TECHNICAL NOTE October 23, 2002 SUBJECT: Procedures for Verifying Temperature Calibration PROBLEM: DATATRACE users frequently need a procedure to verify that the Tracer calibration is in specification. While the following procedures can verify the calibration, in most cases (MPIII Tracers being the exception) Tracers need to be returned to the factory if they are found to be out of calibration. PROCEDURE: It is also recommended that Tracers be recertified on an annual basis. This is also available from the factory, as required. In those cases where the users internal procedures require it, and the factory is notified before Tracers are serviced, an As Received report can be provided. This procedure describes several methods for verifying the temperature measurement accuracy of Tracers. These procedures can be performed on a Tracer any time accuracy needs to be verified. Tracers are normally tested at several temperature points. We recommend that temperature points are selected which are critical in reference to the target process. For example, using Tracers in a sterilization process would suggest using 121 C for a verification point. The temperature sensor in a Tracer is either a precision NTC thermistor or a PT2000 RTD (MPIII only). These particular devices were selected based on their close tolerance and good, long-term stability. DATATRACE Tracers are warranted to remain within accuracy specifications for one year from date of purchase. While the best procedure to verify a Tracer s calibration is the Oil Bath/Oven Test, other procedures can be utilized, albeit with lesser precision. The following procedures are discussed in this document: Oil Bath/Oven Test Ice Point Test Pooling Test
OIL BATH/OVEN TEST: The first procedure is the most precise, but it also requires the most expensive equipment and relies on the availability of trained personnel. However, in many cases, DATATRACE users have this type of facility as part of their operation. The test is performed by immersing the Tracer s sensing element in an environment that provides a uniform, stable temperature, and immersing a temperature reference standard (reference) into the same environment for comparing the measurements of the reference against the Tracer s measurements. 1. Constant temperature liquid circulating baths are recommended to provide the temperature environment up to 150 C (FRB Tracers) or 140 C (MPIII Tracers) with minimum specifications of 0.005 C uniformity and 0.005 C stability. For temperatures above that, an oven with an isothermal block should be used. The isothermal block provides thermal coupling between the Tracer(s) and the reference, and should have uniformity and stability better then 0.050 C. a. Dow 200 silicone fluid with a viscosity of 5 centistokes is recommended as the medium in the constant temperature bath for temperatures below 10 C. b. Distilled water can be used as the medium in the constant temperature bath for temperatures from 10 C to 80 C. c. Dow 200 silicone fluid with a viscosity of 20 centistokes is recommended as the medium in constant temperature baths for temperatures from 80 C to 150 C. Silicone fluid should not be heated above 150 C for testing Tracers. 2. The reference used should be accurate to 0.025 C or better and traceable to N.I.S.T. The constant temperature baths should be installed, operated and maintained in accordance with the bath manufacturer s instructions and with the Material Safety Data Sheets (MSDS) s of the manufacturer of the silicone fluid. The oven shall be installed, operated and maintained in accordance with the manufacturer s instructions. The following procedure details the procedure that should be adhered to for an accurate Tracer verification:
Set up the bath or baths and allow them to stabilize at target temperature for 15 to 30 minutes. 1. Program the Tracers to be tested for 30 second intervals, starting approximately 15 minutes before they will be immersed into the baths or positioned in the oven. 2. Synchronize your computer clock with your time standard or wrist watch. 3. For LoTemp, Standard Temp, and MPIII Tracers to be tested in baths: A. When placing Tracers into the bath, position their probe tips in such a way that when they are submerged the tips will be at least 1 away from the bottom or sides of the bath, and will be submerged at least 1½ below the fluid level. Tie wraps can be used to help position the probe tips of flex Tracers probes. i. Some custom length rigid probes may not fit into the baths. A bath lid should be used that has holes appropriate for inserting the probes into the bath through the lid. Be cautioned, however, that there may be some inaccuracy introduced into the measurement due to the electronics being at a different (close to room ambient) temperature than the temperature being measured. The magnitude of this inaccuracy can be as much as 0.1 to 0.2 C when the differential is large, such as 100 C or more. B. Cover the bath with the lid and insert the reference into the hole in the lid. Slide it in to a level that places its tip near the middle of the bath. For best results try to position the reference probe and the Tracer probes as close to each other as possible without touching. Observe the minimum immersion depth for the reference probe. C. After the Tracers have been immersed for 15 minutes and the temperature indicated on the display of the reference probe has stabilized, record the indicated temperature and time. D. Repeat steps 1 through 3 for each additional temperature point. E. Read Tracer and compare Tracer data to the time and reading from the reference probe. 4. For High Temp Tracers to be tested in the oven: CAUTION: TEMPERATURES OVER 150 C (140 C for MPIII) WILL DAMAGE THE TRACER S ELECTRONICS AND BATTERY. USE A FACTORY REC- OMMENDED THERMAL PACK TO PROTECT THE TRACER S ELECTRON- ICS AND BATTERY.
A. Program the Tracers to be tested for 30 second intervals, starting approximately 15 minutes before they will be placed in the oven or dry well calibrator. B. Insert the programmed Tracers into the isothermal block in the oven or into the holes in the dry well calibrator. C. Insert the reference probe into the isothermal block in the oven or into the appropriate hole in the dry well calibrator. D. Set the oven temperature for 125 C nominal. E. Wait for the temperature indicated on the reference s display to stabilize. When it seems to be stable, write down the temperature indicated and wait 10 minutes. If it has not changed by more than 0.1 C in 5 minutes it is stable enough for a reading. F. Write down the time and temperature, then set the oven for the next desired temperature and repeat step 5. G. Write down the time and temperature, then set the oven for 275 C nominal and repeat step 5. H. Write down the time and temperature, and then turn the oven or dry well calibrator off. CAUTION: BE SURE TO WEAR WELDERS GLOVES WITH GAUNTLETS WHEN HANDLING HOT OBJECTS FROM THE OVEN! I. After the Tracers have cooled enough to handle comfortably, remove them from the oven. J. Read the data out of the Tracers with the DATATRACE for Windows program. K. Compare the temperature the Tracers measured with the recorded temperature the reference indicated. CAUTION: MAKE SURE YOUR PROCEDURE DOES NOT EXCEED THE THERMAL PACK S TIME AND TEMPERATURE LIMITS! A certified calibration bath or oven may not be available to all customers due to its heavy dependence on high precision, expense, and periodic calibration requirements, along with the availability of well trained individuals to perform the procedure. Following are several additional methods to spot check the accuracy of a Tracer, though with a lesser degree of precision.
ICE POINT TEST: The ice point test is a simple and reliable verification for testing MPIII or LoTemp Tracers that is useful in the field. The test is based on using the melting point of water as the reference temperature (0.000 C). The ice point is obtained using simple procedures and easily available equipment and materials. 1. Fill an insulated container with ice shavings (sized 0.25 to 0.5 inches) made from distilled or deionized water. A standard kitchen blender may be used to make the ice shavings, but all inside surfaces must be extremely clean. A Dewar vacuum flask is best but an insulated cooler is acceptable. Make sure that the container is large enough to accommodate the Tracers and enough ice/water mixture to surround them with at least four (4) inches in all directions. 2. Add sufficient distilled or deionized water to fill all voids between the ice shavings. The center of the ice/water mixture will stabilize at a very exact 0.000 C. This is the melting (freezing) point of water. The ice/water mixture must be well stirred and aerated with atmospheric air or inaccuracies will result. There must be enough water to move freely around in the ice shavings as the water is stirred. Motorized stirrers with adequate impellers give the best results. Keep in mind that ice or water that is not made with distilled or deionized water adds impurities to the mixture which will cause the temperature to be different than 000 C. Salts cause the temperature to be lower. 3. Program the Tracers with an interval of 30 seconds. 4. Place the Tracers into the bath positioning the probes directly in the center of the ice/water mixture. Make sure not to overcrowd the bath and ensure there is sufficient distance between the Tracers and the walls of the bath, including top and bottom. 5. Allow about 15 minutes for the bath to stabilize, making sure the mixture is well stirred and aerated. 6. Retrieve the Tracers from the bath, dry them and up load the data. The temperatures indicated at the end of the stabilization should be at or about 0 C. If there is a larger than expected deviation it may be due to impurities in the water or the water that was used to make the ice, inadequate stirring or aeration, too much or too little ice, inadequate insulation of the container, or improper positioning of the Tracers in the container.
POOLING TEST: While metrology standards require that the overall measurement system s uncertainty be no greater than four times more accurate than the specifications of the equipment being calibrated, the pooling method can identify a Tracer that may be substantially out of calibration by comparing its readings with the readings of other Tracers. 1. Program all Tracers with an interval of 30 seconds and a common start time. 2. Place the Tracers in a well stirred bath or oven with the probes positioned as close to each other as possible. If an oven is used, the probe tips should be placed into holes in an isothermal block made of aluminum, copper, or brass. 3. Set the bath or oven to the most critical temperature you wish to monitor and allow 15 to 30 minutes to stabilize. Record the time. 4. Retrieve the Tracers and up load the data. 5. Compare temperatures at the time when they were at a stable temperature. 6. If any of the recorded temperatures at the time when they were at a stable temperature disagree by greater than two (2) times the accuracy specification for that type of Tracer, document it. Repeat the test. If there is a repeatable and substantial disagreement with the recorded temperatures of the other Tracer the subject Tracer should be returned to the factory for service and/or recalibration. It is important to remember that these procedures are not as precise as a calibration, especially a factory calibration. The value of these procedures is to verify a Tracer s calibration on a periodic basis to give the user a sense of confidence that the Tracer is within specification or heads up that a Tracer may need service.