Uniformity A bath can have good stability but poor uniformity. The bath must be homogenous in temperature throughout the test zone where you'll make your comparison measurements. When you place two or more thermometers in the fluid, they should be at the same temperature during your measurement. The uniformity spec defines the peak value for this error source. The more probes you're testing, the larger the test zone and the more important uniformity becomes.
Uniformity depends mostly on the mixing of the bath fluid.
Ask about both vertical and horizontal gradients. This is a problem if your standards probe and the probes under test are not the same length. If you properly immerse the SPRT and your bath suffers from vertical gradients, you won't be measuring the temperature at the 3-inch depth of your probes under test. Equilibration Blocks Accreditation guidelines recommend the use of a metal equilibration block to improve short-term stability during the measurement. However, a block can be inconvenient.