Need help with TC gauge

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Nnnnnnn
Posts: 58
Joined: Tue Jun 13, 2017 6:25 pm
Real name: Niels Geerits

Need help with TC gauge

Post by Nnnnnnn »

I recently bought a granville phillips 270006 TC tube on ebay. According to this document http://www.ancorp.com/familyPDFs/file11 ... 52PM63.pdf the gp 270006 is equivalent to the A&N 6000 Series. Also according to this document pins 3 and 5 are the heater pins while pin 7 is the positive TC pin. Using a multimeter I confirmed that these pins are indeed connected. I found that pin 8 is also connected and must therefore be the negative TC pin. I made a non inverting amplifier (gain is 400) circuit using an lm358 opamp and hooked the output to an arduino. To heat the filament I use a 1.5V battery with 70 ohm in series (this puts 19mA into the filament). I pump down the tube, heat up the filament and nothing happens. I also tried to hook up the TC pins directly to the multimeter, again I´measured 0 volts. Though on the arduino when i turn the pump on I see a short rise in voltage across the TC (filament cold) which drops to zero again. The TC needs less than 1 torr to function so no reading could mean that my pump is no good (but it is new and rated for 22 micron). I can boil water with it at room temperature so I should be in the range of the TC. I will include a few pictures of my circuit, pump, TC and the voltage spike:

spike:
v spike.jpg
circuit:
circuit.jpg
tubing:
tubing.jpg
TC
tc.jpg
I hope someone can help.
John Futter
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Re: Need help with TC gauge

Post by John Futter »

Niels
Try to lose all of those brass fittings and direct connect the gauge to the pump with appropriate adapter using a little
loctite 290 super wickin on the threads --- do not use thread tape
Rex Allers
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Re: Need help with TC gauge

Post by Rex Allers »

Niels,
Not sure if it will help but maybe reading this 2015 thread might give you some insights.
"Fixing an Analog Thermocouple Gauge"
viewtopic.php?f=10&t=10116#p67724

That one was about a 531 sensor, so different from yours but all thermocouple sensors are similar. In my post of Mon May 25, 2015, I worked out some details of the meter circuit and also worked out the basic construction of a 531 sensor I had here. I was able to see the thermocouple effect with a low-scale voltmeter and a heat gun. That technique might give you a way to test your sensor and work out how the wires are arranged inside.

In your setup, if you make the heater current adjustable up to the rated current, you should see the termistor base voltage vary as the amount of heat is changed.

Assuming your sensor is fine, I'm not sure how you translate thermocouple voltage to pressure without a proper meter for your type of sensor or a table or chart of expected outputs.

In that other thread, it turned out the meter and sensor were fine. The problem was the vacuum wasn't anywhere near what was expected. John may have given you good advice on plumbing.
Rex Allers
Nnnnnnn
Posts: 58
Joined: Tue Jun 13, 2017 6:25 pm
Real name: Niels Geerits

Re: Need help with TC gauge

Post by Nnnnnnn »

Thanks for the advice John. I will try that and report back here once I get back from vacation.

As an extra here is my vacuum pump running for 30s with a piece of rubber blocking the inlet. Compared to other pumps I have heard this one sounds too loud https://youtu.be/fdFSK8NDHVY

Just saw you post rex soI got my TC and a hair dryer, shot some hot air into the tube and measured 40mV. The gauge seems to be working so the problem must be the pump.

EDIT: 40mV was a false reading. I now get 0.3mV all the way up to 1mV.
Nnnnnnn
Posts: 58
Joined: Tue Jun 13, 2017 6:25 pm
Real name: Niels Geerits

Re: Need help with TC gauge

Post by Nnnnnnn »

It has been a while and I have made some progress. First of all my TC tube is not one of the simple ones that runs on a DC heater current and has 2 TC output terminals. My tube runs on an AC heater current and has one TC output terminal which is referenced to both heater pins (not sure how to imagine this). See this website for more details http://www.repairfaq.org/sam/lasercva.htm#cvagtc my tube is equivalent to the DV-6M tube. I got a small transformer whose primary and secondary coil are both center tapped. I use two N-FETs and an arduino to create a 1kHz square wave for the transformer. The secondary center tap is connected to ground. The two other terminals are connected to the TC heater via a resistor and a potentiometer. I measure the TC voltage with a multimeter putting one MM terminal to the tubes TC terminal and the other MM terminal to ground. The pump to TC tube adapter stack looks as followed: Pump-> 1/4" Flare -> 3/8" BSPP -> 1/4" BSPP -> 1/8" NPT -> TC. I also used Loctite 290 on the NPT connections like John said. I am still waiting on proper sealing rings for the BSPP connections.

So the circuit is kind of like this http://www.repairfaq.org/sam/thtcro1.gif (the one for DV6).

The results: when I turn the pump on the TC output slowly climbs to around 4mV (FS 10mV) (@360mV dropped across the heater), when I turn off the pump the reading drops quickly to 1mV and then slowly declines further, but I disconnect the power before I let the pressure increase further. I blame the low output on the lack of sealing rings on the BSPP connections for the time being. I also tried to amplify the TC output with an opamp using a simple non inverting amplifier circuit (two resistors and an lm358p nothing else). To my surprise the opamp just clipped (VO=V_supply), so I disconnected the amplifier and looked at the TC output using my scope. I found that the output was not purely DC, but mainly AC (1V peak) with a small DC offset. To work around this with the opamp I could add a cap in the feedback loop turning the opamp circuit into an integrator which filters out the AC signal (any thoughts?).

I checked the documentation from the manufacturer http://www.idealvac.com/files/manualsII ... Manual.pdf (page 63 figure 5-8). It looks like they did use an integrator. I am not sure what the purpose of C27 or C32 is. Perhaps together with the resistors it forms a low pass filter to get rid of that AC. Anyway first I will try a simple integrator and make sure I could the proper sealing rings.
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Richard Hull
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Re: Need help with TC gauge

Post by Richard Hull »

What a wonderful up hill battle you are fighting as you claw your way to a simple TC gauge. A 1518 tube takes DC from a 1.5 v battery to drive the heater and the thermistor drives a 10mv 360ohm meter load to show a nice, Slightly expanded low end range. I have several TC gauges laying around here and I never paid more than $5.00 for the entire meter box complete with socketed cable. I then bought or already had the needed tube.

I'll bet the final issue will be the crappy TC gauge to pump connection. Mostly that black friction tape outgassing its 1.6528 million, trapped, micro air pockets over a number of hours.

One positive thing will come from this, if you are successful......You will know the ins and outs of TC gauges.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
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