As an analytical chemist, I'm always ensuring my glassware is thoroughly clean before I start an analysis. However, my supervisor always states that using ultrasonic bath to clean volumetric glassware damages its calibration, rendering it imprecise for analytical usage. Does that hold true, or is it okay to use an ultrasonic cleaner?
According to technical information for volumetric measurement provided by Brand, it is acceptable to use an ultrasonic bath.
Both glass and plastic labware may be cleaned in an ultrasonic bath. However, direct contact with the sonic membranes must be avoided.
For gentle treatment of labware, clean immediately after use – at low temperatures, with brief soaking times, and at low alkalinity. Glass volumetric instruments should not be exposed to prolonged immersion times in alkaline media above 70 °C, as such treatment causes volume changes through glass corrosion, and destruction of graduations.
If your supervisor is unconvinced by Brand's guidance on this (as supplied by Loong in his answer), you could always test the effect of ultrasonic cleaning directly:
Take, say, six volumetric flasks, and weigh the amount of water each holds when filled to the line. Then subject three to several cycles of ultrasonic cleaning (the other three can be left to sit in the ultrasonic cleaning solution, but without turning the machine on), dry all six, and repeat the weighing measurement. Determine if the before-and-after difference for the ultrasonically cleaned flasks is significantly different from that seen in the control flasks.
While the control flasks do control for temperature differences between the before-and-after measurements, it would be cleaner if you could do the before-and-after measurements at the same temperature. That way the only difference you see should be random experimental error plus any systematic differences due to cleaning.
[Or more simply, if your boss is unconvinced by Brand's guidance, you could ask Brand if they could supply their test data on the effect of sonication.]
EDIT, BASED ON COMMENTS: If cumulative damage is the concern, then one could always employ an accelerated wear technique. In this case, one would increase the sonication time until it matched the total sonication time anticipated over the lifetime of a volumetric flask. E.g., if standard cleaning time is 20 minutes, and glassware is typically cleaned 600 times, then one could leave it in the sonicator continuously for 8 days (assuming the sonicator wouldn't be damaged by this). If you don't have a spare sonicator, you could do this over four successive weekends. Or perhaps one weekend would be sufficient to satisfy your manager.
Note that this accelerated wear technique is a standard method used in product testing -- e.g., if you have a part that over its, say, 10-year lifetime is expected to be flexed no more than 10,000 times, then you put it in a device that does those 10,000 flexes over a very short period of time (or does sufficient flexes on several parts to determine a mean time to failure).