AL


Annual drift is the digital setting repeatability. It predicts how far the electronics (sensor and control board) might move from a calibration over one year, and is expressed as +/- x.x °C. In other words, if a calibration at 70 °C is done today, after one year of use, what is the worst-case drift in temperature from the indicated 70 °C? If it was specified as +/- 0.1 °C, then the actual temperature might be as low as 69.9 °C or as high as 70.1 °C.


Digital setting accuracy is the worst-case difference between the actual temperature and the reported temperature across either the entire range of the bath or circulator, or a predefined temperature range based on what is required. This number is typically worse when it is measured in a differ- ent location than the control sensor, such as when the measuring sensor is placed in the middle of the bath. The number may be very small at ambient temperatures, but as the temperature gets much higher or lower, the ambient room temperature can either add heat or take away heat from the fluid, resulting in a different actual temperature when measured away from the reporting sensor. Also, control sensors, though calibrated, are not perfectly linear in reporting temperature, so some variance can result from the control sensor itself.


Uniformity is the homogeneity of the water within the bath and is stated as a +/- x.x °C deviation from the control sensor temperature. It can be stated at one or more locations and at one or more depths within the bath area.


How Stability is Determined According to the DIN


12876 Standard Stability specifications for standard heated or refrigerated/heated bath circulators and water baths are normally established according to the DIN 12876 standard and are typically not better than +/- 0.01 °C for circulating baths and +/- 0.05 °C for water baths. This standard does not dictate re- quirements for stability; rather, it establishes a standardized test method so that the performance of bath circulators and water baths from one or multiple vendors can be more directly compared.


Conditions set in DIN 12876 for measuring temperature stability are described as follows: the measuring equipment (sensor, amplifier and recording device), or a thermometer with a responding value no bigger than 10-3


°K and a response time of 5 +/- 1 seconds, must be used. Also


required is a constant ambient temperature of 20 °C and a stable supply voltage for the bath circulator or water bath.


Factors that can significantly influence temperature stability include se- lected (setpoint) temperature, bath fluid, temperature-control method, heating and cooling power, as well as the pump performance and length of tubing, when circulating to an external application. Stability for most heated bath circulators, water baths and heated immersion circulators are tested with water and a setpoint temperature of 70 °C, with the measuring device placed in the center of the usable space of the bath.


Heated Bath Circulators and Water Baths The bath should be filled with water to its maximum volume, with the


bath cover in place. AMERICAN LABORATORY


Heated Immersion Circulators The immersion circulator is mounted on a round vessel of approximately


250 mm in diameter and 10-liter volume.


Heated Bath Circulator with External Circulation A Woulff bottle is connected to a circulator with tubing 1 m in length and


inner diameter of 12 mm. Measurements are carried out at the center of the Woulff bottle.


Refrigerated Bath Circulators The bath should be filled to its maximum volume with ethanol and set to


–10 °C, with the bath cover in place. Again, the measuring device is placed in the center of the usable space of the bath.


Conclusion The difference between NVLAP and DIN 12876 is that the DIN standard


only ensures that the displayed temperature is the temperature at the control probe, while NVLAP requires that other points within the bath area are also measured for temperature stability. The performance difference between calibration baths and standard baths is that calibration baths can have temperature stability of +/- 0.001 °C or better and standard baths are usually +/- 0.01 °C at best.


This means that a standard bath circulator or water bath can be used if:


• Application stability rating is not tighter than +/- 0.1 °C (or +/- 2.0 °C for a noncirculating water bath).


• Users establish their own testing, calibration procedure and schedule.


• A calibrated reference temperature device is used to determine the stability and actual temperature at the location used within the bath.


• If the above applies, thousands of dollars may be saved by using a standard bath circulator or water bath.


How to Establish a Testing and Calibration Procedure The minimum details to include in the procedure description are:


• Ambient Temperature: Depending on the bath, a change in ambient temperature can be reflected by 0.3% to 1.5% deviation in the measured temperature value at the calibration point, i.e., a deviation of 10 °C in ambient temperature may result in a 1% or 0.1 °C change of temperature at the calibration point.


• Supply Voltage: A change in supply voltage will be reflected by 10-3 10-5


to


°K per percent deviation, i.e., a 10% change in voltage can affect the actual temperature at the calibration point by as much as +/- 0.01 °K.


• Test Intervals: Recommended recalibration is every 4 to 6 months if a control bandwidth of +/- 0.05 °K or better is needed. For all other cases, annual recalibration should be adequate.


• System Setup: The volume of liquid, fluid level, type of heat transfer medium, length and diameter of tubing and position of the high-temperature cutout should be recorded, as should the exact adjustment of the set temperature and any noteworthy features in the system. A drawing of the setup helps make records clear and memorable.


41 APRIL 2016


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