Electronic International CHT ungrounded?

[Lawson Laslo]

Hello I purchased the CHT gauge as seen in picture below
I was hoping to use the MGL cht spark plug sensors that are type K but there grounded only $35
And I’m aware that EI uses ungrounded Sensors... costing$100
If I where to use the grounded MGL sensors would it throw readings off by much or cause any issues?

 

[Lawson Laslo]

 

[flyingron]

There's an EI guy who is a member here, but I think it shouldn't care. The inputs are differential. You use one for the non-grounded side of the probe and the other for the ground. If the probe doesn't have two wires, you'll need to make one up to connect to a ground close to the thermocouple.

 

[PaulMillner]

There's an EI guy who is a member here, but I think it shouldn't care. The inputs are differential. You use one for the non-grounded side of the probe and the other for the ground. If the probe doesn't have two wires, you'll need to make one up to connect to a ground close to the thermocouple.

That's not how grounded probes work... the thermocouple junction is grounded at the tip of the probe. That's done to improve heat transfer, and allow the thermocouple to respond more quickly. That at least is the claim that JPI and others that use grounded probes make. The disadvantage is that stray current can theoretically be introduced, causing noisy temperature signals.

Thermocouples generate very small millivolt range voltages, so they're always wired two-wire back to the cold-junction (CJ) compensation board where the input is measured. IIRC, EI twists the thermocouple wire pair, JPI does not... in both cases, it's the same metallurgy, in the case of EGTs, chromel and alumel, all the way to the CJ compensation board. JPI uses chromel-alumel for everything (K thermocouples). EI uses iron-constantan (J thermocouples) for the CHT readings... again, iron and constantan wires all the way to the CJ board.

 

[Lawson Laslo]

That's not how grounded probes work... the thermocouple junction is grounded at the tip of the probe. That's done to improve heat transfer, and allow the thermocouple to respond more quickly. That at least is the claim that JPI and others that use grounded probes make. The disadvantage is that stray current can theoretically be introduced, causing noisy temperature signals.

Thermocouples generate very small millivolt range voltages, so they're always wired two-wire back to the cold-junction (CJ) compensation board where the input is measured. IIRC, EI twists the thermocouple wire pair, JPI does not... in both cases, it's the same metallurgy, in the case of EGTs, chromel and alumel, all the way to the CJ compensation board. JPI uses chromel-alumel for everything (K thermocouples). EI uses iron-constantan (J thermocouples) for the CHT readings... again, iron and constantan wires all the way to the CJ board.

Thanks for all the info!
So it sounds like i can not use a grounded cht, is that correct?

 

[PaulMillner]

Thanks for all the info!
So it sounds like i can not use a grounded cht, is that correct?

It depends! If EI doesn't chime in here, give them a call, they pride themselves on their customer support. Some units that use ungrounded TCs will generate an error if you use a grounded couple. I don't know if that's true for the somewhat aged box you are considering, but EI can tell you.

Paul

 

[flyingron]

Strange, I have all my EGT and CHT probes paralleled to the EI MVP-50 and the JPI-830.

 

[PaulMillner]

Strange, I have all my EGT and CHT probes paralleled to the EI MVP-50 and the JPI-830.

What do you view as being strange about that, Ron? Are your probes grounded (JPI) or ungrounded (EI)? If they're EI ungrounded ones, I don't think JPI would care. The question above is if the old EI units care about grounded probes... your MVP50 is a different generation of device, of course.

 

[bnt83]

@Dave Arata

 

[Dave Arata]

Our legacy instruments (such as the C-6 pictured above) require the use of an un-grounded type K thermocouple. Our MVP-50/CGR-30 system can be interfaced with either grounded or un-grounded probes and can be type K or type J.