Accurate Thermal Systems Temperature Baths

Accurate Thermal Systems Temperature Baths

Temperature Calibration Metrology Wells are used to compare a known set temperature with the measurement from the thermocouple, RTD, and liquid filled thermometer under test. The checking of temperature is vital in numerous processes. Temperature is the most widely measured variable, so having a temperature dry well or liquid bath for calibration work is easy to justify.

Considerations when selecting a Temperature Dry Well or Liquid Bath

• Lab or portable.
• Because temperature is measured from cryogenic to extremely high temperatures, they are made to cover these wide ranges. Selection starts with understanding the required temperature range.
• Desired accuracy. Rule of thumb for process instrumentation is to select calibration equipment with an accuracy of 4x better than the instrument being calibrated. Baths come supplied with a digital temperature controller. Look at its accuracy specification. If insufficient, you can purchase an external temperature probe and display with better accuracy and use it as the master meter. It will also be easier to send out for annual re-calibration, than a complete system.
• Watch the stability specification. The benefit of having a high accuracy temperature probe is lost if the dry well or liquid bath temperature is constantly fluctuating.
• While it is important to account for future needs, do not overly exaggerate the range. As the ranges widen, more complicated solutions are required. The following table compares liquid and dry block and various liquid bath mediums.

Technology Typical Temp. Range	Benefits and Drawbacks
Liquid Medium: Water 4-95°C (40-200°F)	Lowest initial cost Wide common temperature range Water is easy to cleanup, non-hazardous, practically no cost Accommodates odd size temperature probes Takes several hours to warm-up or cool-down and stabilize* Water baths evaporate a lot of water as temperature gets closer to boiling point, even with a lid or use of polypropylene spheres (i.e. ball blanket). Be prepared for refilling. Moisture from evaporation may corrode electronics over time
Liquid Medium: Alcohol -40°C/°F to Ambient	Water Bath may be possible to use with denatured ethanol (alcohol)  (check with manufacturer or TEquipment) Evaporation and volume of bath alcohol is concern not only for refilling but flammability Easy to cleanup Takes several hours to cool-down and stabilize
Liquid Medium: Glycol or Oil 0-300°C (32-400°F)	Higher cost than Water Bath and over time the most expensive because of continued replacement of bath medium as it oxidizes from the heat Typical bath mediums: glycol/water mix, mineral oil, Dow Silicone Oil, or polyalphaolefin (PAO) Glycol/Water mix typical range: 0-95°C (32-200°F). Oils intended for temperatures above water boiling Oil fumes must be well ventilated using laboratory hood Accommodates odd size temperature probes Takes several hours to warm up or cool-down and stabilize*
Dry Block -25 to 1200°C (-32 to 650°F)	Bench models have higher initial cost than liquid baths Portable versions possible No evaporation or fuming concerns. Hassle-free Requires inserts for known probe diameters and lengths, some supplied with unit, additional ones purchased separately Fastest warm-up and stabilization time at 20-60 minutes Blocks made of aluminum approximately until its melting point of 660°C (1200°F) . Above that temperature stainless steel is typically used.

Getting sub-ambient temperatures with temperature baths
To lower the bath temperature below ambient several different solutions are possible.
 

Thermoelectric cooling is based on a Peltier principle. It is purely electronic with no moving parts, except a cooling fan. Great benefit over refrigeration compressors, but thermoelectric cooling is very limited in cooling capacity (i.e. BTU’s of heat removal). Because of that limitation, it will only work in very small baths. The technology can be adapted to dry block or small volume liquid baths.

Dip Cooler (“Coldfinger”) is an external refrigerated chiller with a pump that circulates in a closed loop a cooling medium of denatured alcohol or glycol/water mix through a cooling coil that is immersed in the temperature calibration bath. A heat exchanger in the cooler has one side with the cooling medium and another with the refrigerant. Available from Techne.

Flow thru Cooler is a refrigerated chiller without a circulation pump. The a cooling medium of denatured alcohol or glycol/water mix is circulated in a closed loop by a pump in the temperature bath instead. Analogous to a drinking water fountain using city water pressure and no internal pump. A heat exchanger in the cooler has one side with the cooling medium and another with the refrigerant Available from Techne.

Built in refrigeration compressor(s). Having the compressors built into the bath offer the convenience of one instrument and avoid need for circulating cold fluid. Also colder temperatures are possible. A negative is that the combined instrument is larger and heavier, so more expensive to ship for repairs.

When purchasing a Cooler or bath with built in refrigeration compressors, make sure to select a model with the appropriate local voltage (110 VAC or 220 VAC). Refrigeration compressors are only available in one or the other voltage. If the wrong one is purchased, getting a transformer to step up or step down the voltage is expensive because of the amperage that compressors draw.