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Customizing the Temperature Accuracy of Specimen Cup Temperature Labels

Posted by Steve Yeager on June 27,2018

 

The case study “Custom Temperature Labels Ensure Accurate Drug Screenings” taught the importance of temperature labels having the proper temperature profile to match the drug testing cup.  Here is more detail on why and how American Thermal Instruments (ATI) creates that custom temperature profile for drug testing temperature labels.

Customizing a temperature label requires ensuring the label accurately indicates the temperature of the fluid inside the cup and that it will adhere to the cup.  To be accurate, the label should be tested for the need of an offset, proper placement on the cup, and adhesion to the cup.

SpecimenCup

What is an offset?

Temperature labels, like those seen on the specimen cups above, have temperature sensitive liquid crystal dots that are formulated to change color at a specific temperature.  When a specimen cup is filled with a warm fluid, the heat must be conducted through the cup wall to the outside wall onto which the label is adhered. The label is also exposed to the air temperature within the room which is cooler than the liquid inside the cup.  In many cases a 92°F event on the label will have a liquid crystal dot formulated to change at 92F. If the liquid’s heat is conducted rapidly, a 92°F dot will change when the liquid temperature is 92°F. However, if the heat transfer is slow, a 92°F liquid crystal may change when the liquid temperature is 94°F.   For the cups with slow heat transfer, a different temperature liquid crystal dot can be placed on an event in order to accurately indicate the liquid temperature. For instance, in the example above where the heat transfer is slow, the 92°F liquid crystal dot could be placed on the 94°F event mark to accurately indicate the liquid temperature.  This is a 2°F offset. Many of ATI’s temperature labels for specimen cups have some level of offset.

What factors dictate whether a temperature offset is required?

  • Thermal Conductivity of Cup Wall:  Common plastics used for cups are polyethylene and polypropylene.   The table below lists the thermal conductivity of polypropylene (PP) and polyethylene (PE) and some other common materials for comparison.  PP and PE are poor conductors of heat. PE has over two times the thermal conductivity of PP. Equal in importance to cup material is the variation in thickness of container wall from cup to cup in determining thermal conductivity observed at the point of label application.  The thicker the wall, the slower the thermal transfer.

Material

Thermal Conductivity (Watts/(Meter Kelvin)

Copper

401

Carbon Steel

54

Glass

1.05

Polyethylene (high and low density)

0.33-0.51

Polypropylene

0.10-0.21

Styrofoam

0.033


What else affects the accuracy of a temperature strip?

  • Position of Label on Cup:  Placement of the temperature label is dependent on the minimum fill level of the cup and the design of the cup.  Temperature strips are placed near the bottom of the cup to be under the minimum fill level. The lowest a label can be positioned and still be accurate is best to ensure the label is adhered to a cup wall area in contact with fluid.  The design of that bottom level of the cup will then dictate the placement of the label. For instance, labels should not be placed on the bottom lip of the cup that either is not in contact with fluid or may cool the fluid. Labels should not be placed over areas with built in test strips that insulate  the outer cup wall from contact with fluid. The photos below show two different placements of strips on a cup. The first photo is of a low placement and second photo is of a higher placement. The lower placement is near a molded lip underneath the cup and the heat transfer is uneven creating multicolored lines across the temperature dots.   The preferred higher placement shows an even distribution of color across each temp dot and a higher overall temperature.
LabelTooCloseToBottom

Label too close to bottom

 

 LabelInCorrectPosition

Label in CORRECT position

  • Adhesion:  The area for adhesion must be flat so that no air pockets are formed between the label and the cup surface.  ATI can evaluate adhesion of the temperature indicator to the cup. Should the label release over time, this could result in inaccurate readings and tearing of the label.  If a cup design or material is changed, it should be submitted to ATI to ensure continued good adhesion and accuracy.

 

How are offset and placement determined?

The ATI technical group is supplied with cups from the customer.  Two to three labels will be applied to a cup. A comparison is conducted between a calibrated electronic digital thermometer (EDT) with an accuracy of ±0.01°C in the liquid and temperature strip(s) on the outside of the cup.  (See left photo below). Warm water is added to the cup and the temperature from the EDT and temperature strip are recorded. Data is then graphed. The photo below on the right shows the importance of placement (See right photo below). The lower label is too close to the bottom which has a molded lip.  The temperature dots are cooler at the bottom than the top causing two to three colors to be visible in one dot. The higher label has a consistent heat transfer and only one color on each dot.

       TempGage        TempStrips

 

Figure 1 (below) is the graph of data collected from a customer’s cup with their current temperature label.  A difference of about 1 – 1.5°C between liquid (EDT) and label (DDI). An ATI temperature label with proper offset is shown in Figure 2 (below).  The liquid temperature and temperature label match very closely, showing the ATI thermometer more accurately reflects the temperature of the liquid.   

CurrentCustomerTemperatureStripFigure 1

Figure2

Figure 2

Because ATI provides the service of customizing temperature labels, the labels impart the customer a higher assurance that test specimens come from the proper donor.

Topics: Medical Safety, Industry Knowledge, Temperature Monitoring, Medical Devices

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