Confusion in Defining Calibration Range

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Confusion in defining calibration range

Posted by Gopi on 13 October, 2009 - 12:48 am

Hello,

I have a confusion in defining calibration range.

As far as i know calibration range is defining ranges for which corresponding 4 mA or 20 mA should be generated fo

a given sensor. Is this correct?

When comes to my work i am doing data sheets for field instruments in my datasheets like pressure transmitter and

temperature transmitter there are fields like:

Range Limits

Calibration Range

i will define range limits my considering Max operating Pressure/Temp that will come to 66 % of the desired range.i

am clear about range limits.

but for defining calibration range i am not clear. when i made little research on internet some said Min Operating

pressure/Temp and Max operating pressure/Temp will be calibration range. But i feel this not clear because some

people contradicted this.Please clarify me regarding this calibration range.

To add about temperature transmitters, i am using both RTD's and Thermocouple's for my application. when i

interacted with some vendors they said there is no use of defining any calibration range at all for temperature sensors

as everything is predefined in the form of table's and said calibration range is needed only for pressure sensors and

other sensors other than temperature sensors.

But some vendors are contradicting this statement.

i am now totally confused in defining calibration range. If to define please guide me how to decide calibration range

with respect to sensors application (Pressure, Temperature, Flow etc).

Please help me as soon as possible.

Thanks in advance:)

With Reagrds,

Gopi

Posted by Roy Matson on 13 October, 2009 - 11:09 am

It is confusing as you say.

The calibrated range is where you define your 4 & 20 mA points

e.g. it might be possible to set the 20 mA point for a pressure transmitter between 50 and 150 psi, but you set it at

100.


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The calibration range is 0 - 100 psi

The span is 0 - 50 to 150 psi, sometimes on a data sheet you see it expressed as 50 - 150 psi

I normally specify a transmitter so the calibration range ends up near the mid point or higher in the span.

Hope this helps a little.

Roy

Posted by James Fountas on 14 October, 2009 - 8:48 am

Hi Gopi,

The calibration range may not be the same as the span of an instrument. For example, if you want a temperature

sensor with a span of 600 to 1400 C, but the critical process temperature is around 1150 C, you may do the

following. You can order a temperature sensor with a span 600 to 1400 but request a three point calibration at 112

1150 and 1175 C. You want accuracy around 1150 and outside of that you don't need a high accuracy. Calibrating

the instrument across the whole range may not give you a great accuracy around the point that is most critical to

you.

Best to talk to people at the other end about paper work and make sure that everyone understands the paper work

in the same fashion.

Regards,

James Fountas

Posted by ultima on 14 October, 2009 - 9:02 pm

hi,

just added, range limits are basically an limitation for operating range itself. yes some are around 60% some are

up to 75% from its max operating limits. i.e if an xmitter have max operating limits for 100 bar and minimum

operating limits are 0 bar then the range limits will be around 0-60 or 0-75 bar. it depends the vendors. about

calibration range, well I was told that it was actually the ranges where you put 0 to 100% or yes the 4-20mA

equal to the desired values you want the xmitter to operates. you have Temp transmitter have the max op range

for 100 C (use an RTD sensor for example) but you only need it to operate from 12 C to 75 C (it depend on

process, well i don't know what kind of process but it's just an examples) then you calibrate it. (some said it was

re-rangging not calibration because calibration was set the 0-100% values as exact same as max and min limits o

the instruments) to 12 C for 0% or 4mA and 75 C for 100% or 20mA. well that's all I can tell you. maybe want t

try looking at the internet perhaps some instrumentation e-learning center, because I was little confuse about the

differs between calibration and re-ranging itself. sorry if this can't help.

Posted by JMW on 15 October, 2009 - 8:33 pm


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Not all instruments require calibration... PRTs don't need it unless you are being extra fussy about accuracy.

Some instruments do. The factory calibrated range is the range of values over which the instrument has been

tested. This isn't the same as the operating range which will be the range over which it is actually used or can b

used. Nor is it the range of the outputs. This depends on the purpose of the instrument in the application.

So, if you have a PRT temperature sensor and want to use it over the range 50-70degC, then you can set yourspan and bias on the 4 - 20 mA to be whatever you like. It might be you will set them as 0 and 100 deg C for a

indicator (but match the indicator span and bias to your sensor span and bias) or 50 degC as 4 mA and 70 degC

as 20 mA if it is for control.

Posted by Roy Matson on 16 October, 2009 - 10:55 am

And if you weren't confused enough before you will be even more confused now after reading all the previous

posts LOL.

Roy

Posted by Jonas Berge on 3 February, 2010 - 11:12 am

For range, calibration, sensor limits, and trim, please refer to the

technical white paper on this page:

http://www.eddl.org/DeviceManagement/Pages/Calibration.aspx

Range Limits: This should be called sensor limits. This is the minimum and maximum input the sensor can handle.

Calibration Range: The confusion is in the term "calibration" because it means three different things:

- Sensor trim: correct reading when sensor has drifted

- Range setting: set the points corresponding to 4 mA and 20 mA respectively

- Current trim: if output current does not match DCS AI card

For instance, a differential pressure transmitter may have a sensor that can measure from -250 inH2O to +250

inH2O. These are the lower and upper sensor limits.

For that DP transmitter you may set a smaller range: 0-200 inches to measure the level in your tank. These are the

lower and upper range values.

When you receive your DP transmitter from the manufacturer, sensor trim was already done in the factory so no

need to redo that. However, after years in operation it may drift and need sensor trim. In your workshop your

European deadweight tester may use metric weights so you cannot test at 200 inH2O so you pick something close

to your range values: 0 and 5000 mmH2O. These are your lower and upper trim points.


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Cheers,

Jonas

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Confusion in Defining Calibration Range