In-Situ Calibration: Methods & Pitfalls of Thermal Mass Flow Meter Sensor Field Validation

In-Situ Calibration
Methods & Pitfalls of Thermal Mass Flow
Meter Sensor Field Validation
By Matthew J. Olin, President & CEO, Sierra Instruments, Inc.
A S I E R R A W H I T E P A P E R
www.sierrainstruments.com
N O R T H A M E R I C A
5 Harris Court, Building L / Monterey, CA 93940 / USA
800.866.0200 / 831.373.0200 / fx 831.373.4402
E U R O P E
Bijlmansweid 2 / 1934 RE Egmond aan den hoef / The Netherlands
+31 72 5071400 / fx +31 72 5071401
A S I A - P A C I F I C
Second Floor Building 5 / Senpu Industrial Park
25 Hangdu Road Hangtou Town / Pu Dong New District
Shanghai, P.R. China Post Code 201316
+8621 5879 8521 / fx +8621 5879 8586

INTRODUCTION
Mid-to-large size facilities and campuses inevitably have
hundreds of ow instruments to monitor, maintain, and repair.
For a reliability engineer, ensuring that all instrumentation
meets ISO 9000 or similar standards is a time-consuming
responsibility. These standards mandate that precision
instrumentation needs to be checked (validated) or
recalibrated as often as once a year. Sensor elements can
become dirty, plugged, or drift over time. The resistance and
capacitance of electronic components also degrades, leading
to changes in sensitivity or drift.
Once an instrument drifts out of speci cation, it must be
recalibrated to maintain its original accuracy. Thermal mass
ow meters are not immune to these factors. As a precision
instrument designed to measure the molecular mass ow
rate of gases in ducts and pipes, these types of instruments
can require cleaning, veri cation, and recalibration. Many
ow meter manufacturers falsely claim that in-situ (or in-place)
calibration is an easy and inexpensive method for both
verifying the meter’s original factory-calibrated accuracy and
verifying the meter is in calibration. However, when evaluating
thermal mass ow meters for in-situ calibration or validation
capability, be aware that sensor drift will create false positives
that reduce the reliability of the validation.
2

This white paper not only explores the role of stable no-drift
sensor design, but examines ve methods of eld calibration
validation to help end users choose the most accurate, stable,
and cost-effective in-situ calibration solution.
Background: Wet Sensor Design
The stability of all thermal mass ow meter sensors starts with mechanical design. The basic physics of
thermal mass ow meters is attributed to Louie V. King, who published his famous King’s Law in 1914,
mathematically describing heat transfer between a heated wire and the uid ow it is immersed into. King
called his original instrument a “hot-wire anemometer,” which measured the mass velocity at a point in
the ow. The usage of hot wire anemometers grew, in particular, in research environments. This technology
was not widely used in industry because of the fragile nature of the hot wires.
To solve this fragility problem, Sierra Instruments pioneered the development of an industrial-strength
sensor in the ‘80s that could be used in a broad spectrum of industrial process control applications.
The solution was to coil the platinum wire around a ceramic mandrel and mold the wire in place with a
glass coating. This assembly was then placed inside of a thermo-well. However, the gap or boundary layer
between the thermo-well and the platinum-wound mandrel needed to be lled with something other than
air to assure heat transfer from the sensor to the ow. This was the key to assuring an accurate and stable
thermal mass ow meter. The air gap was lled with a potting compound—a conductive epoxy called
thermal grease or cement. This type of sensor is known today as a wet sensor and is used by virtually all
manufacturers of thermal meters (See Figure 1).
The Problem: Wet Sensor Drift
This wet sensor design proved workable, but it had an inherent weakness. The sensor would drift over time
affecting the accuracy of ow measurement readings. As a function of its very principal of operation, the
sensor is heated and cooled over time, expanding and contracting the cement inside the sensor, making it
crack, settle, and shift from its original state. This phenomenon is analogous to freshly poured cement on a
sidewalk. Eventually, the cement hardens and often cracks, shifts, and settles as it is repeatedly heated by
the sun and cooled at night.
3

Since thermal sensors are precisely calibrated to determine the heat transfer versus ow characteristics,
any change in the physical makeup of the sensor layers will invalidate this calibration, resulting in drift or
outright failure. Excessive drift means users must send the meter back to the factory for recalibration.
Dry Sensor: No Drift Thermal Dispersion Sensor
The best way to minimize drift in a thermal sensor is to remove the root cause—the epoxies, cements,
and thermal greases that make up the wet sensor. In March of 1999, Sierra Instruments introduced a
new patented sensor design. Through a proprietary, highly-controlled manufacturing process, the metal
thermowell sheath is tightly formed on the mandrel and platinum-wire assembly. The sensor is designed to
form such close contact that little or no air gap exists and no organic ller cements are needed
(See Figure 2).
4
Figure 1. A Typical Wet Thermal Dispersion Sensor
Sensor Wall
Organic Filler
Sensor Windings

5
Patented Swage
Sensor Wall
Design
Hard Glass
Coating
NO Organic Filler /
NO Air Gaps
Sensor Windings
Figure 2. Patented Dry Thermal Dispersion Sensor
Velocity Sensor
(cutaway)
Temperature Sensor
This innovative new cement-free sensor, known as a dry sensor, was patented by Sierra as DrySense™
Sensor Technology. All materials used to make the sensor are selected to assure that the coef cients
of thermal expansion are approximately the same. As a result, they expand and contract at the same
rate, limiting the stress and cracking. Sierra determined that using a dry sensor was the only way a
manufacturer could claim stability over the sensor’s lifetime.
In-Situ Calibration Veri cation
Despite wet sensor design weaknesses, to this day, all manufacturers of thermal mass ow meters, except
for Sierra, use the wet sensor design because they are easy and economical to build. In addition, all
thermal meter manufacturers have generally the same method of using in-situ validation.
As expected, in-situ calibration veri cation of thermal ow meters is a highly marketed feature that claims
to validate the sensor’s accuracy on location. In-situ veri cation does not replace calibration. If substantial
drift is found, the ow meter must be sent back to the factory for recalibration.

The following section details ve principles of thermal mass ow meter sensor validation to assess which
in-situ veri cation method will result in the most accurate results, thereby saving time and lowering costs.
These ve approaches are: Resistance, Zero-Flow, K-factor, Full-Flow, and Flow-Audit.
Validation Using Resistance
The simplest method measures the resistance across the velocity sensor. Since the velocity sensor is
normally a platinum resistance temperature detector (PRTD), the measured resistance is directly related
to the temperature of the sensor. This temperature should be equal to the space surrounding the velocity
sensor once everything has come to equilibrium (See Figure 3).
6
Velocity Sensor
Platinum Windings
Figure 3. Validation Using Resistance
Resistance of
Windings
(20 typical)
Multimeter
This method only measures the resistance of the platinum wire that is wrapped around the platinum
mandrel. As the dry versus wet sensor discussion illustrates, there is much more to a thermal dispersion
sensor. Resistance measurement makes this a good troubleshooting tool in determining whether the wire
has an open or short circuit and thus the sensor has totally failed.
Power must be removed from the velocity sensor, and it must be allowed to come into thermal equilibrium
with its surroundings. Further, these surroundings must be at a constant temperature. In some cases, the
meter can take as long as 30 minutes to reach thermal equilibrium and, for that period of time, it is not
capable of measuring ow. If the temperature of the process uid is uctuating, this check cannot be done
in-situ.
However, this method does nothing to measure drift since the test doesn’t measure factors related to heat
transfer from the wire through the epoxies and sheath into the owing uid. Therefore, this method can
only be con dently used with dry sensor design which doesn’t drift.

Validation Using Zero Flow
Most manufacturers have realized the limitations of validation using resistance and have various methods
of checking the sensor’s electrical output (either power or raw sensor output voltage) at a zero- ow
condition (See Figure 4). Zero ow is the only truly reproducible point between the factory calibration and
the site where the meter is being used.
To understand how this process works, it is necessary to review the factors that in uence a thermal
dispersion ow meter’s calibration:
n Gas being measured
n Temperature and pressure of the gas
n The pipe the gas is owing inside and the maximum ow rate the meter is expected to measure
If a meter is in the same gas at the same temperature and pressure as factory calibration and the ow is
zero, it should read the same sensor output voltage or dissipate the same power as it did at the factory. If
it does not, it is because the sensor, or the electronics that drive the sensor, have drifted over time.
There are a variety of reasons why this measurement can be problematic:
n As stated, this measurement is only valid at zero ow, meaning the ow in the pipe must be either
shut off or the ow meter partially removed from the pipe with a hot-tap.
n Even if the meter is at zero ow, it still must be in the same gas at the same temperature and
pressure as factory calibration.
7
Velocity Heater
Coil
Power to
Heater Coil
(500mW typical)
Multimeter
Figure 4. Validation Using Zero Flow Calibration
For these reasons, many manufacturers provide data for checking zero at another set of more reproducible
conditions: zero ow at atmospheric pressure and temperature. This requires the meter to be completely
removed from the process and allowed to come to equilibrium at ambient conditions. At best, this
stretches the de nition of in-situ veri cation, as it is not “in place.”
The key drawback of validation using zero ow is that it is only valid at a single ow point. While this is a
good indicator of the type of offset that can be caused by drift, it does nothing to validate the accuracy of
the ow meter through its calibrated range.

Field Adjustment Using K-Factors
As an interim step, many manufactures enable the application of a global K-factor that works as a
multiplier to the observed ow value. This is simply a linear offset most often employed to make the meter
reading agree with another device. The problem with K-factors is that the inherent response curve of a
thermal sensor to ow is non-linear and is best represented by a complex polynomial function, typically at
least to the fth order (See Figure 5).
8
60
50
40
30
20
10
0
0 0.5 1 1.5 2 2.5 3
Electrical power, Watts (W)
Mass velocity, Standard m/s (Vs )
Figure 5. Sensor Output Versus Increasing Flow Rate
In other cases, the manufacturer may allow several points on the calibration curve to be adjusted. This is
typically done for large ducts and pipes as part of a ow transit. This is sometimes erroneously called an
in-situ calibration.
In this procedure, the ow pro le inside a large duct or pipe is characterized by measuring the velocities at
various points, generally along horizontal and vertical lines. Since an thermal ow meter is a point velocity
device, it can only measure the velocity at a single point in the total ow and is affected by ow pro le
disturbances. A ow traverse can determine the best placement of the ow meter, and may suggest that
multiple points are needed. Some manufacturers offer multipoint thermal ow meter averaging systems for
this purpose (See Figure 6). A ow traverse is not an in-situ calibration. It simply re nes the placement of
the meter, or determines a gross correction K-factor to bring the existing calibration in line with observed
results.
Figure 6. Multipoint Flow Meter System

Validation Using Full-Flow
One complex and expensive technique that validates beyond a zero ow condition checks the full- ow
range by generating a series of known ow rates, from zero to full scale (See Figure 7). The system uses a
small sonic nozzle opening that directs a known ow past the velocity sensor. The diameter of the nozzle is
xed, and by applying a known differential pressure across the nozzle, the ow through the nozzle can be
calculated.
Pressure
Regulator
Test Valve
Internal Flow
Tube
Calibrated
Nozzle
Known Flow
Rate
As with the other techniques discussed, this method has its drawbacks:
n It depends on the nozzle not becoming plugged or dirty (and thus changing the size of the nozzle
from when it was calibrated) and requires precision pressure gages, which themselves need periodic
recalibration.
n The meter must be removed from the process (although not necessarily the pipe), so a hot tap
system is required.
n This is a rather complex and expensive technique, requiring a source of pressurized air or nitrogen,
a variable pressure regulator, tubing, and the nozzle. Such a system cannot be back- tted, and the
nozzle is a permanent xture of the probe assembly.
9
Figure 7. Validation Using Sonic Nozzles
Temperature
Sensor
Velocity Sensor

Validation Using Actual Flow-Audit Method
The ow-audit method is perhaps the very best in-situ calibration veri cation. This method uses a
high-accuracy ow standard to prove the accuracy of the ow device under test (DUT). A ow-audit is
performed with a similarly calibrated meter that is installed into the pipe via hot-tap near the DUT, or
even at the same measurement point if the meter under test can be removed. The key words above are
“similarly calibrated;” a meter calibrated for natural gas cannot be used to check a meter on compressed
air for instance. Likewise, the temperature and pressure as well as pipe size must be matched.
The ideal meter for the ow audit method has the application exibility to work on different gases and
pipe sizes and dynamically compensate for temperature and pressure differences. Many companies buy
thermal insertion mass ow meters as audit meters because of their ability to insert the sensor into the
ow via hot tap. This adds convenience and avoids costly process shutdowns. However, traditionally, a
thermal meter needs to be purchased for each speci c application at the facility. For the majority of users,
this is cost prohibitive.
For gas ow auditing, a solution now exists that allows a single thermal ow meter to be used across
multiple pipe sizes and gases. Released to market in 2011, Sierra’s QuadraTherm® 640i insertion thermal
mass ow meter has been rapidly adopted as a ow-audit meter to check other thermal meters at a
facility. Due to its high accuracy of 0.75% of reading, it is also commonly used to check many different gas
mass and volumetric ow technologies.
Coupled with a hot-tap insertion point located near the DUT, the 640i is a “universal” ow meter that
can be recon gured in the eld to match nearly any ow measurement point in a facility. The 640i has
Sierra’s patented no-drift dry sensor as discussed earlier in this whitepaper. The result is a stable reliable
measurement. As seen in Figure 8, the user programs the instrument to the exact gas and pipe size of the
device under test and inserts the 3/4” (19mm) sensor probe into the pipe near the DUT. Engineering units
can even be programmed to match the DUT.
The 640i ow-audit meter will immediately start reading ow. Compare this ow to the DUT. If the two
units read close to each other, the DUT can be signed off as validated and reading properly.
10
Figure 8. Audit-Meter with Hot-Tap
Device Under
Test
Flow
Flow Audit
Meter
Low Pressure
Hot Tap

In-Situ Validation Isn’t Calibration
For four of the calibration validation methods, if the meter does not pass the validation, it generally must
be returned to the factory for recalibration. However, using the ow–audit method does allow the end user
to adjust a DUT using the K-factor method discussed earlier in the whitepaper to adjust the DUT to match
the exact ow reading of the audit meter.
Precise thermal ow meter calibration occurs under tightly controlled temperature and pressure conditions
using the same gas and the same size pipe section or ow body that the meter will be used in.
As you can imagine, such a facility is a large and expensive asset and certainly not portable. Consequently,
if you nd your meter is out of calibration, it is highly recommended to send it back to the factory or
accredited ow calibration service center for recalibration.
Validate, Don’t Calibrate
How can you validate a sensor that will drift out of spec due to the very nature of its mechanical design?
You can’t. All validation methods assume that there is no drift. As described earlier in this white paper,
wet sensors are prone to drift and dry sensors do not drift.
Dry no-drift sensors have a big advantage during in-situ calibration validation. The allmetal, epoxy-free
mechanical design provides the con dence that the in-situ calibration validation is actually valid. Dry
sensors are validated in the same way as a wet sensor, although in this case, it is not drift that is expected,
but rather dirt or mechanical damage. For this reason, Sierra offers a lifetime warranty on its patented dry
sensor and guarantees that there will be no drift.
As a result, there is no need to buy expensive in-situ calibration instruments. Sierra offers a free in-situ
calibration validation software package called ValidCal™ Diagnostics. Unlike other validation methods, the
ValidCal™ Diagnostics program provides a complete check of all meter components including the velocity
and temperature sensors, the sensor drive circuitry, the accuracy of the pressure transducer (if applicable),
and all digital and analog outputs and alarm relays. This capability is included free with each meter and
provides a printed calibration certi cate and diagnostics report. All of this can be accomplished without
removing the meter from the process piping. This capability can be found in all Sierra thermal meters,
including the latest high-accuracy QuadraTherm meter (See Figure 9, which is multivariable and has 0.5%
of reading accuracy).
When evaluating thermal mass ow meters for in-situ calibration validation capability, be aware that
sensor drift will create false positives that reduce the reliability of the validation resulting in reduced
measurement quality. Assure that the instrument has a dry sensor and that the manufacturer backs up
their sensor with a no-drift guarantee before you run an in-situ calibration validation procedure.
11

Summary and Conclusion
In-situ calibration validation is one of the great bene ts of thermal mass ow technology. This whitepaper
reviews ve in-situ calibration validation approaches. These are: Resistance, Zero-Flow, K-factor, Full-Flow,
and Flow-Audit. Each method has varying cost and complexity, but does offer the end user the advantage
of proving some aspect of ow meter performance in the eld to ful ll quality requirements.
When evaluating thermal mass ow meters for in-situ calibration validation capability, beware that
sensor drift will create false positives that reduce the reliability of the validation. The assumption by all
manufacturers, including Sierra, is that their sensor does not drift. Only with sensor stability can users truly
validate a sensor’s factory-calibrated accuracy in the eld. Assure that your thermal mass ow meter has a
drift-free, dry sensor, which has no organics and cements that drift over time.
Finally, it is highly recommended to use the ow–audit method for the highest quality calibration
validation. All forms of in-situ calibration validation discussed in this whitepaper give the end user
information about the thermal meter’s operating performance, but only the ow-audit method actually
validates the calibration at actual owing conditions.
Acknowledgements
I would like to thank Scott Rouse, Erica Giannini and Charlotte Chapman for their contributions to the
white paper.
12
Figure 9. Sierra’s QuadraTherm® Mass Flow Meter Featuring DrySense™ Sensor Technology

In-Situ Calibration: Methods & Pitfalls of Thermal Mass Flow Meter Sensor Field Validation

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