“Do I have to maintain inspection gage calibration and certification?  Why don’t they retain their calibration over time? Do I need to verify gage accuracy on a regular basis? How often? What is the difference between calibration and verification of accuracy?”

These are common questions in the coatings industry.  A simple answer is that without routinely calibrating/certifying coating inspection gages using standards traceable to a national metrology institution and verifying the accuracy of your equipment prior to use, the gages only reveal values, and there is no way to determine whether those values are representative. Quality Assurance and Quality Control inspectors have an obligation to make certain that the values being displayed by the gages are accurate and represent the quality of the work performed, as decisions regarding acceptability of work performed, or the need for rework are made based on gage readings. So, calibration and verification of accuracy are both important, but are distinctly different.

Differentiating Calibration from Verification of Accuracy and Adjustment

Calibration is defined as a controlled and documented process, and is performed by the gage manufacturer, their authorized agent, or by an accredited calibration laboratory.  Calibration must be performed in a controlled environment that is not typically found in a shop or in the field.

Verification of accuracy is performed by the gage operator and does not need to be performed in a controlled environment. Based on the accuracy verification process, adjustments may be necessary to compensate for shop or field conditions during the measurement process. An example, based on a dry film thickness gage is provided below.

ASTM D7091, Standard Practice for Nondestructive Measurement of Dry Film Thickness of Nonmagnetic Coatings Applied to Ferrous Metals and Nonmagnetic, Nonconductive Coatings Applied to Non-Ferrous Metals and SSPC-PA 2, Procedure for Determining Conformance to Dry Coating Thickness Requirements both contain information on calibration, verification of accuracy and adjustment (incidentally, all three are required prior to obtaining coating thickness measurements when one or both of these standards are invoked by the project specification). For electronic gages (known as Type 2 gages), verification of accuracy is performed using traceable, certified coated standards or using certified shims placed on smooth metallic substrate. This is typically accomplished by using coated standards or certified shims that are slightly below and slightly above the anticipated dry film thickness range (known as two-point verification).

SSPC-PA 2 states that verification of accuracy should be performed (at a minimum) at the beginning and end of each work shift, and recommends verifying accuracy during measurement acquisition, especially if a large data set is being obtained or the gage is dropped or suspected of being out of tolerance.  This step makes certain that the gage is working properly, but another step, adjustment, is necessary before using the gage to measure coating thickness.

Adjustment is the act of aligning a gage to correct for substrate metallurgy, curvature, roughness (including surface profile), and other characteristics that may affect the measurements. This is accomplished by placing certified or measured shims onto the prepared, uncoated substrate and adjusting the gage to align with the shim value. One point or two point adjustments using shims may be performed.  After this step, the instrument is ready for use in measuring coating thickness.

Alternatively, after verification of accuracy is performed a series of 10 or more Base Metal Readings (BMRs) can be obtained from the prepared, uncoated surface. The average BMR is deducted from the coating thickness. BMR is not the same as surface profile. Surface profile is defined as a measurement of the maximum, peak-to-valley depth created by abrasive blast cleaning and some rotary impact power tools. BMR is the effect of this roughness on a coating thickness gage. For example, a 3-mil surface profile may yield an average 0.7 mil BMR. Never deduct surface profile from coating thickness measurements.

By adjusting the gage to a known thickness over the prepared surface (i.e., using a measured shim) or by measuring and deducting a BMR, the thickness of the coating above the peaks of the surface profile is revealed.

While the focus of this column has been on dry film thickness gages, any gage that takes a measurement should be calibrated (typically annually). This includes temperature gages, micrometers, pressure gages, conductivity and pH meters and any other instrumentation used to verify the quality of workmanship.

Matt Fajt is a Vice President and Business Unit Manager for the Instrument Sales and Service Group for KTA-Tator, Inc.  He is an NACE Level 2 certified coatings inspector, SSPC PCI Level 1 and a frequent workshop facilitator on inspection instrument use.  He can be reached at mfajt@kta.com.

Why do we need to measure ambient conditions?

Ambient conditions are the prevailing conditions of air temperature, the moisture content of the air (relative humidity), and the temperature at which condensation will occur (dew point).  Most coating specifications have set requirements for monitoring and documenting results for surface and air temperature, relative humidity and dew point.  These conditions are to be measured and recorded in the specific areas where surface preparation and coating application will occur, then compared to the specified ranges and/or the coating manufacturer’s restrictions listed on the product data sheet.  

While theoretically a surface temperature only slightly above the dew point temperature would preclude condensation, the 5°F safety factor accounts for instrument inaccuracies and changing or varying conditions.

You should not rely on prevailing conditions from a local weather service or from the internet as conditions at the project site and the specific work area can vary considerably. And surface temperature won’t be reported. Ambient conditions should be measured where the work will occur and recorded prior to start-up of operations and at 4-hour intervals thereafter, unless conditions appear to be changing. In this case, more frequent checks may be required.

Using Instruments for Assessing Prevailing Conditions

Whirling (Sling) Psychrometer: When discussing the measurement of ambient conditions using a whirling psychrometer (ASTM E337, Standard Test Method for Measuring Humidity with a Psychrometer (the Measurement of Wet- and Dry-Bulb Temperatures), you hear the terms wet bulb temperature and dry bulb temperature used on a regular basis, but how are these terms defined?  Wet bulb temperature is an indication of the latent heat loss caused by water evaporating from a wetted sock or wick on the end of a bulb thermometer mounted in the psychrometer housing.  While whirling the instrument away from your body in 20-30 second increments, the water evaporates from the wetted sock into the air, so there is a cooling effect on the thermometer causing a decrease in temperature.  This process is repeated until two temperature readings from the wet bulb thermometer are with 0.5° of one another. The depression of the wet bulb thermometer from the dry bulb (air) thermometer is the calculated difference between the air temperature and the stable wet bulb temperature. For example, a dry-bulb temperature of 70°F and a wet-bulb temperature of 60°F nets a difference of 10°F. this is known as the wet-bulb depression.

Psychrometric tables are used to look-up the relative humidity and dew point temperature.  First choose the table of interest (relative humidity or dew point temperature, then select the table corresponding to the prevailing barometric pressure for the geographical location that the project is in.  Intersect the dry bulb (air temperature) with the difference between the dry and wet bulb temperatures, known as the depression of the wet bulb to determine the relative humidity or dew point temperature. A separate thermometer is used to measure the temperature of the surfaces to be prepared and/or coated. The temperatures and the relative humidity can then be compared to the requirements listed in the specification to determine conformance.

Digital Psychrometer: The use of a digital psychrometer for assessing prevailing ambient conditions and surface temperature is a much simpler process compared to the use of a whirling psychrometer, psychrometric charts and surface temperature thermometer. Most of the digital psychrometers will display the relative humidity, air temperature, surface temperature, dew point temperature and the difference (spread) between surface temperature and dew point temperature.  Data are constantly updated and displayed simultaneously for easy recognition.  This eliminates the need to use psychrometric tables to determine the relative humidity and dew point temperature, as well as any need for a separate surface temperature thermometer. The data can be auto-logged and uploaded to cloud-based software or downloaded to a device using USB or Blue Tooth®.

Which Method Wins the Duel?

Whirling psychrometers were first invented in the 1600’s (see image to right), and the US Weather Bureau Psychrometric Tables were first published in 1941. So, one may conclude that newer technology wins the duel. Not so fast! Digital psychrometers also have limitations and without user knowledge they too can produce erroneous data.

While having all the ambient conditions and the temperature of the surface readily displayed is a great benefit, there are important steps that must be followed when using these electronic instruments.  It is very important that the digital psychrometer be allowed to ‘stabilize’ to the atmospheric conditions where the work is occurring.  This could take anywhere from 20 to 30 minutes.  That is, accurate readings are not possible immediately after departing an air-conditioned vehicle and walking onto the jobsite.  Additionally, the humidity sensor used by most instrument manufacturers has a tendency to dry out during periods of inactivity, resulting in false, low humidity readings.  To re-saturate the sensor, the manufacturers recommend placing the probe of the digital psychrometer in a re-sealable plastic bag or sealed container with a damp (not wet) cotton cloth for 24-hours.  This will extend the life of the sensor and help ensure representative readings. And most instrument manufacturers recommend annual calibration.

Whirling psychrometers also have their limitations and the potential mis-use. These instruments cannot be used in freezing temperatures and proper use (thorough saturation of the wick with deionized water and reading the wet-bulb temperature after several 20-30 second increments of whirling until the wet bulb temperature stabilizes) is very important.

Despite the availability and apparent convenience of the digital psychrometers, many quality control and quality assurance personnel still rely on older “tried and true” technology. Both will work well when used properly.