Magnetic Surface Contact

5.2.2 Magnetic Surface Contact

Repeatability is the ability of the instrument

Thermometer

to give the same readings under similar The magnetic surface contact thermome-

ambient and target conditions. The Elcome-

ter is one of the most common instruments ter 214 has a repeatability of ±0.5% in

used to determine substrate temperature. reading ±1 digit with a response time of 1

The instrument ( Figure 5.5 ) consists of a second.

bimetallic sensing element, which is pro- Anyone, particularly inspectors, should

tected from drafts. It also includes two mag- question a noticeably high or low reading.

nets on the sensing side which are attracted Take spot readings around the area to set a

to the steel surface.

benchmark. This helps discern a reading that is outside the limits, or the possibility that the reading is incorrect. Have other instru- ments at hand, such as a sling psychrometer,

Environmental Testing 5-5

netic surface. Two high-temperature mag- nets act as the base holding the instrument to the surface. The bimetallic sensor (located in a draft shield) is in virtual ther- mal contact with the surface to be mea- sured. This gives the instrument a quick response time.

• The standard for using the magnetic sur- face contact thermometer in a coating application calls for the manufacturers’

Figure 5.5 Magnetic Surface Contact Thermometer

instruments to meet all Coating Thickness standards for quality and use, and be in

For proper use: (1) ensure the substrate is

accordance with ANSI/NCSL Z540-6

clean and dry to enable good magnetic

(National Calibration Standard).

attraction, (2) place the magnetic back of the Calibration of the magnetic surface ther- thermometer on the surface, and (3) allow

mometer is done at the manufacturers’ facil- the instrument to stabilize before taking

ity and cannot be done at the jobsite. The measurements. Stabilization time varies, but

instrument’s low cost indicates that it should is usually 2 to 3 minutes.

be replaced if there is any doubt about its Once the thermometer is stabilized, read the

accuracy, rather than send it to a third party dial and record the results in the daily log.

laboratory.

With any instrument used to determine sur- The operating parameters of this instrument face conditions, measurements should be

vary with the temperature range needed on made at the actual work locations. The

the substrate.

steel surface temperature should be mea- sured at a variety of points within the area to

The accuracy and precision range most man-

be coated, including those that are likely to ufacturers for the coating industry use is

be hotter or colder than the norm ( Figure ±5%. This can be confirmed by the serial

5.6 ). number each manufacturer is required to place on the instrument (IAW ANSI/NCSL Z540-6 2006 version).

Question the readings that are very high or low variations compared with the actual temperature of the substrate. Before taking

actual measurements, know what the

Figure 5.6 Magnetic Surface Contact Thermometer

In Use

approximate surface temperature range should be on the surface. This information is

When using the magnetic surface thermome- found on the manufacturers’ product data ter, be aware of, or have in your possession:

sheet.

• The manufacturers’ instructions which are simple to understand and use. The

Some common errors are operator-based and

thermometers are specifically designed for

some are equipment-based. Operator-based

use on any horizontal surface or any mag-

reading inaccuracy examples: (1) reading

5-6 Environmental Testing

taken in direct sunlight, (2) instrument left in place too long, or (3) instrument removed before it stabilized (most manufacturers rec- ommend 3 to 5 minutes for the best accu- racy).

It is very possible that erroneous readings are equipment-based due to calibration or equipment malfunction and, therefore, the instrument should be replaced.

Many surface temperature gauges lose their accuracy easily and should be checked regu- larly (perhaps daily) against a known stan- dard. Mercury or spirit thermometers used with sling psychrometers (whirling hygrom- eters) are generally much more accurate and can be used as a convenient reference.