The liquid-in-glass thermometer is one of many precision instruments calibrated by the NBS. A complete description of calibration services offered by the NBS appears in NBS Special Publication 250 [2], which can be obtained by writing to the Office of Technical Information and Publications, National Bureau of Standards, Washington, D. C. 20234. Supplements to NBS Special Publication 250 are issued periodically indicating any changes in these services.

2.1. Kinds of Thermometers Accepted for Calibration

Not all thermometers classified as liquid-in-glass are acceptable for calibration by the NBS. In general they must be of an acceptable design and workman

'Figures in brackets indicate the literature references at the end of this Monograph.

ship, and capable of being placed in the existing calibration facilities. The type known as laboratory or chemical thermometers, consisting of a solid stem with graduation lines and numbers permanently etched or placed on the stem, are regularly accepted. Others approved for calibration are Beckmann, calorimetric, and enclosed scale (Einschluss) laboratory thermometers. Industrial or mechanical type thermometers with special mountings can be accepted for test if they can be inserted in the comparison baths, or if the thermometer can be easily detached from the mounting.

Any thermometer, such as a household thermometer, consisting of a paper, plastic or metal mounting containing the scale graduations and attached to the unmarked glass thermometer by metal clips, is not acceptable for calibration. Maximum self-registering mercury-in-glass clinical thermometers are no longer tested at the NBS.

mometry laboratory at NBS if there is any doubt concerning the acceptability of a thermometer for calibration. Every effort will be made to assist and satisfy the needs of the scientific and industrial community with problems involving temperature

measurement.

2.2. Preliminary Examination

Every thermometer submitted for test must be uniquely identified by a serial number and pass a preliminary examination before final acceptance. These thermometers are viewed under a microscope having a magnification of 15 or 20X to assure that the mercury and capillary are clean. Any foreign matter found in the capillary or bulb that, in the judgment of the laboratory personnel, might tend to make the thermometer readings erratic, is reason for rejection.

Glass chips or oxides of mercury in the capillary or bulb are considered foreign matter that can cause the thermometer to indicate different readings at different periods of time. [For example, if a small chip of glass is located in the capillary at the 30 °C indication, each time the mercury column advances past this point, it could cause the reading of the thermometer to vary for temperatures above 30 °C, because of different quantities of gas that may be trapped around the chip. The movement of the chip to different locations in the capillary, or the trapping of mercury around the chip when the mercury_column recedes, would also cause erroneous readings.]

Other reasons a thermometer may be ineligible for test are given below:

2.3. Reports of Calibration

In most instances a Report of Calibration will be issued by NBS for every liquid-in-glass thermometer submitted for test that is found free from serious defects as determined by the preliminary examination. In order to receive a Report of Calibration, the thermometer must be calibrated at two or more calibration points. If only one point is requested, a Report of Test will be issued.

The Report of Calibration contains the corrections determined for each point requested and an estimate of the uncertainties associated with the corrections. Also stated on the document are the agency or firm requesting the test, the trade mark and serial number

pletion date, and explanatory notes defining the conditions under which the results of the test are applicable. When necessary, accompanying the Report of Calibration is a sheet explaining how to calculate the corrections for emergent stem. If the thermometer is of the metastatic (Beckmann) type, the report will be accompanied by a second sheet containing a table of setting factors (See Sec. 6.4) enabling the user to apply the calibration results for settings other than the 20 °C setting for which the corrections are reported. A facsimile of a Report of Calibration is shown in figures 1 and 2.

Any departure from the conditions under which the corrections were obtained, as stated on the Report of Calibration, may significantly change the values of the corrections. Conditions of immersion are particularly important (Sec. 6). It should be emphasized that the estimates of error assigned to the scale corrections do not assure the user of this accuracy in a temperature measurement.

2.4. General Instructions to Applicants
for Tests

Testing will be conducted in accordance with the policies of NBS as described in NBS Special Publication 250. The cost of calibration will depend on the number of calibration points requested, the temperature range, and the quantity of thermometers submitted. For tests not specifically outlined in the above publication, NBS should be consulted. If the required measurements appear feasible, and, in the opinion of NBS, sufficiently important to justify the work, such tests will be undertaken for a special fee determined by the nature of the work and time involved. In all requests for test the following procedures and information are pertinent.

a. Initial Arrangements

A letter or purchase order requesting the test must be sent to NBS either with the thermometer or separately. No work can be undertaken unless both the thermometer and letter or purchase order have been received. Information in the request for test should include the quantity and serial numbers of the ther mometers sent, the calibration points required, (See Sec. 5.4) and the suggested method for return shipment. If it is desired that the thermometer be insured upon return, a statement to this effect must appear on the request, and a value assigned; otherwise, the thermometer will be returned uninsured.

NBS will assign a test number to each calibration request and acknowledge the receipt of the thermometer. On the acknowledgment will be stated the estimated completion date and estimated cost of calibration. Any foreseeable difficulty that may be encountered involving the calibration will be mentioned on the acknowledgment, as well as the name of someone in the testing laboratory who can be contacted if questions arise.