This is a neat little bench multimeter which you can purchase for about £100

If you have a reference its fairly easy to calibrate but be careful to note down the factory setting before you start so if it goes south you can start again :)

The voltage reference I use is a REF102CU for the 10V which is accurate at 10V + 0.0025% and drift of 2.5ppm/degrees centigrade so an order of magnitude better than the OWON :)

The 5V is derived from the 10V using an INA105 trimmed to 4 digits so 5.0000 or 1 ten thousandth of a volt but derived from the 10V so in reality 5V + 0.0025%

I manufacture these boards and mount them in an enclosure with USB supply. Unfortunately the REF102CU is extremely hard to source and expensive.

N.B. All the REF102CUs obtained from China are fakes or the low spec REF102K so please don't waste your money, only the BB ones seem to be genuine

Calibration
Utility Math up Range down
then select debug

Apart from size, the very low power consumption is a real plus as the meter can be left on without damaging your finances...