Magnetrol Thermatel series User manual

Thermatel®
Thermal Dispersion Mass Flow
Measurement Handbook

Tabl of Cont nts
Introduction ....................................................................................................1
What is Mass Flow M asur m nt?...................................................................1
Typ s of Flow Transmitt rs..............................................................................4
Diff r ntial Pr ssur .........................................................................................4
Orific .............................................................................................................4
V nturi ............................................................................................................6
Av raging Pitot Tub .......................................................................................7
Vort x Sh dding ..............................................................................................7
Turbin Flow M t rs .......................................................................................8
Ultrasonic Flow ...............................................................................................9
Coriolis............................................................................................................9
Pr ssur /T mp ratur Corr ction...................................................................10
Multi-Variabl Transmitt rs ...........................................................................10
Th rmal Mass Flow.......................................................................................11
Diff r nt Typ s of Th rmal Mass Flow M t rs ..............................................13
R spons Tim ..............................................................................................14
T mp ratur Comp nsation ..........................................................................15
Pr ssur Eff cts ..............................................................................................17
Calibration ....................................................................................................18
Calibration vs. Configuration........................................................................20
Accuracy........................................................................................................20
Flow Profil ...................................................................................................21
Installation Options.......................................................................................25
Accuracy vs. R p atability .............................................................................27
Limitations of T chnology .............................................................................28
Applications...................................................................................................29
Difficult Applications ....................................................................................30
Uniqu F atur s.............................................................................................30
R f r nc s......................................................................................................31
Common Conv rsion Factors........................................................................32
Gas D nsity at Standard Conditions .............................................................33

1
Introduction
Accurat mass flow m asur m nt of gas is difficult to obtain. Th main r ason
is that gas is a compr ssibl fluid. This m ans that th volum of a fix d mass
of gas d p nds upon th pr ssur and t mp ratur it is subj ct to.
Consid r a balloon containing on actual cubic foot of gas at room t mp ra-
tur (70° F) and atmosph ric pr ssur . An incr as in th room t mp ratur
caus s th balloon to xpand. An incr as in th pr ssur surrounding th bal-
loon r sults in a d cr as in volum . Although th volum of th balloon
chang s with variations in pr ssur and t mp ratur , th mass of th gas insid
th balloon has r main d th sam . This illustrat s how pr ssur and t mp ra-
tur aff ct th actual volum .
Th r ar many w ll stablish d m thods of m asuring th actual volum tric
flow rat . How v r, th m asur d flow rat will vary with chang s in t mp ra-
tur and pr ssur . For virtually all industrial proc ss op rations, th us r wants
to m asur th mass flow rate inst ad of th actual flow rat . Ch mical r ac-
tions work on th basis of mass r lationships of ingr di nts. Combustion is
bas d upon th mass flow rat of th air and th fu l. Gas consumption in a
facility is bas d upon mass flow rat . To accurat ly m asur mass flow, th
actual flow rat must b adjust d to corr ct for any chang in t mp ratur
and pr ssur .
Th rmal mass flow t chnology is a m thod of gas flow m asur m nt that does
not require corr ction for chang s in proc ss t mp ratur or pr ssur . Th rmal
mass flow t chnology also has a b n fit of m asur m nt at low v lociti s and
gr at r turndown capabiliti s than thos obtainabl with oth r gas flow
m asur m nt d vic s.
What is Mass Flow Measurement?
Mass Flow is th m asur m nt of th flow rat without consid ration of th
proc ss conditions. Mass flow is quival nt to th actual flow rat multipli d
by th d nsity of th gas. M = Q ×wh r Q is th actual flow and is th
d nsity. As th pr ssur and t mp ratur chang , th volum and d nsity
chang , how v r th mass r mains th sam .
To obtain standardization of gas flow m asur m nt, Standard conditions of
T mp ratur and Pr ssur (STP conditions) ar utiliz d. Gas flow m asur d at
STP conditions is corr ct d from th actual proc ss conditions to standard
conditions; this will b discuss d in mor d tail lat r.

2
Th simpl st way of m asuring mass flow of gas is in units of cubic f t p r
minut or cubic m t rs p r hour, corr ct d to STP conditions. This is r f rr d
to as SCFM (standard cubic f t p r minut ) or th m tric quival nt of
Nm3/h (normal cubic m t rs p r hour). Th d nsity of a gas at standard con-
ditions is known, thus providing a r lationship b tw n SCFM and pounds
p r hour or b tw n Nm3/h and kg/h.
Th conv rsion b tw n th volum at actual conditions and th volum at
standard conditions is bas d on th id al gas law — actual volum incr as s
in dir ct proportion to an incr as in absolut t mp ratur , and d cr as s
in dir ct portion to an incr as in absolut pr ssur . Consid r th balloon
xampl — as th t mp ratur incr as s, th volum xpands; as th pr ssur
incr as s, th volum shrinks.
Absolut pr ssur of z ro psia (pounds p r squar inch at absolut conditions)
is a p rf ct vacuum. On atmosph r of pr ssur is d fin d as 14.69 psia or
z ro psig. Th conv rsion b tw n psia and psig is asy: PSIA = PSIG + 14.69.
If you hav a pr ssur gaug calibrat d for psig, it will r ad z ro at s a l v l
and only m asur gaug pr ssur abov atmosph ric pr ssur . Th following
chart will h lp clarify this.
Absolut z ro is d fin d as th t mp ratur wh r mol cular motion stops. It
is d fin d as 0 K (K lvin) which is -273.16° C or 0° R (Rankin ) which is
-459.67° F. To conv rt b tw n actual t mp ratur and absolut t mp ratur ,
simply add 460 to th t mp ratur in d gr s Fahr nh it or 273 to th t m-
p ratur in C lsius.
Vacuum (negative gauge pressure)
Absolute Pressure (psia)
Gauge Pressure (psig)
Atmospheric Pressure (psia)
System Pressure (1)
Atmospheric Pressure
System Pressure (2)
Absolute Zero Pressure
Figure 1

3
Onc w stablish a s t of conditions as a standard t mp ratur and pr ssur
(STP conditions), w can conv rt b tw n th flow rat at actual conditions
and th flow rat at standard conditions.
Th subscript (a) r f rs to actual conditions; th subscript (s) r f rs to stan-
dard conditions.
Unfortunat ly, not all STP conditions ar univ rsal. Many us rs consid r on
atmosph r and 70° F as STP. Som industri s us on atmosph r and 60° F
as standard; oth rs us on atmosph r and 32° F as standard. Th m tric
quival nt is Normal conditions which ar bas d on a pr ssur of on bar
(14.5 psia) and 0° C or 1 bar and 20°C.
Th important issu is that Standard Conditions ar not Standard and a mass
flow m t r n ds to b abl to p rmit th us r to s l ct th d sir d STP condi-
tion. An rror of approximat ly 8% will occur if th r is a diff r nc in STP
conditions b tw n 70° F and 32° F.
Onc a s t of standard conditions is id ntifi d, th d nsity of that gas at th s
conditions is known. Th r for , it is a simpl matt r to conv rt from SCFM
to mass in pounds p r hour:
In this formula, th d nsity in pounds p r cubic foot is th d nsity at th sp c-
ifi d STP conditions. A list of common gas s and th ir d nsity is in th
app ndix.
SCFM = ACFM T(s)
T(a)
P(a)
P(s)
()( )
pounds
hour = SCFM x pounds
cubic foot x60 minutes
hour

4
Types of Flow Transmitters
Th r ar many typ s of flow transmitt rs — som ar us d for both liquid
and gas flow m asur m nt, whil oth rs ar sp cifically us d for on fluid. Th
following tabl id ntifi s many of th diff r nt typ s of flow transmitt rs and
th ir us in liquid, st am, or gas s rvic , and if th y m asur actual flow or
mass flow.
As shown, th r ar many t chnologi s to m asur th flow rat of gas. Most
of th s m thods m asur th flow rat at th actual op rating pr ssur and
t mp ratur and r quir pr ssur and t mp ratur corr ction to obtain th
mass flow.
Th following discuss s som of th common typ s of flow m asur m nt:
Differe tial Pressure
M asuring th pr ssur diff r nc across a flow l m nt is th most common
m thod of flow m asur m nt. Th r ar s v ral typ s of flow l m nts.
Orifice
Th most common flow l m nt is th orific plat as shown in Figur 2.
Technology Li uids Steam Gas Actual Flow Mass Flow
Differential
Pressure XXXX
Vortex XXXX
Turbine XXXX
Magnetic XX
Positive
Displacement XXX
Variable Area XXXX
Coriolis XXX
Ultrasonic XXX
Thermal XX
Differential Pressure
Differential Pressure
Pressure
Flow
Reference
Orifice Plate
Figure 2

5
This m thod of flow m asur m nt utiliz s a prov n t chnical conc pt call d
“B rnoulli’s Equation.” This r lation stat s that th pr ssur drop across a flow
r striction is bas d upon th squar of th flow:
Th flow rat is bas d upon flow at actual op rating conditions.
Th pr ssur drop across th flow l m nt is proportional to th squar of th
flow. If th pr ssur drop (∆P) is qual to “a”, and th flow rat doubl s, th
pr ssur drop incr as s to 4a. If th flow rat tripl s, th n th pr ssur drop
incr as s to 9a. Th r for th signal str ngth incr as s as th flow rat
incr as s. At z ro flow th r is no signal, and th signal str ngth slowly
incr as s as th flow incr as s as shown in th chart b low. This r sults in
poor low flow s nsitivity.
Basic limitations of this typ of flow m asur m nt ar :
1. Limit d turndown. A 3:1 turndown caus s a 9:1 r duction in pr ssur
drop. A 4:1 turndown caus s a 16:1 r duction in pr ssur drop.
Th r b com s a limit on th turndown ratio of any diff r ntial
pr ssur m asur m nt instrum nt du to th DP transmitt r. Typical
turndown of a DP transmitt r is 10:1.
2. Limit d low flow m asur m nt. Th r is a minimum flow rat
which g n rat s a pr ssur drop that can b m asur d with any
diff r ntial pr ssur transmitt r. This is illustrat d in th abov graph.
0
2
4
6
8
10
12
14
16
0 0.5 1 1.5 2 2.5 3 3.5 4
Pressure Drop
Flow Rate
Flow = constant ×∆P
()
1⁄2
Figure 3

6
3. Cr at s pr ssur drop. B caus th flow m asur m nt is bas d upon
m asuring a diff r ntial pr ssur , th flow m asur m nt r quir s that
additional pr ssur drop b add d to th proc ss, which incr as s
op rating cost. Th pr ssur drop cr at d by th primary l m nt is
fr qu ntly ov rlook d wh n consid ring op rating cost.
4. M asur s actual flow rat . Any diff r ntial pr ssur flow d vic m as-
ur s the actual flow rate in ACFM. To d t rmin th mass flow rat ,
corr ction from actual conditions to standard (mass) conditions is
r quir d. To d t rmin mass flow from actual flow, it is n c ssary to
m asur :
— Diff r ntial pr ssur
— Absolut pr ssur
— Absolut t mp ratur
Th s m asur m nts ar th n s nt to a flow comput r that calculat s th mass
flow rat in SCFM, using th quations pr viously discuss d.
Ve turi
A v nturi flow l m nt is v ry similar to th op ration of th orific plat ,
xc pt that, du to th construction of th v nturi, th r is a r cov ry of n r-
gy r ducing th ov rall pr ssur drop. A typical v nturi flow l m nt is shown
in Figur 4.
Whil r duction in n rgy consumption is d sirabl , a v nturi flow l m nt is
consid rably mor xp nsiv than an orific plat ; and, still suff rs similar lim-
itations, with r sp ct to low flow s nsitivity and s nsitivity to chang s in
proc ss pr ssur and t mp ratur .
Venturi Flow Element
Figure 4

7
Averagi g Pitot Tube
Th av raging pitot tub can b an ff ctiv diff r ntial pr ssur flow m asur -
m nt d vic . It is an ins rtion d vic that m asur s th av rag v locity across
th pip as shown in th illustration b low. Whil this typ of instrum nt is
ff ctiv for liquid and st am flow, it has limitations with gas flow m asur m nt,
particularly low flow s nsitivity and turndown.
Th flow m asur m nt is bas d on d t rmining th v locity pr ssur , which is
th diff r nc b tw n th total pr ssur (m asur d on th upstr am sid ) and
th static pr ssur (m asur d on th downstr am sid ). Th v locity pr ssur
provid s an indication of th actual v locity at th op rating t mp ratur and
pr ssur . Th av raging pitot tub sampl s at various points across th pip or
duct in ord r to r duc th ff ct of flow profil . R f r to Figur 5.
Vortex Sheddi g
This t chnology is w ll acc pt d for flow m asur m nt at high flow rat s. Th
principl of op ration is bas d upon th sh dding or th cr ation of vortic s
from a blunt l m nt. Th instrum nt counts th numb r of vortic s cr at d;
and, through a known r lationship, conv rts this to th actual flow rat
through th l m nt. R f r to Figur 6.
Total
Pressure Static
Pressure
Averaging Pitot Tube
Flow
Still Fluid
Shear Layer
High Velocity Fluid
Alternate
Vortices
Vortex Shedding
Figure 5
Figure 6

8
Th r ar limitations wh n using this t chnology for m asur m nt of gas
flow rat s:
1. Vort x flow m t rs m asur th actual flow rat moving past th flow
l m nt. To conv rt to mass flow, it is n c ssary to m asur pr ssur ,
t mp ratur , and us a flow comput r to calculat mass flow.
2. Minimum flow rat . Th r is a minimum flow v locity that g n rat s
vortic s. At flow rat s b low this minimum flow rat , th instrum nt
will not g n rat vortic s. This is why som vort x flow m t rs ar
r port d to drop off at th low nd. This r duc s th ability to d t ct
low flow rat s and r duc ov rall turndown of th instrum nt. This is
a physical limitation that xists for all vort x flow m t rs. Som tim s
th pip siz is r duc d in ord r to incr as th v locity through th
vort x flow m t r to improv low flow s nsitivity. This complicat s an
installation— sp cially if don as a r trofit on an xisting installation.
Whil vort x flow transmitt rs ar occasionally us d for high v locity gas flow,
th ir most common applications ar for liquid or st am flow.
Turbi e Flow Meters
Fr qu ntly us d for air and gas flow m asur m nt, turbin flow m t rs ar
availabl as both an in-lin body and an ins rtion prob .
Th van s of th turbin flow m t r spin as th fluid mov s — th gr at r th
flow rat , th fast r th van s will turn. Various m thods ar us d to count th
puls rat (numb r of turns) of th van s.
Turbin m t rs can b accurat for m asuring th actual gas flow rat ; but,
lik oth r d vic s, th y r quir pr ssur and t mp ratur corr ction to obtain
mass flow m asur m nt. Turbin m t rs also hav a minimum v locity that
th y can d t ct, and may r quir som typ of lubrication of th b arings.
Th y ar also known to continu spinning aft r th flow has stopp d r sulting
in high flow m asur m nts.
Many applications for turbin flow m t rs ar for small r, in-lin flow bodi s
wh r pr ssur and t mp ratur ar fairly constant. B caus of th moving
parts th gas do s n d to b cl an.

9
Ultrasonic Flow
Ultrasonic t chnology is us d to m asur th v locity of th fluid in a clos d
pip . Th s nsors can b ith r ins rtion typ or a non-invasiv typ , which
clamp onto th sid of th pip .
Wh n th r is flow, th tim for a puls to go b tw n th downstr am and
upstr am transduc r will b long r than th tim from th upstr am to th
downstr am transduc r. Th fast r th v locity th gr at r this diff r nc in
tim . This tim diff r nc can th n b us d to calculat th v locity and flow
in th pip at op rating conditions. M asur m nt of both pr ssur and
t mp ratur is us d to obtain mass flow.
Th accuracy of an ultrasonic flow m t r can b improv d by providing multi-
pl s ts of transduc rs or paths which m asur th flow in diff r nt ar as of
th pip .
Coriolis
Coriolis is a dir ct form of mass flow m asur m nt, which do s not r ly upon
th physical prop rti s of th fluid. Wh n first d v lop d, Coriolis flow m t rs
did not hav th s nsitivity for gas flow m asur m nt; how v r, th t chnology
has improv d and Coriolis is now us d to m asur gas flows.
Th r ar a vari ty of d signs of Coriolis Mass Flow M t rs. Th s includ singl
straight tub , multipl straight tub , curv d tub , S-tub and oth r variations.
Th y work on th basic principl that th th Coriolis forc is d p nd nt on
th mass flow through an oscillating s nsor tub .
Th advantag of Coriolis is that it is a tru , high accuracy, mass flow
m asur m nt, ind p nd nt of th prop rti s of th fluid. Disadvantag s ar
that it is availabl only as an in-lin d vic with limitations in siz . Coriolis
flowm t rs ar xp nsiv to purchas and install. May not b suitabl for low
flow, low pr ssur applications. Pr ssur drop across a Coriolis flowm t r is
also a consid ration.
Downstream
transducer
Upstream
transducer
Flow direction Flow direction
D
L
tu
td
Figure 7

10
Pressure/Temperature Correctio
As pr viously discuss d, many flow m t rs m asur th actual flow rat at
op rating conditions rath r than th mass flow rat . In ord r to obtain a mass
flow m asur m nt, it is n c ssary to corr ct for th actual pr ssur and t m-
p ratur . Figur 8 illustrat s what is n c ssary to obtain a mass flow m asur -
m nt from a diff r ntial pr ssur flow m asur m nt d vic (th most common
form of flow m asur m nt). Th diff r ntial pr ssur across a flow l m nt
must b m asur d to obtain th actual flow rat ; th n, th actual flow rat
must b corr ct d for pr ssur and t mp ratur . This r quir s thr s parat
instrum nts to m asur diff r ntial pr ssur , pr ssur , and t mp ratur . Th s
signals ar s nt to a flow comput r to d t rmin th mass flow rat . Th rmal
mass flow t chnology r plac s all of this with on instrum nt that dir ctly
m asur s th mass flow rat of th gas.
Multi-Variable Transmitters
A multi-variabl transmitt r combin s th function of th thr transmitt rs
(∆P, P, and T) and th flow comput r into on transmitt r. Th multi-variabl
transmitt r m asur s th pr ssur and t mp ratur of th gas, th pr ssur
drop and th n calculat s th mass flow.
Whil only on 4–20 mA signal is g n rally availabl , m asur m nt of th
oth r variabl s is availabl via HART®communication. Th functionality of
thr transmitt rs in on d vic r duc s th installation cost of diff r ntial
pr ssur mass flow m asur m nt.
Multi-variabl transmitt rs ar fr qu ntly us d with av raging pitot tub s.
Turndown ratios in flow up to 8:1 ar obtainabl . This turndown ratio is
bas d on th maximum allowabl flow rat that th av raging pitot tub can
d t ct. Th actual turndown rat for th us r’s sp cific application will b l ss,
d p nding upon th application. Limitations in low flow m asur m nt
capabiliti s still xist.
Multi-variabl transmitt rs ar also int grat d into som vort x flow transmit-
t rs. Th s instrum nts m asur th actual flow rat using a vort x; th n, also
m asur th pr ssur and t mp ratur and provid a corr ction to mass flow.
Th s instrum nts t nd to b rath r xp nsiv .
Flow
Computer
P PT
Mass Flow
Actual to Mass Flow Conversion
Figure 8

11
Thermal Mass Flow
Th rmal Mass Flow off rs many advantag s ov r oth r, mor traditional,
m thods of m asuring gas flow.
1. Mass flow m asur m nt. Th rmal mass flow transmitt rs provid a
m asur m nt of th mass flow rat of th gas bas d upon h at trans-
f r. Th gas flows past a h at d surfac cr ating a cooling ff ct. H at
transf r is caus d by th mass (or mol cular) flow of th gas providing
a mass flow m asur m nt. Corr ction of th gas flow rat for pr ssur
and t mp ratur is not r quir d.
2. Exc ll nt low flow s nsitivity. Th rmal t chnology can m asur v loci-
ti s down to 10 standard f t p r minut —much low r than any
oth r flow d vic . Th h at transf r rat is gr at st at low flow rat s
and d cr as s as th flow rat incr as s. This mak s this t chnology
sp cially s nsitiv for low v locity m asur m nt and high turndown
r quir m nts.
3. Exc ll nt turndown. Th Magn trol®Th rmal Mass Flow Transmitt r
off rs th ability to m asur th low v lociti s as w ll as high flow
rat s. This can provid a turndown rat of 100:1 or mor d p nding
upon th application r quir m nts and calibration of th instrum nt.
4. Low pr ssur drop. Th ins rtion prob has littl blockag of th
pip , th r by, cr ating v ry low pr ssur drops.
5. Eas in installation. Using an ins rtion prob , th instrum nt can
asily b install d in a pip or duct. Many installations us a compr s-
sion fitting or a r tractabl prob ass mbly for ins rting th prob
into th pip .
6. Factory Calibrat d. Each instrum nt is calibrat d by MAGNETROL
for th application sp cific r quir m nts and configur d to th us r’s
sp cifications. Th instrum nt can b install d and plac d dir ctly
into s rvic without any n d for fi ld s t up, calibration, or
adjustm nt.
7. Low st install d cost. Wh n consid ring options to m asur mass
flow, th TA2 has th low st install d cost whil providing xc ll nt
p rformanc . No additional instrum ntation is r quir d to obtain a
mass flow m asur m nt.

12
MAGNETROL off rs th TA2 th rmal mass flow m t r to m t a broad
rang of us r’s application r quir m nts:
Th s nsor us d with th MAGNETROL Th rmal Mass Flow m t r is
illustrat d in Figur 9.
Th s nsors on th TA2 ar prot ct d to pr v nt possibl damag from “bot-
toming out” wh n ins rt d into a pip . This is an important consid ration if
th prob is install d into th pip with a compr ssion fitting. Th r ar two
s nsors — a h at d s nsor (shown on th right) and a r f r nc s nsor which
m asur s th t mp ratur of th gas. Pr cision match d Platinum RTDs ar
us d for th t mp ratur m asur m nt. A mass balancing l m nt is us d to
nsur that both s nsors will r spond th sam to chang s in t mp ratur .
RTD
Heater
Mass Balancing
Element
Reference
Sensor
TA2 Sensor
Model TA2 with
Insertion Probe
Model TA2
with Flow Body
Figure 9

13
Different Types of Thermal Mass Flow Meters
Th r ar two diff r nt t chnologi s utiliz d for th rmal mass flow m asur m nt.
Both m thods obtain th sam r sults, and th us r should not b conc rn d
which m thod is us d. MAGNETROL has xp ri nc with both t chnologi s.
Co sta t Temperature Differe ce
This t chnology maintains a constant t mp ratur diff r nc b tw n th
h at d s nsor and th r f r nc s nsor. Th instrum nt controls th amount of
pow r to th h at r to maintain this t mp ratur diff r nc . Th t mp ratur
diff r nc is s t during th calibration to optimiz th p rformanc for th
sp cific application. As th flow rat incr as s, mor pow r is r quir d to
maintain this constant t mp ratur diff r nc . R f r to th chart shown in
Figur 10. This is th t chnology us d in th TA2.
As shown in th illustration, at low mass flow rat s, th r is littl h at transf r,
and thus th amount of pow r r quir d to maintain th d sir d t mp ratur
diff r nc is low. As th mass flow rat incr as s, th amount of pow r
r quir d to maintain a constant t mp ratur diff r nc incr as s.
Chang s in pow r ar gr at st at low v lociti s thus providing xc ll nt low
flow s nsitivity. As th mass flow rat incr as s, th pow r incr as s as shown.
This p rmits flow m asur m nt at v ry high flow rat s providing high turn-
down capabiliti s.
Th r is an inh r ntly non lin ar r lationship b tw n mass flow rat and signal.
Th instrum nt lin ariz s th signal to produc a lin ar 4-20 mA output signal
ov r th dynamic rang of th instrum nt.
Calibration is an important part of th op ration of th TA2. Each instrum nt
is calibrat d to stablish this r lationship b tw n pow r and mass flow.
Th us r can asily modify th 4 and 20 mA points within th calibration
rang of th instrum nt. Oth r fi ld configuration may includ changing
th pip or duct siz , units of m asur m nt, damping, and oth r sp cific
application factors.
Power
Mass Flow Rate
Figure 10

14
Co sta t Power
This t chnology us s a constant pow r to th h at r. Th instrum nt m asur s
th t mp ratur diff r nc b tw n th h at d s nsor and th r f r nc s nsor,
which m asur s th proc ss t mp ratur . Th t mp ratur diff r nc d cr as s
as th flow rat incr as s. This was th m thod utiliz d in th TA1, th first
g n ration th rmal mass flow m t r of MAGNETROL.
At low flow rat s th t mp ratur diff r nc b tw n th s nsors is gr at st.
As th flow rat incr as s, th t mp ratur diff r nc d cr as s. A curv of
t mp ratur diff r nc v rsus mass flow rat is shown in th chart b low.
Th chang in t mp ratur diff r nc with mass flow rat is v ry larg at th
low flow rat s – this provid s xc ll nt low flow s nsitivity as pr viously m n-
tion d. As th flow rat incr as s, th t mp ratur diff r nc d cr as s y t th
curv still shows good s nsitivity at v ry high flow rat s providing high turn-
down capabiliti s.
Response Time
On of th main diff r nc s b tw n th Constant T mp ratur Diff r nc
and th Constant Pow r T chnologi s is th r spons tim . Th r is som
th rmal mass which has to h at up or cool down r sulting in a slight d lay in
tim b tw n wh n a chang in flow occurs and th instrum nt r ports th
flow rat .
Constant Pow r op ration is a passiv op ration. Th t mp ratur diff r nc is
d p nd nt upon how long it tak s for th h at d s nsor to h at up or cool
down with flow chang s. Th r spons tim d p nds upon th amount of th
st p chang , if th r is an incr as or d cr as in flow rat , and th typ of gas.
Tem perature Difference
Mass Flow Rate
Figure 11

15
The constant temperature difference operation used in the TA2 has a faster
response time to changes in flow The TA2 controls the power to the heater
to always maintain a constant temperature difference between the reference
RTD and the RTD measuring the temperature of the heater There is a PID
control circuit in the TA2 with parameters set to provide the fastest response
possible while maintaining steady operation once flow has been reached The
response time of the TA2 to increasing and decreasing flow is shown below:
Temperature Compensation
Thermal Mass Flow Transmitters measure heat transfer and infer the mass
flow This relationship between heat transfer and mass flow is obtained during
the calibration which will be discussed in more detail later
However, the gas properties that affect convective heat transfer are also affected
by changing temperature These properties include thermal conductivity, gas
viscosity, density and specific heat
MAGNETROL has done extensive testing and analysis on the effect of
changes in flow at different temperatures and has developed a proprietary
method of providing temperature compensation over the entire operating
range of the instrument The chart below shows typical curves for the TA2
showing the change in flow rate which occur at different process temperatures
due to changes in gas properties
Figure 12

16
MAGNETROL t mp ratur comp nsat s ov r th ntir op rating
t mp ratur rang of th instrum nt. The TA2 measures the temperature and
then applies a correction in the flow measurement based upon the operating
temperature.
Th chart b low shows th sam data aft r w apply our t mp ratur
comp nsation.
Th s illustrations d monstrat th ff ctiv n ss of MAGNETROL
t mp ratur comp nsation of th mass flow m asur m nt bas d upon varying
gas prop rti s.
Som th rmal mass flow manufactur rs just t mp ratur comp nsat th l c-
tronic circuit. What th s manufactur rs ov rlook is that th gas prop rti s
that aff ct conv ctiv h at transf r ar t mp ratur d p nd nt. Thus changing
t mp ratur chang s h at transf r. MAGNETROL provid s r al tim t mp r-
atur comp nsation that m asur s th t mp ratur of th gas and automatically
corr cts th mass flow m asur m nt bas d on t mp ratur variations.
Without r al tim t mp ratur comp nsation, th accuracy of th flow m as-
ur m nt will d grad with t mp ratur chang s. In som comp titiv d signs,
th rat d accuracy is only good within 50° F of th calibration t mp ratur .
Power mW
Mass Flow
100°F
200°F
300°F
400°F
TA2Uncompensated
Flow Data
Power mW
Mass Flow
100°F
200°F
300°F
400°F
TA2Compensated
Flow Data
Figure 13
Figure 14

17
If th instrum nt do s not provid a t mp ratur m asur m nt, th instru-
m nt cannot provid r al tim t mp ratur comp nsation. This is sp cially a
consid ration with oth r manufactur rs’ constant t mp ratur diff r nc op r-
ation; th s d signs hav a r f r nc RTD and a s lf h at d RTD. Th r f r-
nc RTD is us d in th l ctronic circuit and do s not provid t mp ratur
m asur m nt. Thus th s instrum nts do not provid r al tim t mp ratur
comp nsation.
Anoth r consid ration in using a s lf h at d RTD is that th r sistanc of th
s lf-h at d RTD chang s with t mp ratur . Without knowing th t mp ra-
tur , th instrum nt cannot comp nsat for th changing r sistanc of th
h at d RTD.
MAGNETROL advanc d r al tim t mp ratur comp nsation is an important
f atur which provid s sup rior p rformanc for th Mod l TA2.
Pressure Effects
H at transf r is aff ct d by changing t mp ratur . This is bas d upon both
th ory and th xp ri nc of MAGNETROL. How v r, h at transf r is not
aff ct d by changing pr ssur s. Th chart b low d monstrat s that pr ssur
do s not aff ct th rmal mass flow m asur m nt.
An incr as in pr ssur will incr as th gas d nsity – th r is th sam
amount of h at transf r with a low v locity, high d nsity gas as th r is with a
high v locity, low d nsity gas.
Signal
Mass Flow
14.7 psia
25 psia
100 psia
Figure 15

18
Calibration
Each instrument is calibrated for the gas and the specified flow rate.
Calibration involves placing the sensor in a flow bench, flowing a known
amount of gas over the sensor and measuring the signal. This is repeated for at
least 0 different flow rates. The calibration data of signal and flow rate is then
installed in the instrument. The calibration is NIST traceable. A calibration
certificate is included with the instrument and all calibration data is retained
at MAGNETROL for future reference.
When installed and placed into operation, the instrument measures a signal,
and then converts that signal to the flow rate for the user’s application. The
instrument adjusts for differences in area and blockage effect between the
calibration fixture and the field installation.
Each gas has different thermal properties that effect convective heat transfer.
An instrument calibrated for air will not provide accurate measurements if
used for natural gas. Each instrument is calibrated for a specified gas over the
maximum flow rate specified.
It is possible to have two sets of calibration data in the TA2. Thus the same
unit can be calibrated on two different gases such as natural gas and Oxygen
allowing the user to swap between calibration tables. If the instrument is cali-
brated for a specific gas, it is also calibrated on air. The gas data goes into one
table and the air data into the second table. If the user ever needs to check the
calibration on a flow bench, it would be possible to check the air data rather
than the more expensive third party calibration for different gases.
Direct calibration on the actual gas is commonly performed and will provide
best results. There are occasions where the user is looking for close and repeat-
able flow measurements. In these cases MAGNETROL can provide an Air
Equivalency calibration. Every time we perform a direct calibration on a gas,
we also calibrate on air. This has given us an extensive database which compares
the velocity of air to the velocity of gas to produce the same signal. The
following shows a typical air equivalency curve.
Figure 16