Transmitter Scaling

[Dale Hayes]

How is the 4-20mA points determined for the 4mA (deg F) and the 20mA (deg F) values? Is there any standard or formula for determining the transmitter temperature span?

In my process I am using a thermocouple. My process temperature range is 60F to 110F.

 

[petersonra]

How is the 4-20mA points determined for the 4mA (deg F) and the 20mA (deg F) values? Is there any standard or formula for determining the transmitter temperature span?

In my process I am using a thermocouple. My process temperature range is 60F to 110F.

There is no "standard" as such.

Most times one would set the span of the xmtr so that

1) the actual expected temperatures including unusual conditions will always be within the span.

2) the "normal" reading will be perhaps 50-75% of full span.

It is not always possible to get #2 depending on #1.

I try to use something that is reasonably straightforward like 0-100 deg C. However, I have a customer who always uses strange spans on temp xmtrs like 20-220 deg F. No idea where he came up with that, although it is not far off from 0-100 deg C.

 

[Dale Hayes]

Part 2

Part 2

Another Question:

Some Temperature Transmitter vendors offer the output signal at (4 to 20)mA or (20 to 4)mA. What is the purpose of a 20-4mA?

 

[petersonra]

Another Question:

Some Temperature Transmitter vendors offer the output signal at (4 to 20)mA or (20 to 4)mA. What is the purpose of a 20-4mA?

I have never seen a temp xmtr that went 20-4 mA. I have no idea why someone would want to do that.

 

[GoldDigger]

I have never seen a temp xmtr that went 20-4 mA. I have no idea why someone would want to do that.

Measuring cold instead of heat?
I can imagine situations where it would simplify the downstream control logic, but not by much....

 

[petersonra]

I am trying to imagine some scenario where in the case of T/C loss you would want the thing to go full scale and full scale is cold.