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he Electronic Bell Jar - Thermocouple Vacuum Gauge

Building a Thermocouple Vacuum Gauge

he operating principle of the thermocouple vacuum gauge and instructions for build

a low-cost power supply & readout that is compatible with commercial gauge tube

The full version of this article appeared in Volume 1, Number 4 of the Bell Jar.

troduction

he thermocouple (or T/C) gauge is one of the more common and cost effective gauges for vacuum press

easurement in the 1 Torr to 1 milliTorr range. The T/C is usually found in the forelines of high vacuum

stems (i.e. between the roughing and diffusion pumps) as well as in single pump systems of the sort use

acuate sign tubes.

ke most vacuum gauges, the T/C gauge does not measure pressure directly as do, for example, manome

the McLeod or Bourdon type. Instead, these vacuum gauges depend on changes of a physical

aracteristic of the residual gas within the gauge tube. In the case of the T/C gauge, and all other therma

nduction gauges, that characteristic is the thermal conductivity of the gas.

thermal conduction gauge may be thought of as a defective vacuum insulated thermos bottle (refer to

gure 1).

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ach has a hot element (coffee for one, a filament in the case of the other) within a vacuum wall. There a

o ways of removing heat: conduction (molecule to molecule) and radiation. For both coffee and warm

aments the primary path at atmospheric pressure is conduction. As it turns out, the thermal conductivity

r is nearly constant down to a fairly low pressure - about 1 Torr. Then it begins to change rather linearly

th pressure down to a value of about 1 mTorr, whereupon conduction through the gas ceases to be a ma

ctor. At that point, the dominant loss factors are conduction through wall and leads, and radiation.

hat might be surprising to many people is that a fairly good vacuum is needed in a thermos. With a bit

gher pressure, you might as well have no vacuum. In the case of the thermal conduction gauge, operatio

ll only occur within the sloped portion of the curve. An interesting experiment would be to nick open a

ermos bottle refill and measure the cool-off rates for hot water with the bottle evacuated to a number of

essures. The result would be a useful, but very slow, thermal conduction gauge.

he T/C gauge contains two elements: a heater (filament) and a thermocouple junction which contacts th

ament. With the filament current held constant, as the pressure within the tube is decreased the filamen



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ll become hotter because of the improved thermal insulation provided by the increasingly rarefied gas.

his temperature is sensed by the thermocouple junction. Measurement is accomplished by reading the

ermocouple junction voltage on a sensitive meter which has previously been calibrated against a

anometer. Simple T/C gauges may be obtained from a variety of sources such as Duniway Stockroomo

urt J. Lesker Co.These gauges consist of the gauge tube itself, a power supply for the filament, and a

oving coil (d'Arsonval) meter for displaying the pressure. Tubes usually have a 1/8" male pipe thread fo

upling to the vacuum line and an octal (vacuum tube) base for mating with a socket. In newer gauges, t

wer supply is usually nothing more than a plug-in type ac adapter with a potentiometer for adjusting thrrent. Each type of T/C tube has its own calibration curve. Also, as there are some structural variations

om tube to tube within a type, each has its own filament current rating. The current at which the gauge w

nform to the calibration curve is imprinted on each tube. The gauges are calibrated for air. As different

ses have varying thermal conductivities, the gauge will not be accurate when working with, for exampl

gon or carbon dioxide.

aking Your Own Gauge Controller

s was previously noted, complete basic T/C gauges are available from a variety of suppliers. Typical pre in the $200 to $250 range, new. Given the basic simplicity of a T/C gauge, building one from availab

rts would not seem to be difficult. The gauge tube and the readout (thermocouple) meter are the only

ecialty components. For the tube, I don't think that there is much purpose to trying to build your own. B

e of the cheaper ones (some suggestions will follow below). They can be had for about $40 new. The

eters are specialized items in that they have to be compatible with the millivolt level, low impedance ou

the gauge's thermocouple.The more commonly available milliamp/microamp meters have coil resistan

any times the 55 ohms of the meter used in a typical gauge controller. Connect up a standard microamp

eter to the gauge tube and it might budge, but probably not much. If you buy a meter as a subassembly

unaway sells the meters separately, but they are not cheap if your reference point is your junk drawer o

rplus catalog) you will get a very professional and calibrated readout as long as you use the tube for whwas intended. Even with this route, the complete gauge should come in at half the price of a new

mmercial unit.

very satisfactory alternative involves the placement of an IC amplifier/buffer between the gauge and th

eter. By selecting the right values of components, almost any meter can be coupled with any gauge tube

ng as you know the tubes maximum output and calibration curve. The next section will detail how to bu

op-amp based T/C gauge using either of two inexpensive tubes.

n Op-Amp Based T/C Controller

he meter side of this controller is based on a single stage op-amp amplifier configured in the inverting

ode. To establish the component values in the circuit (the values of the input and feedback resistors) on

eds to know the load resistance for which the T/C tube was calibrated and the maximum output voltage

e thermocouple at fullvacuum. The latter corresponds to a full scale deflection of the meter and is taken

pressure of 10-4Torr. The tubes we shall consider are the 531 and 6343 (and their Kurt J. Lesker

uivalents). Relevant data on these tubes is shown in Figure 2.

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he circuit is shown in Figure 3.



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nce the input impedance of an inverting amplifier is set by the input resistor, the value for this should b

ms. I elected to measure the output with a 30 k-ohm/volt multimeter set on the 1 volt scale. Thus the ga

the amplifier would have to be set to up the 14 mV T/C output to 1 volt, a gain of 71.4. As the amps ga

set by the value of the feedback resistor, Rf, divided by the value of the input resistor, Ri, Rfshould be

out 3.9k. As it turned out, the closest values I had on hand were 47k and 3.3k which would give a gain

.2. Close enough, I figured.

he op-amp used was a 741 and the circuit was assembled on a Radio Shack proto pc board, catalog num

6-159. I used a regulated +/- 15 volt supply but a couple of 9 volt batteries would work as well. Likewi

mA meter (surplus of course) with a series resistor could be used in place of the multimeter. Do includ

e offset pot for zeroing.

n the filament supply side it does not matter which pin you select as the positive pin. However, it is

sential that the filament supply be independent of the amplifier circuit (i.e. no common ground). Otherw

u will end up just amplifying the filament voltage (the filament and the thermocouple are electrically



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nnected). The filament pot (as well as the offset pot) are 10 turn wirewounds. Fair Radio Sales and othe

rplus electronics houses have them for about three dollars per.

o get the gauge going, connect the tube to your system with the threaded connection (use Teflon tape or

her sealant) or just slip it into a piece of tight fitting rubber vacuum tubing and tighten with a hose clam

ctal sockets are available from Fair Radio and 4 conductor telephone type cable is good for the tube to

ntroller connection (this should be no longer than 10 feet or so). Be sure to have the filament current

ntrol at the lowest setting so you don't burn out the tube. Begin to pump down the system and set the oft for a 0reading on the T/C meter. Then begin to bring the filament current up to the value marked on t

be. The T/C meter should begin to creep up indicating that (1) the circuit is working and (2) that you ar

lling a vacuum.

any T/C tubes dont do well when operated at atmospheric pressure. To preserve your tube, dont apply

ament power until you are sure that you are drawing a vacuum in the system. Also, avoid getting

ntaminants in the tube and position it at a location in the system plumbing where oil cannot back up int

alibration

ow, all you need to know is the correspondence between the meter reading and pressure. The table of

gure 2, with data points scaled directly from production gauges, gives a reasonably accurate set of poin

th which to develop a calibration curve. Even with the sloppy resistor selection, my prototype controlle

acked a commercial gauge pretty well. Also, bear in mind that T/C gauges are not particularly accurate

struments. Most often they are used only as rough indicators of pressure where 10 to 20 percent accurac

ceptable.

ther Thermal Conductivity Gauges

here are two other common types of thermal conductivity gauge.

he Pirani gauge has a fine wire filament that has a high temperature coefficient of resistance. The wire a

both the heater and the sensor. Usually a Pirani gauge is part of a Wheatstone bridge circuit that also

cludes a temperature compensating element. Well designed Pirani gauges offer better accuracy and

sponse time than do thermocouple gauges (often tens of milliseconds vs. several seconds).

he thermistor gauge is the least common of this class of gauge. Using a small thermistor, the principle is

ry similar to that of the Pirani. However, the element is more massive and the response time is slower. hmaus of the University of Alberta described thermistor gauge in Volume 4, Number 2 of the Bell Jar

etails are also provided on Roys Web Site.

ome-made Pirani gauges can be made from small light bulbs, carefully opened, or even from model airc

gine glow-plugs. (See Volume 4, Number 1 of the Bell Jar.



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hermistor Vacuum Gauge

Thermistor Vacuum Gauge

ntroduction

ere is a simple, low cost rough vacuum gauge that you can build. The unit is intended toe small and light for mounting as close to your vacuum feedthroughs as possible to eliminatehe need for expensive shielded cabling. A well regulated 15 volt D.C. supply and either a

and calibrated analog meter or an ADC and computer are required. Vacuum range is from 5o well over 1000 millitorr and response time is fairly slow with a glass bead thermistor.

Part 1: The Self Balancing Bridge

ircuit Description

What follows assumes you have used an NTC thermistor, i.e. resistance drops withncreasing temperature.

nitially the thermistor will be a room temperature and at a resistance of about 2000hms. Current from start-up resistor R6 will drive the non-inverting input of the op ampositive turning the power F.E.T. on to start heating the thermistor. Thermistor resistancerops during heating until it equals the resistance of reference resistor R3 which has beenhosen to match thermistor resistance at 100 degrees C. The op-amp will now do everythingcan to maintain thermistor resistance at the R3 value resulting in a constant 100 degree. thermistor temperature.

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ridge output voltage is at maximum at Atmospheric pressure and decreases in vacuum dueo diminishing heat losses to gas molecules around the thermistor. Minimum voltage issually around 5 volts at .005 torr vacuum.

Part 2 . The Level Shifter

he subtractor circuit shown above shifts the 5 to 8 volt output from the bridge down to a

round referenced 0 to 2.5 volts. Normally R7 to R10 would all be the same value with9 connected to a reference voltage equal to whatever the minimum voltage from the bridgeas. R9 is connected to the regulated +15 volt supply to reduce component countnd consequently is a higher value. R9 will most likely have to be hand trimmed forfferent thermistors. Don't use programming resistors of less than 100k for the singleower supply circuit shown or you will have problems getting the output to go near ground.

Part 3. Results

he graph below is a "quick and dirty" attempt at calibration against a thermocouple gauge onhe only vacuum system that was available for testing.



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he previous gauge was calibrated against a Granville Philips 'Convectron' (TM) Piraniauge. Results are here

Part 4. Construction Hints

nd a suitable thermistor that is vacuum compatible and with a resistance of about 2000 ohmst 25 degrees C and decreasing resistance with increasing tenperature. Some manufacturer'snd supplier's web page links with curves, etc. are included below.

acuum feed throughs for the thermistor leads can be quite expensive and hard to find sohat one is left up to the user's ingenuity with low vapor pressure epoxy or hermeticallyealed surplus connectors.

he electronics is easily assembed on an electronic 'bread board' for initial testing. Be sure toeat sink the power F.E.T. and allow for ventilation of the F.E.T. and R3. Themistor

manufacturers warn of drift above 105 degrees C so consult a TC curve and take care inhe selection of reference resistor R3.

nce you have the basic gauge working it will require calibration against a reference gaugef some sort. Alternatively if your bridge output is between five and eight volts attmospheric pressure other readings may be calculated using the graphs above as aeference. Low cost thermistors vary between individual units so your readings will not behe same as the graph readings but will be shifted up or down.

he bridge circuit shown in Part 1 can also be used with a single filament Pirani gaugeconnections to the inverting and non inverting op amp inputs are switched and R3 is reduced

o 115 ohms ohms for the old CVC "Autovac" gauge tubes. Outdoor Christmas lightaments seem to have about the right resistance and might also be worth trying forhose inclined to experiment.

Conclusion

riginally the control circuit was a Pirani gauge controller we developed to replace somencient Consolidated Vacuum Corporation Autovacgauge control units. With the demise ofVC and with two of their Pirani heads still on a difficult to modify glass teaching system

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replacement for the original vacuum tube unit was necessary. The Pirani controller hasroven to be very reliable and accurate and has been in service for ten years with no problems.

low cost replacement roughing gauge was in demand so we decided to try some glassead thermistors as vacuum sensing elements and developed the very low-cost, simplercuit shown above. Accuracy and response time are not as impressive as that of the Piraniauge but still adequate for non-critical applications.

oy Schmaus September 1999

Thermistor Manufacturer's Links

Alpha SensorsExcellent technical information including thermistor curves.

General Information About Thermistorsfrom Wuntronics Gmbh.

Thermometrics

ources

ost local electronics jobbers should be able to supply thermistors from a variety ofuppliers. Here are a few suppliers that list low cost glass and epoxy thermistors in their catalogs.

Digi-Key

Thermometrics, Keystone Thermometrics, Panasonic thermistors.

Electro-Sonic

Stock # 135-202FAG-J01Fenwal Glass encapsulated chip thermistor, 2k at 25 degrees C. $5.92 Cdn.

Newark

Philips, Thermometrics thermistors

References

The Burr Brown Handbook of Operational amplifier Applications The National Semiconductor Corporation Linear Data Book The International Rectifier HEXFET Power MOSFET Data Book

1999,2000 by Roy Schmaus

Back
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