Don’t get fooled: what to look for when buying an isothermal bath

If you’re in the market for an isothermal bath (temperature calibration bath), you may have already noticed that many suppliers provide technical specifications that are confusing or incomplete. You might only be able to find a performance spec for a single temperature point, not for the bath’s whole temperature range. Or they don’t say which bath fluid was used in testing.
 

An isothermal calibration bath is a significant investment so you need to get enough information – and the right information – to help you make the best decision. Here are four key specifications to evaluate:

Often no one bath fluid will work well over an isothermal bath’s entire temperature range. For example, a Fluke Calibration 7341 Deep-Well Compact Bath has a range from -45 °C to 150 °C. Below 0 °C Ethanol is a good fluid, but you’d need another fluid, like silicone oil, at temperatures above that. If you need to cover your bath’s full temperature range, you’ll want to decide between changing bath fluids or using more than one bath unit.

When a bath maintains a constant temperature over time, we say it is stable. Stability varies at different temperatures but some suppliers only give out one spec at or near ambient, and they don’t mention that it only applies to one temperature or a narrow range. Be sure you understand a bath’s stability over the entire temperature range that you care about.
 

You also need to ask what fluid the manufacturer used when they defined the stability specification, because bath fluid affects stability. For example, at 37 °C a bath is more stable with water as the fluid. If you use oil, you can expect somewhat more instability, and if that oil has high viscosity at 37 °C you can expect even greater degradation.

A bath must be the same temperature throughout the test zone where you’ll make your measurements. The uniformity specification defines the peak value for this error source. The more probes you test, the larger the test zone – and the more important uniformity becomes.
 

Uniformity depends mostly on the way the bath fluid is mixed. Does the bath use a circulator pump for mixing? If so, are there thermal flow patterns in the bath that interfere with uniformity? Check both the vertical and horizontal temperature gradients.

How many probes and sensors do you need to calibrate? If your workload is large, you may want to look for an isothermal bath with a large tank size.  What about the length of your sensors? Longer SPRTs, PRTs and liquid-in-glass thermometers require a bath with plenty of immersion depth. What about oddly-shaped sensors? If you are calibrating tri-clamp sanitary sensors, for example, you should look for a bath that accommodates them (by the way, we make baths like that: the 6109A and 7019A Portable Calibration Baths).
 

You’ll find them here:

How to select a calibration fluid application note
 

You can also get live help from one of our isothermal bath application experts who will help you evaluate specifications to ensure you’re covered.