Cropico Type P.4/E Thermocouple Potentiometer

[Superscope]

Well, I know what a Thermocouple is, but I've never had the need
to check one to the level this Equipment seems designed for.

Has anybody on the Forum ever used one of these pieces of Equipment in anger?

Where exactly would this have been used?

I'm guessing it wouldn't have been used for checking your domestic Gas Boiler!
Or would it?

I guess this is just a Bridge at the end of the Day, but it is an impressive piece of Kit.

Is it of any use today?

What type of Thermocouples was this designed for?
I assume not all Thermocouples were made Equal.



Ian

[Neutrino]

I guess it would have been used as a calibration standard in a laboratory. The reference junction can be put in a bucket of melting ice. Alternatively the temperature of the reference junction can be measured and the corresponding voltage from the thermocouple tables added to the measured voltage.

It is a collector’s item. Nowadays you can easily measure microvolts with a multimeter.

This can be used with any type of thermocouple. You need to know the type of thermocouple and refer to the corresponding table that shows the relationship between temperature and voltage when the reference junction is at 0°C.

David

[G4_Pete]

Hi these type of devices were used extensivly in the chemical industries mainly for calibrating temperature instruments connected to thermocouples and occasionally for measuring temperatures on process conected thermocouples as a cross check. My Apprentice diary shows I must have been shown how to use them in 1974.

I dont think I used this exact model but have used many similar. Normal operation was to standardise the instrument using the reostat control against an internal mercury standard cell and then balance the millivolts on inject or measure using the mv scales. Most of these are a bridge type of balance as you mentioned. Actual temperature is taken from charts depending on the thermocouple type. Also as the voltage obtained is a function of the difference between the hot end vs the cold end of the disimilar metals junctions a compensation temperature would need to be applied. Looks like you have the thermometer in the lid from the photograph.

Have a look on the lid instructions and this should make more sense now.

Providing the internal reference is good this instrument could still be used either for straight mv checks or temperature via the charts for the thermocouple types.

Pete

On the subject of thermocouples and the zero degree reference, one of the most ingenious I saw was metal bellows full of water, cooled by a Peltier device, switched by the expansion of the ice. The face opposite the Peltier was used for the zero reference.

Lovely bit of kit too, they don't make them like that anymore.

[MotorBikeLes]

When our kiln control instruments were serviced, a potentiometer like this was used. Usually, a Cambridge, but the "poorer" chaps turned up with the Cropico. Inside, there will (likely) be a Weston Cadmium standard cell. Terminal emf 1.0186v from memory. You do NOT draw current from it. Instead, there is also a normal 1.5v Leclanche cell, which is got to "null balance" against the standard cell. That is periodically used to ensure calibration is correct.There thermocouples used in our industry were normally Pt against Pt/13% Rh, and for some lower temperature purposes, chromel/alumel. On the cheaper Cropicos, they had circular printed scales which were fitted for each thermocouple type. For higher temperature use, one with 10% Rhodium was used. Yours is a more expensive model, presumably with one or more built in scales. You can have one only calibrated in mv, when you have to use your thermocouple tables.
Les.

[David G4EBT]

Originally Posted by Superscope

Well, I know what a Thermocouple is, but I've never had the need
to check one to the level this Equipment seems designed for.

Has anybody on the Forum ever used one of these pieces of Equipment in anger?

Where exactly would this have been used?

I'm guessing it wouldn't have been used for checking your domestic Gas Boiler!
Or would it?

What type of Thermocouples was this designed for?
I assume not all Thermocouples were made Equal.

Ian

I spent my career with British Gas, (actually, until privatisation, The Gas Board'). In 1968, when the natural gas conversion programme was underway, I spent a year in the laboratories as part of a team designing conversion kits to convert obsolete gas appliances of every type to safely burn natural gas, which British Gas would otherwise have had to replace.

Converting gas cookers was quite a challenge, both the hotplate burners and the oven. I'll skip the detail of what was involved in burner design, such as natural gas being twice the calorific value of coal gas, a much slower flame speed and narrower limits of combustion or I'll go too far off topic and exceed my bandwidth, something on which I've got form.

Controlling the temperature in an oven (especially obsolete ones dating back to the 1940s) was problematical as they used a bimetallic (brass and steel) tubular thermostat to control the flow of gas. Once a conversion kit had been designed, the ability of the thermostat had to be checked to see that it would control the temperature within certain limits. That's where the Cropico Thermocouple potentiometer came into it.

A thermocouple in a blacked copper ball about 2" in diameter was placed in the centre of the oven, and another thermocouple was placed is a flask of ice cubes. The potentiometer was brought into balance and the reading was compared with a chart which indicated the oven temperature. That had to correspond with the expected temperature on thermostat knob.

Why was the thermocouple in a a black ball?

Kirchhoff I think, (whose chum Mr Bunsen invented the Bunsen burner, which every gas appliance to this day uses):

https://en.wikipedia.org/wiki/Gustav_Kirchhoff

Assuming that the oven worked correctly and didn't generate carbon monoxide, it then had to pass the standard test for ovens, which involved baking a Victoria Sponge, created from a standard set of ingredients. There weren't many perks of the job, but 'let them eat cake' was one of them.

Still the standard test for checking the performance and temperature regulation of ovens - gas or electric:

https://www.which.co.uk/reviews/buil...s-aqQBj7y8PqF2

A long-winded way of saying yes, I used a Cropico Thermocouple Potentiometer many times a day for a year in the early stages of my career.

Hope that's of interest.

[McMurdo]

I think to simplify matters, it needs to be said that the gas thermocouple in the OP which just tests for presence of a flame as part of a flame failure device (safety cut off found in boilers, cookers, ovens, fires) is different to an instrumentation thermocouple which is used to accurately measure, monitor or control temperature, based on the predictable relationship between the thermocouple temperature and the amount of millivolts it generates over a defined temperature range. The cropico will be aimed at the latter class of devices.

Of course these days it's all done digitally and thermocouple calibrators consist of a peltier temperature source. You just stick your thermocouple into a hole, key in a temperature and the unit will heat or cool the thermocouple to the required level to be read off on another display.
Temperature displays are likewise tested by dialling in the thermocouple type (based on the metals used) and a test temperature. The calibrator injects the correct millivolts into the display with automatic CJC compensation taken into account.

The cropico is a nice relic but would be a real PITA to anyone used to modern equipment.

[Superscope]

Thanks for all the feedback, especially yours David.
I always enjoy reading your extra Long replies.

Well, I was very tempted to keep this Meter myself,
but I have resisted.
It is a very well built piece of equipment, but in all honesty,
not my area of interest, but for sure would make a nice addition
to somebodies collection.

It is now posted on the "Parts Offered" Section.
https://www.vintage-radio.net/forum/...d.php?t=184174



Ian