Pressure Transducers

On my last post I discussed common refrigeration pressure switches. As the name implies, these are switches which are opened and closed by pressure changes. They can make or break circuits, but they cannot indicate pressure. Pressure transducers are often used for electronic controls because they can actually indicate system pressures.

The word “transduce” means to change from one form to another. A pressure transducer turns pressure changes into analog electrical signal changes. This is most often a change in a DC voltage, typically 0 – 5 volts DC. This changing voltage can then be interpreted as a pressure by the electronic control to which it is connected.

The most common pressure transducers used in HVAC use a small stainless-steel diaphragm with strain gauges bonded to it. A change in pressure causes the diaphragm to bend, which causes the strain gauges to change resistance. These transducers have three leads: two are wired to DC+ and DC- and the third carries the signal. Pressure transducers ohm out like a potentiometer. On diagrams this looks like a potentiometer with a pressure bellows connected to the wiper arm. The resistance between the two leads that connect to DC voltage should stay the same regardless of the pressure. The third lead changes resistance relative to the two other leads as the pressure changes.

When the two other leads are connected to 5 volts DC, the signal connection will vary between 0 and 5 volts DC depending on the pressure. The control then interprets this voltage and controls the system based on the board’s program. If you want to check the transducer signal, read the voltage between the signal lead and DC- and then compare this voltage to a chart published by the manufacturer. Here are a couple of links to more information on presure transducers

Omega Transducers

Emerson Climate Technologies

 

Technically Speaking

Throughout my entire career in HVACR, technology has been changing how we work and what we work on. During this time, people have been continually complaining about the changes wrought by new technology. I can still remember an older technician in the late 70’s (yes I was actually working way back then). He was bragging in the parts house about replacing the primary control on someone’s oil furnace. It had a newer cad-cell type sensor which he did not understand, so he retro engineered the system to use a stack switch. Basically, he charged the customer to downgrade their equipment because he did not understand the controls. Another tech agreed, saying those cad cell things were not trustworthy. Then he added that he did not work on heat pumps – if a customer had a heat pump, they had to call somebody else. Today these guys would not be able to work – there just are not enough systems that meet their limitations. Many folks today don’t feel comfortable with communicating systems, but these systems are here to stay. In unitary product lines, communicating systems are still just the higher end. The thing is, most air conditioning manufacturers are offering them now. Virtually all mini-split, multi-split, and VRF systems use communicating control technology. And in case you haven’t noticed, those systems are here to stay as well. Sure, new technology can be a pain – especially brand new technology. Early adopters often pay a price for being the first with the new stuff. However, bitching about having to learn something new won’t keep the world from changing. You will just be left behind. As I age, I understand the pain of the old techs who just want to do the stuff they with which they are comfortable. I become less flexible both physically and mentally as I age, so I find it necessary to study harder to keep up with current field technology. As I see it, I have a choice – I can complain that the future will be different than the past, or I can work hard to keep up with changes in this field I love.    

Tracing a Communications Problem

One bad apple don’t spoil the whole bunch girl …
You may know the Jackson Five hit that line comes from.
(The first time I heard it was NOT on an oldies radio station.)  Unfortunately, when it comes to communicating controls, one bad communicating board CAN take down the whole network. If the communicating portion of a board is sufficiently damaged, it can prevent communication to ANY of the components on the communicating network. This is because they are all tied together in parallel. A sufficiently low resistance can pull down all communication signals on the whole bus. We saw this first hand this week in our lab. A system that had been operating correctly started having problems communicating. A lab instructor had put in a problem and the student tried to troubleshoot by swapping wires around, resulting in a dead board. (In my experience, swapping around wires just to see what will happen almost never ends well!) Now, the ENTIRE SYSTEM stopped communicating and the control could not find the furnace, which did not have any problems. When the communicating wires to the condensing unit were disconnected, the thermostat found the furnace and was able to operate it. The bad condensing unit board managed to pull down the entire communication bus. When you have a communicating control system that will not communicate, the first step should always be to check your connections. Make sure they are correct and making good contact. Then, try to isolate different parts of the system by connecting one part of the system at a time to the communication bus and seeing which specific component will not communicate. Unlike standard 24 volt controls, you cannot read a voltage to determine if a board is communicating. Isolating the components and checking them individually usually identifies the component which is causing the problem.