An automatic controller for liquid cooled space suits is reported that utilizes human sweat rate as the primary input signal. The controller is so designed that the coolant inlet temperature is inversely proportional to the subject's latent heat loss as evidenced by evaporative water loss

Blackaby, J. R.

Chambers, A. B.

English

NASA Technical Reports Server

' N72-27125
A LIQUID COOLED GARMENT TEMPERATURE CONTROLLER
BASED ON SWEAT RATE
Alan Chambers and James Blackaby
Ames Research Center
INTRODUCTION
When a man is isolated from his environment-as is an astronaut in a pressure suit-auxiliary cooling
is required to remove excess metabolic heat to maintain thermal balance (ref. 1). The major portion
of this cooling is often accomplished by a liquid cooled garment (LCG), and, in current operations
with the Apollo LCG, the astronauts maintain comfort by manually adjusting the inlet temperature
of the coolant. This method of control has been adequate, but it would be preferable to have
automatic temperature control. In an emergency, an astronaut should be free to concentrate
entirely on his immediate situation; furthermore, man may be a poor judge of his own thermal
state, especially when his attention is distracted so that his reactions to sensations of warmth and
cold are delayed. The development of automatic LCG temperature controllers, responding to an
astronaut's heat production rate, has been the subject of several studies. Three of these efforts are
reviewed, and the design and operation of an Ames-developed controller based on sweat rate is
presented in detail.
REVIEW
Three LCG temperature controllers that have been reported are a "metabolic rate" controller (ref. 2
and fig. 19.1), a "constant skin temperature" controller (ref. 3 and fig. 19.2), and a "differential
temperature" controller (ref. 4 and fig. 19.3). In the "metabolic rate" controller, the subject's
oxygen consumption is monitored and the controller automatically adjusts the LCG inlet
temperature to remove the appropriate amount of heat. A linear relation between the metabolic
heat produced and oxygen consumption is assumed. Difficulties develop with this approach during
periods of rest when slight overcooling
sometimes brings about lowering of skin
temperature and results in shivering;
oxygen consumption then increases, and
the controller lowers the temperature
still further. Another feedback element
would have to be added to the controller
to correct this difficulty. In addition,
this controller requires a satisfactory
method for continuous monitoring of
the oxygen consumption of the
astronaut in his pressure suit.
The second proposed controller
attempts to maintain a constant skin
temperature. This system has proved to
be stable, but the basic philosophy of a
BASAL 02
CONSUMPTION
COOLANT
co°'  T ITEMPERATURE SUIT HEATCONTROLLER , TRANSFER
02 CONSUMPTION
Q
TO SUIT
i  ooYTHERMAL --PROCESSES
Figure 19.1 "Metabolic rate" temperature controller (ref. 2).
283
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constant skin temperature is at
odds with the findings of an Ames
study (ref. 5) illustrated in figure
19.4, which shows that for
optimum comfort, skin
temperature should decrease with
an increase in heat production. The
"constant skin temperature" con-
troller should be adequate for low
metabolic rates; however, it may
allow an excessive amount of sweat-
ing at high metabolic rates when a
decrease in skin temperature is
desirable.
The "differential temperature"
controller attempts to regulate
cooling as a function of skin
temperature and heat removal. A
linear relationship between the heat
removal rate and comfortable skin
temperature is assumed. Skin
temperature is determined by
averaging the temperatures at four
selected points (right calf, over the
right kidney, right lower abdomen,
and lift bicep). The heat removal
rate is determined by measuring the
difference between the LCG inlet
and exit water temperatures. For
any given heat removal rate the
optimum skin temperature is
compared with the actual skin
temperature, and the LCG inlet
temperature is adjusted
accordingly.
A disadvantage of the
"differential temperature" con-
troller (and also of the "constant
skin temperature" controller) is the
requirement that transducers or
sensors be affixed directly on the
body of the subject. Skin sensors,
when used for extended periods,
are likely to cause discomfort; thus,
the acceptability of these con-
trollers is likely to be limited. The
COOLANT
BIAS_ TEMPERATURE ___R_
PRESSURE_¢._ APt, I COOLANT
-,vL --, SUIT HEAT
TRANSFER
: CONTROLLER SKIN
TEMPERATURETEMPERATURE
CONTROL [ SENSOR
PRESSURE I
Q
TO SUIT
BODY _
THERMAL
PROCESSES
Figure 19.2 "Constant skin temperature" thermal controller (ref. 3).
BASAL
SKIN TEMPERATURE
AND
HEAT PRODUCTION_ COOLANT
-- TEMPERATURE
CONTROLLER
SKIN TEMPERATURE
COOLANT
TRANSFER
t BODY
THERMAL
PROCESSES
O TO SUIT
Figure 19.3 "Differential temperature" thermal controller (ref 4).
,oo _
36
_ j0.22 Ib/hr
COMFORT ZONE / "_.._////7_////_ COMFORT
- 24 _- ""_///'_//.'-0.11 Ib/hrCOLD UNCOMFORTABLE SWEAT RATE
75 _- COLD UNCOMFORTABLE_ ZEswEROAT22 R E
I I I I
70 - O IOOO 2000 3000 4000
HEAT LOAD, Btu/hr
Figure19.4 Conduction cooling comfort zone (ref 5).
284
Environmental Control Research Branch at Ames Research Center has, therefore, undertaken the
development of an LCG automatic controller that does not require skin sensors.
AMES CONTROLLER DESIGN
The comfort and sweat rate data in figure 19.4 illustrate three interrelated concepts applicable to
subjects wearing a cooling garment such as the Apollo LCG.
1. Skin temperature should decrease with increasing heat load
2. Zero sweat rate is not necessary and may not be preferable at the higher heat loads
3. Sensible cooling of the skin can be directly related to the increase of sweat rate with increasing
heat load
With these concepts defined, the Ames controller development has been based on the utilization of
sweat rate as a primary input signal.
Three approaches were considered in formulating the logic for a sweat rate controller.
1. The controller could be designed to adjust coolant inlet temperature TIN to the LCG such that
the subject's sensible heat loss is kept proportional to his latent heat loss, thereby maintaining
an acceptable balance between the normal modes of heat transfer. The heat losses would be
determined from measurements of the inlet and outlet LCG and ventilating air temperatures
and the subject's sweat rate. The TIN to the LCG would be increased or decreased depending
on whether sensible heat loss was too large or too small with respect to the latent heat loss.
2. Sweat rate could be kept constant at a set level (i.e., 100 g/hr). The controller would vary TIN
inversely with respect to any change in sweat rate. To perform correctly, the controller set
point would have to be above the subject's normal insensible water loss rate; therefore, a slight
amount of sweating would occur at all times.
3. The controller could be designed such that TIN would be inversely proportional to the
subject's latent heat loss as evidenced by evaporative water loss. This logic is similar to that of
approach 2, but sweat rate would be permitted to increase with increasing work rate.
In the study described here, the third controller logic approach was used because the
instrumentation and computation requirements for its implementation were considered to be the
simplest. A block diagram of the logic is shown in
figure 19.5. The controller in this system regulates
LCG coolant inlet temperature TIN as a function
of two parameters: (a) a basal inlet water temper-
ature TIN and, (b) the increment a_ in evaporative
water loss rate with respect to a basal rate S_ The
TiN is the coolant temperature that provides
comfort for the subject in a sedentary mode, and ,_'
is the corresponding rate of evaporative water loss
T_IN
_ CONTROLLER
Figure 19.5 Temperature controller logic.
(which includes insensible water loss as well as loss due to active sweating). The controller algorithm
is
TIN = TIN - KI(AS)
Evaporative water loss (which is representative of latent heat loss) can be determined from
measurements of specific humidity (dewpoint) in the outlet and inlet air lines of the subject's
pressure suit or similar ventilating garment. If inlet air dewpoint is maintained at a constant low
285
value, then A_ can be expressed simply as a function of air flow rate _, the outlet air dewpoint Tdp
during exercise, and the outlet air dewpoint T_p measured with the subject in the sedentary mode
described above
AS = K2(w)(Tdp - T'dp )
ECRB CONTROLLER OPERATION
Experiments with the controller were conducted with subjects wearing a whole-body air-ventilated
suit over an Apollo LCG. The dewpoint of the suit inlet air was kept approximately constant at
-2 ° C. The inlet air temperature was maintained at 20 ° C with an air circulation rate of 29 m3/hr.
The dewpoint of the suit outlet air thus became the only variable used in controlling TIN. The
subjects exercised on a treadmill at various metabolic rates; the metabolic rates were calculated
based on oxygen consumption measurements during the tests. During initial tests, subjective
evaluations of thermal comfort were recorded while several values of the proportionality constants
K1 and K2 were set in the analog computer, which served as the controller for the tests. Based on
these evaluations proportionality constants selected for the controller equations were K1 =
0.09 [(°C • hr)/gm] and K2 = 0.40[gm/(m 3" °C)I.
The coolant supply system (fig. 19.6) introduced considerable mechanical delay (of the order of
3 min) into the overall controller response time because of backlash in the hot and cold metering
valves, slow reaction in the heat exchanger, and a
long coolant supply line to the LCG. Nonlinearity
of the hot and cold metering valves also affected
the response of the controller. The problems of
nonlinearity and delay were overcome, for these
tests, by operation of the controller in a sample
data system mode. The sampling rate was made
proportional to the difference between LCG inlet
temperature TINhactual and the desired inlet
temperature TiNbcontrol as computed by the
controller. At each sample time the hot and cold
metering valve motors were energized for a short,
WATER IN
11
HEAT
EXCHANGER
IT
TO LCG
RETURN
HOT SUPPLY
¢"
F
1
CONTROLLER
COLD SUPPLY
Figure 19.6 Coolant supply system.fixed length of time (approximately 50 msec) so
that TINlaetual was, in effect, caused to approach
TINIcontrol in steps. This mode of operation reduced the effects of the inadequacies of the coolant
supply components and resulted in relatively quick and stable response of the system.
Figures 19.7 and 19.8 show recorder tracings of treadmill speed, TINIcontrol, and TINlactual for
two levels of exercise. The subjects were kept thermally comfortable during the tests illustrated, and
for all tests with work rates up to
about 3000 kJ/hr. The only major
difficulty with the controller occurred
for a subject exercising at a high work
rate (4200 kJ/hr) for 1/2 hr. In this
test, the subject could not be ade-
quately cooled, especially around the
head and neck (the Apollo LCG
provides no head or neck cooling). A
TIN IAcTuAL'°C3o :i5 :
o
TIN ICONT_L,°C 3_ ___
TREADMILL 3 _ 1SPEED, m/sec 0 _'?--= _ , i
I I ] I I I I I I I I I I I I I I I I I I I I I I
TIME, min
Figure 19.7 Test results-work rate 1250 kJ/hr.
286
portion of the subject's sweat was not evaporated by the
ventilating air, but was absorbed by the fabric of the LCG.
When the subject stopped exercising, the absorbed sweat
began to evaporate; the controller sensed this as continued
sweating and maintained an LCG temperature lower than
was comfortable for the subject. In other respects, the
performance of the sweat rate automatic controller was
promising. Continuing tests are scheduled with the hard-
ware modified to eliminate the causes of most of the
nonlinearities and transport delays of the original system.
REFERENCES
1.
2.
TI N o C 30
ACTUAL' 0 [-_- ...... _ :-'--:_'_'i ]
TINIcoNTROL, °C :30 [ --
0 I •
TREADMILL 3 [:- - _ . 4
SPEED, m/sec 0 _ --_ - J
TIME, min
Figure19.8 Test results-work rate
2900kJ/hr.
Nunneley, S. A.: Space Life Sciences, vol. 2, 1970, pp. 335-360.
Webb, P.; Annis, J. F.; and Troutman, S.J.: Automatic Control of Water Cooling in Space
Suits, NASA CR-1085, 1968.
3. Merrill, G. L.; and Starr, J. B.: Automatic Temperature Control for Liquid Cooled Flight Suits,
NADC-AC-6702, August 3, 1967.
4. Troutman, S.J.; and Webb, P.: Automatic Control of Water Cooled Suits from Differential
Temperature Measurements, Report for NASA Contract NAS12-682, 1969.
5. Unpublished study in Environmental Control Research Branch, Ames Research Center, NASA.
287


A liquid cooled garment temperature controller based on sweat rate

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