmeter, using the nickel bellows, now qualifies to meter ammonia gas and lower-purity propanes that corroded the earlier unit. Since Servometer's assembly process includes leak tests, Liqua-Tech saves operating costs by eliminating this testing from the current manufacture of their flowmeter.

Circle 201--Liqua-Tech, or connect directly at www.rsleads.com/ 508df-201

Circle 202--Servometer, or connect directly at www.rsleads.com/ 508df-202