SureTemp and SureTemp Plus Water Bath Use in the Verification of Calibration

Welch Allyn SureTemp and SureTemp Plus Thermometers

Water Bath Use in the Verification of Calibration TO: Welch Allyn Thermometer Users & Biomedical Engineers FROM: Lari Shreffler Occasionally, Welch Allyn receives phone calls from SureTemp” and SureTemp” Plus customers questioning the proper method for verifying calibration of their thermometers. Of these customers, many have indicated that they are trying to or would like to use a water bath test. It is IMPERATIVE to note that the SureTemp and SureTemp Plus thermometers can only be tested in monitor mode, when being tested in a water bath. There are a couple of reasons why the SureTemp and SureTemp Plus thermometers (as well as any other electronic thermometer that gives a reading in under 3-5 minutes) cannot be tested in their normal fast mode in a water bath. 1. The thermometers are electronic predictive thermometers that utilize a software algorithm to “predict” what the patient’s temperature would have been after 3-5 minutes. Most electronic predictive thermometers also have a monitor mode, which allows the thermometer to be used in a fashion similar to traditional glass mercury thermometers, where the thermometer can be left in place for 3-5 minutes to allow it to come to thermal equilibrium (or for the probe and the mouth to come to the same temperature) and give the patient’s “true” temperature. 2. A water bath test does not have the same rate of heat transfer to the probe as does a human mouth and the thermal recovery times for water versus human tissue are different. 3. Probe contact with the heat source is more constant when fully submerged and surrounded by water than when placed in a mouth that has various anatomical structures that have different levels of blood perfusion and biological activity and, therefore different levels of heat emissions. As a person breathes and as blood is pumped through the body, the heat transfer is more pulsitile than in a water bath. To understand the differences between the human subject and a water bath, let’s look at the following example. Assumptions: Probe cover = room temperature (approximately 70 °F) Human mouth = 98 °F Water bath = 98 °F Human subject: Heat energy moves from the tissue of the mouth that is touching the probe cover (MT) into the probe cover to bring thermal equilibrium between the probe cover and the mouth tissue. The tissue temperature begins to go down while the probe cover temperature begins to rise. The body restores the tissue with heat energy at some rate (R1). The ability of the tissue to heat up the probe cover depends on (R1) the rate that the body can restore heat energy into the tissue. Water bath: Heat energy moves from the water that is touching the probe cover (WT) into the probe cover to bring thermal equilibrium between the probe cover and the water. The water temperature (WT) begins to go down while the probe cover temperature begins to rise. The water bath restores the water (WT) with heat energy at some rate (R2). The ability of the water (WT) to heat up the probe cover depends on (R2) the rate that the bath can restore heat energy into water (WT). Summary: (R1) does not equal (R2) therefore the rate of thermal change, or the time to reach thermal equilibrium with the probe, is different for the bath and the mouth. This is the fundamental reason why Welch Allyn does not design their software by “training” thermometers in water baths. It is also why testing the thermometers in a water bath can only be done in monitor mode. In monitor mode, you are essentially allowing the thermometer the full time required (3-5 minutes) for the water bath and the probe to come to the same temperature. This is also why one should be suspicious of any predictive thermometer that gives good readings in the predictive mode in a water bath. Thermometers that perform well in water baths do not usually perform well in human subjects. Water baths basically don’t reflect what actually occurs in the mouth.