Chromatography Forum

This is a new one on me. I've noticed during the last month, after compiling statistics, unusually high variability in my internal standards, surrogates and CCV.

Just did a turbopump replacement in the connected 5973inert and in the course of it noticed that my 6890 inlet is showing at the called for temperature but is cold (or at least much cooler), being warmed only by the transfer line. I assume I need to get a new inlet heater assembly. But how can this be happening?

Temperature is proportional to resistance of platinum-rhodium thermocouple.

At 20C it is about 100 Ohm. 200C can be simulated by 150-200 Ohm. You can trick GC by replacing thermocouple with resistor. Obviously there is no need in turning on heater when readings states 200C.

Just an oxygenated terminals of thermocouple can make this.

It does change in reading in response to changing the set temperature. I set the temperature for 150C and it dropped to 150C. But I am reading 65C at the flip top with a calibrated lab thermometer. I'm going to run it down to "40C" then back up to "200C" and see what happens. I guess I'd better order a new heater/sensor and maybe a new block just in case the thermocouple is stuck in there for good.

So, if it is changing but not accurately then its still a replacement I suppose. No sensor calibration function for temperature.

Just let it get down to 40 then set it to 200 Set the external probe close to the top of the inlet next to the heater...
Inlet T vs external probe
40C vs 31C
100C vs 40C
140C vs 50C
170C vs 60C
190C vs 70C
200C vs 80C
ultimately at equilibrium 200C vs 100C.

So, variation but time for a new heater/sensor cable.

https://www.restek.com/row/products/acc ... erm=heater

Or maybe the electronics board problem. Better check with definitely working sensor.
As far I know heaters can be reconfigured - e.g. inlet heater+sensor connected to AUX.

Just let it get down to 40 then set it to 200 Set the external probe close to the top of the inlet next to the heater...
Inlet T vs external probe
40C vs 31C
100C vs 40C
140C vs 50C
170C vs 60C
190C vs 70C
200C vs 80C
ultimately at equilibrium 200C vs 100C.

So, variation but time for a new heater/sensor cable.

You could just make a calibration table for it to correct for the wrong readings.

Have you put the external probe down inside the inlet to make sure what the actual temperature of the inlet is and not just at the top?

You may want to send a message to Agilent and see if there is a calibration function in the electronics that isn't one of the main settings. There used to be some maintenance only functions you could access using a code on the keypad, but not sure if the newer units still have that. On the 5890 the .5 then Enter would reboot the main board, but that is the only one I remember.

I'm going to try putting a thermocouple into the inlet.

Also going to put thermocouple in the insulation of the top of the inlet of another functioning 6890 and see how that plots from room temperature to 200C.

Well, direct thermocouple in the well after taking out the liner...
Touching the sides: near the top ~160C and near the bottom ~186 close enough to 200C.

So, I'm going to have to assign the ISS variation to the filament being really old and gnarly.

Over the weekend my 6890N mainboard has apparently failed. I get the Fault:
Thermal Shutdown
Mux ADC offset value

From the Service Manual...

"Fault 240—Mux ADC thermal shutdown
Status message: Mux ADC offset value
The multiplexor, which processes the 6890 GC electronic signals, is not
functioning. Most likely, the multiplexor’s circuitry is defective. Power will be
shut off to the oven and other heated zones. The 6890 GC will not be ready.
❐ The main board must be replaced."

I wonder if this is related to the same power failure that took out my 5973 turbo pump.

With a lot of 'how to do electrostatic control' advice from a friend and some advice from GTLS; I got a new mainboard from Agilent and did the swap Friday. Kudos to Agilent for their 10 page illustrated walk through the exchange process. It was pretty painless. The pamphlet even gave all the keystrokes to reenter the serial number and mfr date.

Hi Who is GTLS? The reason I ask is we had a big thunderstorm and now I have the same fault.

https://www.greatlakestechnical.com/
I think mine was affected a few weeks earlier when I lost my GC-MS turbopump. But I have no proof. Or perhaps there was a power glitch the night it happened because my other 5971/5890 GC-MS had a flow controller board failure at the same time.

https://www.greatlakestechnical.com/
I think mine was affected a few weeks earlier when I lost my GC-MS turbopump. But I have no proof. Or perhaps there was a power glitch the night it happened because my other 5971/5890 GC-MS had a flow controller board failure at the same time.

I have had good results from Great Lakes myself. I they are the ones who upgraded our old 5973 from HPIB to LAN a few years back and some other repairs also.