Hi there,

I'm having difficulty in an instrumentation system that I’ve installed on a trailer. What this trailer does is it basically heats up a bunch of tar and then pumps it into cracks on the ground.

The instrumentation consists of the following

4 Hobo Data Loggers
4 PD-785 Temp Displays
3 Type K Thermocouples
-1 For Measuring the Heating Oil Temperature (Oil that surrounds the tar is heated by a diesel burner that in turn heats the tar tank)
-1 For Measuring the Tar Temperature
-1 For Measuring the Pump Temperature (The temperature of the tar as it is pumped into the crack).
1 RTD (Ambient Temp Sensor)

All thermocouples are wired properly with Type K thermocouple wire.

The Precision Digital Displays are wired to a common ground (the display housing box which is then wired to the frame of the trailer) and are powered from a common power line running from the ignition switch on the trailer that starts the burner (each meter is wired with a separate 5amp fuse coming off this main line).

The problem is that after first installing these probes on the unit and firing the unit up and letting it get up to temperature, the temperature for the Tar is approximately 50'C below what it should be (We used two different bi-metal gauges and they both measured around 150ish 'C while the thermocouple read 100'C)

As well, the heating oil temperature read about 110'C which is also impossible, as the heating oil temperature is always above what the tar temperature is (in this case it is 150'C)

We were unable to test the pump temperature as the pump failed on the unit during testing.

I got a separate supply of type K extension wire and re-wired up the tar tank to see if perhaps it was a bad shipment of wire, and unfortunately that is not the case (the reading stayed the same). So now the only two possibilities that I can think of are a damaged thermocouple giving incorrect readings, or the precision digital displays are not calibrated correctly (According to the manufacturer they are pre-calibrated at the manufacturing plant though).

As this is the first instrumentation setup I’ve worked on I'm not really sure what to do. I've been searching google for some information on this problem, and another possibility seems to be a ground loop error. Would this be possible?

Any input would be much appreciated.

Thank you for your time,
Michael Cowie

 

First, check the TC and check the instrument. If you do not have a proper calibrator, make a 12 inch type K sensor, hook it to controller #1, and insert it in ice water and then boiling water. It should display about 0C and 100C. If so, the instrument is probably OK. Make sure that it is programmed for type K.

If your thermocouple has a second junction (bad connection, nicked wires, etc) it will supply the average of 2 temps -- the process temp and the temp at the bad connection. 150C process temp and ~75C ambient = about 110C sent to the controller.

If you plug the TC into a hand held display (such as an Omega handheld or equivalent), then you eliminate ground loop question on the power line.

These are the first 3 steps that I'd do before digging in deeper.

 

You are getting consistently low thermocouple readings.

Any given mV reading will be interpreted as a lower temperature for a type J T/C than for a type K T/C. The error is not as large as you describe, 22°C low at 100°C, 34°C at 150°C; not the 50° you claim.

100C = 4.095mV(K) = 78+C for type J
110C = 4.508mV(K) = 86+C for type J
150C = 6.137mV(K) = 116C for type J

But, I suspect your PD meter is configured for some other thermocouple type than type K. I've used PD, but I have no idea what a 785 model is, whether it is configurable or not..

There is the possibility of damaged T/C wire. Any point at which the two thermocouple wires touch each other is a junction. There should only be one junction, at the hot end. But if you're using plain wire, not sheathed insulated sensors, then the wire could be damaged resulting in a 'false' junction at some point other than the hot end.

Extension wire is intended for extending wiring from the point of measurement to the instrument, not for actual sensing use. Vendors have thermocouple wire with which you can make your own thermocouples, but you might consider commercial, sheathed, mineral insulated thermocouples that have a ruggedness that simple wire does not. T/Cs designed for bash-around applications usually are sheathed and the extension wire is protected by some form of armor.

Extension wire has a higher error than standard limit-of-error T/C wire, but nothing in the range you're seeing.

You don't mention the type of power you're using. AC? DC? generator?

The low dielectric (fairly good insulation properties) of heating oil and tar, along with the fact that you are heating with a diesel burner (not electric) makes signal ground loops a somewhat remote possibility.

You don't mention the RTD. Is its reading OK? Does it go to a PD meter?
Why would the RTD read OK and the others don't, if they use the same meter?

By the way, in the instrumenation world, 'calibration' and 'configuration' are two very different beasts. Just because your meter is calibrated for type K, doesn't mean that it's configured/programmed for type K.

I believe the tech when he said your meter was calibrated at the factory. But doesn't necessarily mean that your meter was configured at the factory for the way you are using it.

A lot of temperature instrumenation can take multiple types of thermocouples, and the instrument must to be configured (or programmed) for the type which is actually connected. If your model PD meter can take one of several types of thermocouples, then it needs to be configured, which is not the same as "calibrating".

Bud

 

If you are to eliminate grounding problems you need to have a common ground however if your trailer does not touch the ground other than through the rubber tires it is NOT grounded to earth. (This is why if your car gets struck by lightning and you are in it you will not get electrocuted.) Therefore you need to be sure everything is referenced to a common point to start. Things should improve after that.

- Ian

 

Solved the problem after a week of troubleshooting.

First, we ran a line of new thermoucple wire to the probe (just a patch cord of sorts so that it would eliminate any potential grounding issues if the insulation was damaged during installation) and made sure that this did not remidy the problem. Unforutunately it did not, so now there are two possibilities, the Probe and the Displays.

All displays were correctly configured for type K thermocouples. (we doublechecked this just to make sure.)

I contacted the thermocupole manufacturer and he said that he would send out a test probe that he tested to rule out a damaged probe. We tested this new probe with our thermocouple reader and found that the thermoucple was within 1'C or so of what it should've been @ 150'C, so this ruled out the display as the cause of error.

We then connected the alleged damaged thermocouple and the working probe (To two different meters) and stuck them both in an oil bath. A Bimetalic thermometer was also put into the oil bath as a reference. We then heated up the oil bath to 150'C (As shown on the BiMetal thermometer). The new test probe that we recieved read 149'C, and the broken thermocouple read 89-90'C (Constant).

So it looks like this is a simple case of being sent damaged thermocouples. We then tested the other two thermocuples on the trailer. One thermoucple was accurate (the pump temperature one) and one was reading about 50-60'C too low (The heating oil one). I've never heard of a damaged thermoucple doing this before. Would anyone know a way that a thermocuple can malfunction in this way? Perhapse the wires inside the thermoucple crossed at two points making two junctions?

Anyways, the manufacturer is sending out two new probes and he will test them before they are shipped out to us. Hopefully this will be the last of the long list of equipment difficulties we have had. (10 DOA displays from Precision Digital, Broken (Open junction) RTD sensor, and now two damaged thermocouples.)

Thank you for your comments, they helped a lot.

Thanks,
Michael Cowie