I’m a big fan of calibration baths. When I managed our temperature calibration laboratory, I relied on them. With a series of baths and a liquid nitrogen comparator, we were able to perform a large volume of accredited temperature calibrations with lower uncertainties from -197 °C to 500 °C than would have been possible in any other way. Sometimes we would get a probe that was large and bent, or very small, or had a large handle and it would slow us down, but it would not stop us, because the right bath with a large working area can be very flexible.
Don’t get me wrong. There are some limitations. As I explained in “Calibration baths, physics and the black arts” the fluid that fills a bath limits its temperature range. So, covering a wide range of temperatures is likely to require more than one bath. Choosing a bath shouldn’t be that difficult but there are a lot of choices. For some, it’s unfamiliar territory and in that case, I thought you might like to have a map.
The 4 basic bath designs and their strengths and weaknesses
Bath products fall into 4 basic categories:
- Utility baths
- Parallel tube
- Concentric tube
- Heat port
Utility
Utility baths are low cost but not designed for calibration and have some important flaws when temperature calibration is the use case:
- Poor temperature uniformity because circulating fluid is inadequately stirred
- Separate heating and cooling elements force a temperature gradient
- Poor temperature stability due to insufficient controller resolution
Fig. 1 Diagram of a utility bath
Parallel Tube
Designed for calibration work, this design solves many of the limitations of the utility bath. Heating and cooling elements are separate from the calibration working area which allows the fluid to be mixed efficiently before flowing into the calibration area. The temperature gradient in the working area is only affected by
- Heat losses along the length of the tube
- Velocity of the fluid as it moves through the tube
However, the fluid level is susceptible to level changes due to fluid expansion and contraction as the temperature changes, and the working area is small.
Fig 2. Parallel tube and concentric tube design baths
Concentric tube
The concentric tube within a tube design addresses some of the limitations of the parallel tube design. Heating and cooling takes place in the outer area where the fluid is stirred and the temperature controlled. The fluid is pumped into the working area which is guarded from the effect of ambient heat losses by the outer tank. It also solves problems related to stirring problems at low fluid levels. Unfortunately, the concentric tube design still has a limited working area.
Heat port
This secret to the ultra-stable and uniform calibration baths from Fluke Calibration is the heat port design and custom temperature controller. The cooling coil and the heater are sandwiched to the outside of the bath’s stainless steel tank to form heat port. The heat port provides a large working area without forcing temperature gradients like a utility bath does. The extraordinary uniformity in these baths is specified over the entire working area which is defined over the entire volume of the bath beginning 1 inch from the bottom and 1 inch from the sides and 3 inches below the fluid’s surface. The design has the following important features:
- Large unobstructed work space
- Carefully balanced stirring mechanism and mixing scheme
- Thermally engineered insulation to minimize heat leaks
Fig 3. Diagram of heat port standard bath construction
The Fluke Calibration temperature controller is also a proprietary design. The hybrid analog and digital controller is designed for calibration delivering stability to ±0.0001°C, and set-point resolution to 0.01°C (0.002°C on some models).
How to select a calibration bath
First consider the temperature range and accuracy that you need. That should narrow things down considerably. Some manufacturers may not provide sufficient information in their literature to accurately predict what your uncertainties are likely to be. To understand how the bath will affect the accuracy of your calibration, you will need to combine the stability and uniformity of the bath.
Bath stability describes the variation in temperature over time and is influenced by the design of the bath, and the design of the controller. Bath stability is also sensitive to the fluid you choose and the temperature you operate at. So, if that information is not in the literature you need to ask for it.
Bath uniformity describes vertical and horizontal temperature variation in the working volume of the bath. Uniformity is going to vary with temperature, and is influenced significantly by the design of the bath and the viscosity of the fluid. If information about stability and uniformity are not present then you are probably not looking at a calibration bath. If minimal information is present but the stability and uniformity are not actually specified then in purchasing that bath you are assuming some risk that is not covered by the manufacturer.
Next you should consider how much space you will need. If you need to be able to calibrate whatever probe your customer sends you then you will need more working area. If you calibrate a large volume of probes then it will be more efficient to use a larger working area.
A larger bath will tend to have better temperature stability but be slower to change temperatures when adjusted. In that case it may be more effective to standardize the temperatures you measure in a way that minimizes how frequently you must change the temperature of the bath. For example, you could have a dedicated station for 0°C and a dedicated station for 110 °C. Another option is to use just one bath with a carefully selected fluid and automate the process so that work can be done even if a technician is not present.
If you calibrate liquid in glass thermometers then you will need to be able to adjust the fluid level of the bath so that the thermometer is both properly immersed and able to be read through a scope on top of the bath at a right angle to the thermometer. Fluid level adapters and a carousel are useful accessories for this task.
Want to learn more?
Here are some links that can help you learn more about selecting a calibration bath.