offset head pressure on transducer

[aircon1]

Hi, i need some help. i've recently started to work on chillers. My background is vrf systems so this kit is alien to me. i'm ok with it except i have come across hp offsets, on a carel controller which is on a multiclima chiller, can someone explain this to me as i need to set up a new transducer. does offsets mean i have to tell the transducer what my hp reading on my analogue gauge is? e.g. if my gauge reads 19bar and the controller reads 17.8bar does this mean i have to enter an offset of +1.2bar?

any info would be appreciated .
thanks guys.

[frank]

It will all depend on your gauges being calibrated or not.

When was the last time they were calibrated?

In other words......is the pressure transducer reading the correct pressure or your gauges - how do you decide?

[aircon1]

hi frank, there are built in gauges on the chiller itself that read exactly the same as my manifold so i think they're ok. they are 3 years old and have never been calibrated. i have new ones coming soon. i hope they will be spot on.
was i thinking along the right lines then frank?

[RONB]

On some controllers you have to put the full range of the xducer e.g if it is an absoloute xducer you would put a minimum press at -14.7 and lets say it was a hp xducer with a max pressure of say 300 psi you would set the max range to 300 so the controller knows what range it has to work to. Then you may have an offset parameter to calibrate the xducer input as well which as you say can be set up via your manifold.

[Tayters]

The only ones I've messed with were Danfoss ones on an Elm controllers. Basically you set the max and min range of the transducer in the controller settings. Elm called 'offset' the minimum value the transducer would read and 'span' the highest. This would then match pressures the controller would interpret with the mA output of the transducer (4-20mA if I remember rightly).

Cheers, Andy.

Here's a bit from their manual on it.



Calibrating the pressure transducers

1 Set

probe offset to the minimum value of the transducer’s pressurerange and probe span to its maximum.2 Open the transducer to the atmosphere and note its reading.3 Put the transducer back in place, fit a pressure gauge andpressurise the system to half the value of probe span. Note thedifference between the pressure indicated by the transducer and thetrue gauge pressure.4 Alter probe offset — if the atmospheric reading was +0.1 bar reduce

probe offset

by 0.1 bar5 Alter probe span — if the half-range reading was 0.2 bar below the
true gauge pressure, increase probe span by 0.2 bar.

[da_fitz_man]

Agree with the prior two posts, the controller will most likely be using 4 - 20mA input for pressure reading.
ie for a Carel transducer with a range 0 to 30 Bar, 0 would equate to 4mA, 20 mA equates to 30 Bar.

Understandably the controller could use a variety of probe input types - 0-5V Ratiometric, 0-10V, 0 -20mA.
(Typically 4-20mA though, but not always)
Also remembering the range of the transducer varies aswell, -0.5 to 9.5 Bar through to 0 to 45 Bar types.

The transducer selection should not always be the biggest which covers every scenario, as the scaling range will be 'fuzzy'. ie small increments in pressure are not as easily recognised with a 0 -30Bar range compared to a 0 to 10Bar transducer.

Describing an offset setting sounds like a calibration setting.

Typical programming would display -
Type of Input B1: 4 - 20mA ( B1 = Probe input 1 for example. )
minimum value of Probe B1: 0.0 Bar
maximum value of Probe B1: 30.0 Bar
Probe offset: 0.0

As with any sensor which drifts to far out of calibration, it will most likely need changing.

Another note from experience is if the pressure rises or falls out of range it can appear as an intermitting 'transducer fault'. ie the plant is shut down and the pressure equalises for whatever reason, upon restarting the control may not 'see' the transducer. eg. the pressure is 12bar on a 10 Bar transducer.