BL633 Low Temperature Calibrator (-80℃ ~ 105℃)

How to choose a correct liquid bath for your temperature laboratory?

Four key specifications when buying a calibration bath: temperature range, stability, uniformity, and tank size.

1. Temperature range

Bath suppliers normally publish and advertise a temperature range for each model. However, often no single bath fluid works well over the entire temperature range. For example, Ethanol is a good fluid for this bath below 0 °C, but at temperatures above 0 °C another fluid such as silicone oil would be required. As a result, calibration labs must choose between changing bath fluids or using multiple baths to cover the full temperature range of their application.

At temperatures below 0 °C, Halocarbon, HFE, methanol, ethanol, ethylene glycol, Dynalene, and some silicone oil types are all candidate bath fluids. At temperatures above 0 °C, there are several silicone oil types available plus water and mineral oil that could be used. For extremely hot temperatures above 300 °C, salt is a preferred option.

Viscosity is a measure of a fluid’s resistance to flow -we often think of it simply as “thickness.” It is commonly measured in “centistokes” (cSt). The higher the number of centistokes, the more viscous (or thick) a fluid is. Bath fluids which are too viscous create strain on stirring and pumping mechanisms and don’t adequately transfer heat uniformly from temperature sources to thermometers. We recommend using fluids whose viscosity is 50 centistokes or less at the desired control temperature. A homogeneous temperature within the “calibration zone” of a bath is required to achieve a calibration with low uncertainty. Low viscosity fluids reduce bath temperature gradients and contribute to better calibration uncertainties.

2 Stability

Stability is the ability of a bath to maintain a constant temperature over time. A bath’s stability will vary at different temperatures.

Bath fluid also affects stability. The higher a fluid’s viscosity and the lower its heat capacity, the larger the effect on stability will be. In addition to asking about the temperature range, ask what fluid was used when the specification was defined. For example, at 37 °C a bath will be more stable with water as the medium. If you’re going to use oil, expect somewhat larger instability. If your oil has high viscosity at 37 °C, expect even greater degradation in stability.

3 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 probes 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. Does the bath use a circulator pump for mixing? If it does, are there thermal flow patterns in the bath that interfere with uniformity? Be sure to check both vertical and horizontal temperature gradients.

A calibration bath may not have a horizontal temperature gradient, but still have a vertical gradient between different depths. This is a problem if your reference probe and the probes under test are not the same length. For example, you may be testing three-inch long probes and your reference is a 19-inch SPRT. You can only immerse the test probes to three inches, but if you immerse the SPRT to only three inches you don’t have sufficient depth to avoid stem effects that will cause measurement errors. If you properly immerse the SPRT and your bath suffers from vertical gradients, you won’t be measuring the temperature at the three-inch depth of your probes under test.

4 Tank size

The number of temperature probes and sensors to be calibrated should be considered. A bath with a larger tank size that allows large batches to be calibrated may be appropriate for laboratories that calibrate many probes and sensors each year. On the other hand, a bath with a smaller tank size would be a better fit for laboratories with a lower calibration volume.

When longer SPRTs, PRTs, and liquid-in-glass thermometers need to be calibrated, a bath with plenty of immersion depth should be evaluated. Tank opening size is an important factor when odd-shaped sensors (e.g. tri-clamp sanitary sensors) and sensors with large transmitter heads need calibration. When analyzing the tank size required for your application, be sure to allow for proper sensor immersion depth and fluid space below the sensors and between the sensors being tested and the tank wall.

What is the reason the lowest temperature is set at -80 deg. C , thebath go down -90 deg. C

Regularly, -80°C is a commonly adopted standard calibrationpoint in the industry, corresponding to the temperature of dry ice (-78.5°C)and ultra‑low‑temperature applications at -80°C.

Technically, to achieve stable and uniform calibration reliablyat -80°C, the equipment must have a lower cooling capacity of -90°C as a"performance margin", ensuring optimal operating conditions at -80°C.

In terms of cost, stable operation at -90°C is far more costly than at -80°C, while mainstream market demand centers on -80°C.

Therefore, specifying a minimum temperature of -90°C in product specifications and setting -80°C as a key calibration point represents the optimal engineering choice balancing performance, reliability, cost, and real‑world application requirements.

Why the auto level proteciton is so important?

Safety: Prevent probes from being exposed above the liquid level due to oil contraction at low temperatures, which may cause cracking or damage from freezing.

Accuracy: Maintain stable liquid level to ensure temperature uniformity and repeatable calibration results.

Equipment service life: Avoid catastrophic failures such as dry running of circulation pumps and dry burning of heaters.

Operational convenience: Eliminate the inconvenience, hazards and time‑consuming manual oil refilling under ultra‑low temperatures.

Without this function, calibration at -80°C becomes extremely difficult. Operators must manually refill oil at the risk of frostbite after each cooling cycle, and liquid level fluctuations may damage probes or invalidate calibrations at any time. Therefore, automatic liquid level adjustment is a standard feature rather than an optional extra for professional low‑temperature calibration baths.